Assertions

Assertions are a way of breaking out of code when there is an error or an unexpected input.

Some languages throw exceptions and some treat it as a break point.

Task

Show an assertion in your language by asserting that an integer variable is equal to 42.

11l

Translation of: Python

V a = 5
assert(a == 42)
assert(a == 42, ‘Error message’)

68000 Assembly

CMP.L #42,D0
BEQ continue
    ILLEGAL   ;causes an immediate jump to the illegal instruction vector.
continue:
; rest of program

Ada

Using pragma Assert:

pragma Assert (A = 42, "Oops!");

The behavior of pragma is controlled by pragma Assertion_Policy. Another way is to use the predefined package Ada.Assertions:

with Ada.Assertions;  use Ada.Assertions;
...
Assert (A = 42, "Oops!");

The procedure Assert propagates Assertion_Error when condition is false. Since Ada 2012 one may also specify preconditions and post-conditions for a subprogram.

procedure Find_First
     (List     : in     Array_Type;
      Value    : in     Integer;
      Found    :    out Boolean;
      Position :    out Positive) with
      Depends => ((Found, Position) => (List, Value)),
      Pre     => (List'Length > 0),
      Post    =>
      (if Found then Position in List'Range and then List (Position) = Value
       else Position = List'Last);

The precondition identified with "Pre =>" is an assertion that the length of the parameter List must be greater than zero. The post-condition identified with "Post =>" asserts that, upon completion of the procedure, if Found is true then Position is a valid index value in List and the value at the array element List(Position) equals the value of the Value parameter. If Found is False then Position is set to the highest valid index value for List.

Aime

integer x;

x = 41;
if (x != 42) {
    error("x is not 42");
}

Executing the program will produce on standard error:

aime: assert: 5: x is not 42

ALGOL 68

The "Revised Report on the Algorithmic Language - ALGOL 68" suggest that ASSERT may be made available by a particular implementation, quote: "Pragmats may ... convey to the implementation some piece of information affecting some aspect of the meaning of the program which is not defined by this Report,..."

Example given[1]:

INT a, b; read((a, b)) PR ASSERT a >= 0 & b > 0 PR;

This works with neither ELLA ALGOL 68 nor ALGOL 68G.

The standard alternative would be to implement the assertions as an exception as per the Exceptions sample code.

In ELLA ALGOL 68 the ASSERT is implemented as an operator in the environment prelude:

OP      ASSERT = (VECTOR [] CHAR assertion,BOOL valid) VOID:
IF      NOT valid
THEN    type line on terminal(assertion);
        terminal error( 661 {invalid assertion } )
FI;

And can be "USEd" as follows:

PROGRAM assertions CONTEXT VOID
USE standard,environment
BEGIN
  INT a := 43;
  "Oops!" ASSERT ( a = 42 )
END
FINISH

ALGOL W

Assertions were added to the 1972 version of Algol W. If the tested condition is false, the program terminates. In the following, the write does not get executed.

begin
    integer a;
    a := 43;
    assert a = 42;
    write( "this won't appear" )
end.

Apex

Asserts that the specified condition is true. If it is not, a fatal error is returned that causes code execution to halt.

String myStr = 'test;
System.assert(myStr == 'something else', 'Assertion Failed Message');

Asserts that the first two arguments are the same. If they are not, a fatal error is returned that causes code execution to halt.

Integer i = 5;
System.assertEquals(6, i, 'Expected 6, received ' + i);

Asserts that the first two arguments are different. If they are the same, a fatal error is returned that causes code execution to halt.

Integer i = 5;
System.assertNotEquals(5, i, 'Expected different value than ' + i);

You can’t catch an assertion failure using a try/catch block even though it is logged as an exception.

Arturo

a: 42
ensure [a = 42]

AutoHotkey

Exceptions

Works with: AutoHotkey_L

a := 42
Assert(a > 10)
Assert(a < 42) ; throws exception

Assert(bool){
    If !bool
        throw Exception("Expression false", -1)
}

Legacy versions

if (a != 42)
{
OutputDebug, "a != 42" ; sends output to a debugger if connected
ListVars ; lists values of local and global variables 
Pause ; pauses the script, use ExitApp to exit instead
}

AWK

AWK doesn't have a built-in assert statement. It could be simulated using a user-defined assert() function defined as below. The BEGIN section shows some examples of successful and failed "assertions".

BEGIN {
	meaning = 6 * 7
	assert(meaning == 42, "Integer mathematics failed")
	assert(meaning == 42)
	meaning = strtonum("42 also known as forty-two")
	assert(meaning == 42, "Built-in function failed")
	meaning = "42"
	assert(meaning == 42, "Dynamic type conversion failed")
	meaning = 6 * 9
	assert(meaning == 42, "Ford Prefect's experiment failed")
	print "That's all folks"
	exit
}

# Errormsg is optional, displayed if assertion fails 
function assert(cond, errormsg){
	if (!cond) {
		if (errormsg != "") print errormsg
		exit 1
	}
}

The above example produces the output below, and sets the program's exit code to 1 (the default is 0)

Ford Prefect's experiment failed

Axe

A=42??Returnʳ

BASIC

BaCon

' Assertions
answer = assertion(42)
PRINT "The ultimate answer is indeed ", answer

PRINT "Now, expect a failure, unless NDEBUG defined at compile time"
answer = assertion(41)
PRINT answer
END

' Ensure the given number is the ultimate answer
FUNCTION assertion(NUMBER i)

    ' BaCon can easily be intimately integrated with C
    USEH
        #include <assert.h>
    END USEH

    ' If the given expression is not true, abort the program
    USEC
        assert(i == 42);
    END USEC

    RETURN i
END FUNCTION

Output:

prompt$ bacon -q assertion.bac && ./assertion
Converting 'assertion.bac'... done, 24 lines were processed in 0.006 seconds.
Compiling 'assertion.bac'... cc  -c assertion.bac.c
cc -o assertion assertion.bac.o -lbacon -lm 
Done, program 'assertion' ready.
The ultimate answer is indeed 42
Now, expect a failure, unless NDEBUG defined at compile time
assertion: assertion.assertion.h:16: assertion: Assertion `i == 42' failed.
ERROR: signal ABORT received - internal error. Try to compile the program with TRAP LOCAL to find the cause.

prompt$ bacon -q -o '-DNDEBUG' assertion.bac && ./assertion
Converting 'assertion.bac'... done, 24 lines were processed in 0.003 seconds.
Compiling 'assertion.bac'... cc  -DNDEBUG -c assertion.bac.c
cc -o assertion assertion.bac.o -lbacon -lm 
Done, program 'assertion' ready.
The ultimate answer is indeed 42
Now, expect a failure, unless NDEBUG defined at compile time
41

BASIC256

Works with: BASIC256 version 2.0.0.11

subroutine assert (condition, message)
 if not condition then print "ASSERTION FAIED: ";message: throwerror 1
end subroutine

call assert(1+1=2, "but I don't expect this assertion to fail"): rem Does not throw an error
rem call assert(1+1=3, "and rightly so"): rem Throws an error

BBC BASIC

      PROCassert(a% = 42)
      END
      
      DEF PROCassert(bool%)
      IF NOT bool% THEN ERROR 100, "Assertion failed"
      ENDPROC

BQN

BQN has a primitive for logical assertions called Assert(!).

Called with a single argument, it throws an error if its argument is not 1.

Called with a left argument, it prints the left argument, and then throws an error.

   ! 2=3  # Failed
Error: Assertion error
at ! 2=3  # Failed
   ^
   "hello" ! 2=3  # Failed
Error: hello
at "hello" ! 2=3  # Failed

Brat

squish import :assert :assertions

assert_equal 42 42
assert_equal 13 42  #Raises an exception

C

#include <assert.h>

int main(){
   int a;
   /* ...input or change a here */
   assert(a == 42); /* aborts program when a is not 42, unless the NDEBUG macro was defined */

   return 0;
}

To turn off assertions, simply define the NDEBUG macro before where <assert.h> is included.

There is no mechanism to add a custom "message" with your assertion, like in other languages. However, there is a "trick" to do this, by simply logical-AND-ing your condition with a string constant message, like in the following. Since a string constant is guaranteed to be non-NULL (and hence evaluated as True), and since AND-ing with True is an identity operation for a boolean, it will not alter the behavior of the assertion, but it will get captured in the debug message that is printed:

assert(a == 42 && "Error message");

This trick only works with messages written directly in the source code (i.e. cannot be a variable or be computed), however, since the assertion message is captured by the macro at compile-time.

C# and Visual Basic .NET

.NET provides the Debug.Assert and Trace.Assert methods, which notify TraceListener instances subscribed to the program's trace output if the specified condition is false. Both methods also have overloads that allow a specified string to be added to the default message of the assertion, which consists of "Assertion Failed" and a stack trace for the location of the assertion.

The behavior of a failed assertion is controlled by the listeners in the TraceListeners collection shared by the Debug and Trace classes. By default, the collection contains an instance of the DefaultTraceListener class, which uses functions in the Windows API that notify attached debuggers, if any. Additional behavior depends on the framework version that the application is running in:

In .NET Core applications, if no debuggers are attached, failed Debug.Assert assertions for non-UI applications terminate the program and write the assertion message to the console, while failed Trace.Assert assertions do not affect execution. In this respect, a failed Debug assertion behaves similarly to an exception.
In .NET Framework applications, for both types of assertions, a special instance of the Abort-Retry-Ignore message box containing the assertion message is displayed (even with a debugger attached). "Abort" terminates the program; "Retry" switches to the location of the assertion in source code if the application is running in a debugger, or, if none are attached, prompts to launch a just-in-time debugger; and "Ignore" continues execution past the assertion.

Calls to methods of the Debug class are only compiled when the DEBUG compiler constant is defined, and so are intended for asserting invariants in internal code that could only be broken because of logic errors. Calls to methods of the Trace class similarly require the TRACE constant, which, however, is defined by default for both debug and release builds in Visual Studio projects—trace assertions can thus be used for various logging purposes in production code.

using System.Diagnostics; // Debug and Trace are in this namespace.

static class Program
{
    static void Main()
    {
        int a = 0;

        Console.WriteLine("Before");

        // Always hit.
        Trace.Assert(a == 42, "Trace assertion failed");

        Console.WriteLine("After Trace.Assert");

        // Only hit in debug builds.        
        Debug.Assert(a == 42, "Debug assertion failed");

        Console.WriteLine("After Debug.Assert");
    }
}

Imports System.Diagnostics
' Note: VB Visual Studio projects have System.Diagnostics imported by default,
' along with several other namespaces.

Module Program
    Sub Main()
        Dim a As Integer = 0

        Console.WriteLine("Before")

        ' Always hit.
        Trace.Assert(a = 42, "Trace assertion failed: The Answer was incorrect")

        Console.WriteLine("After Trace.Assert")

        ' Only hit in debug builds.        
        Debug.Assert(a = 42, "Debug assertion failed: The Answer was incorrect")

        Console.WriteLine("After Debug.Assert")
    End Sub
End Module

Output (for .NET Core debug builds when outside of a debugger):

Before
After Trace.Assert
Assertion Failed
Debug assertion failed

   at Program.Main() in FILENAME:line 21

Output:

In .NET Core applications, this is the output

when a debugger is attached and is used to continue past both assertions when they fail, or
in release builds of the program, where the call to Debug.Assert is removed and the Trace.Assert assertion is hit but has no visible effects.

In .NET Framework applications, assertions never show up in the console and so the output is this when a debugger or the "Ignore" option used to continue past the assertions.

Before
After Trace.Assert
After Debug.Assert

Displaying Trace assertions in console:

To "see" the Trace.Assert assertion, additional TraceListener instances must be subscribed by the program. In the .NET Framework, there are several built-in subclasses of TraceListener, including ConsoleTraceListener, which writes trace messages to the console. In .NET Core, these classes are available starting from .NET Core 3.0.

Subscribing an instance involves adding the following line to the beginning of Main() (with a semicolon in C#, of course ;)

Trace.Listeners.Add(new ConsoleTraceListener())

C++

Translation of: C

#include <cassert> // assert.h also works

int main()
{
  int a;
  // ... input or change a here

  assert(a == 42); // Aborts program if a is not 42, unless the NDEBUG macro was defined
                    // when including <cassert>, in which case it has no effect
}

Note that assert does not get a std:: prefix because it's a macro.

Clojure

(let [i 42]
     (assert (= i 42)))

Common Lisp

(let ((x 42))
  (assert (and (integerp x) (= 42 x)) (x)))

Component Pascal

Works with BlackBox Component Builder

MODULE Assertions;
VAR 
	x: INTEGER;
PROCEDURE DoIt*;
BEGIN 
	x := 41;
	ASSERT(x = 42);
END DoIt;
END Assertions.

Assertions.DoIt

Output:

TRAP 0

 Assertions.DoIt   [0000001DH] 
 Kernel.Call   [00001A7CH] 
	.adr	INTEGER	1685454913
	.kind	INTEGER	0
	.n	INTEGER	0
	.p	INTEGER	0
	.par	ARRAY 256 OF INTEGER	elements
	.r	REAL	8.70603013185328E+175
	.sig	POINTER	[64760018H]
	.size	INTEGER	2287288
	.sp	INTEGER	256
	.typ	POINTER	NIL
 Meta.Item.ParamCallVal   [00002B5EH] 
	.adr	INTEGER	1685454913
	.data	ARRAY 256 OF INTEGER	elements

Crystal

Crystal doesn't have an assert statement. the spec module provides a testing DSL, but a simple assert can be created with a function or macro.

class AssertionError < Exception
end

def assert(predicate : Bool, msg = "The asserted condition was false")
  raise AssertionError.new(msg) unless predicate
end

assert(12 == 42, "It appears that 12 doesn't equal 42")

D

import std.exception: enforce;

int foo(in bool condition) pure nothrow
in {
    // Assertions are used in contract programming.
    assert(condition);
} out(result) {
    assert(result > 0);
} body {
    if (condition)
        return 42;

    // assert(false) is never stripped from the code, it generates an
    // error in debug builds, and it becomes a HALT instruction in
    // -release mode.
    //
    // It's used as a mark by the D type system. If you remove this
    // line the compiles gives an error:
    //
    // Error: function assertions.foo no return exp;
    //   or assert(0); at end of function
    assert(false, "This can't happen.");
}

void main() pure {
    int x = foo(true);

    // A regular assertion, it throws an error.
    // Use -release to disable it.
    // It can be used in nothrow functions.
    assert(x == 42, "x is not 42");

    // This throws an exception and it can't be disabled.
    // There are some different versions of this lazy function.
    enforce(x == 42, "x is not 42");
}

Dart

Assert

main() {
  var i = 42;
  assert( i == 42 );
}

Expect

Testing assertions can be done using the test and expect functions in the test package

import 'package:test/test.dart';

main() {
  int i=42;
  int j=41;

  test('i equals 42?', (){
    expect( i, equals(42) );
  });

  test('j equals 42?', (){
    expect( j, equals(42) );
  });
}

Delphi

Assert(a = 42);

If an assertion fails, EAssertionFailed exception is raised.

The generation of assertion code can be disabled by compiler directive

{$ASSERTIONS OFF}

Here is a simple console demo app which raises and handles assertion exception:

program TestAssert;

{$APPTYPE CONSOLE}

{.$ASSERTIONS OFF}   // remove '.' to disable assertions

uses
  SysUtils;

var
  a: Integer;

begin
  try
    Assert(a = 42);
  except
    on E:Exception do
      Writeln(E.Classname, ': ', E.Message);
  end;
  Readln;
end.

DWScript

Simple assertion, with a custom (optional) message

Assert(a = 42, 'Not 42!');

Other specialized assertions can be used in contracts, for instance this function check that the parameter (passed by reference ofr the purpose of illustration) is 42 when entering the function and when leaving the function

procedure UniversalAnswer(var a : Integer);
require
   a = 42;
begin
   // code here
ensure
   a = 42;
end;

Dyalect

Dyalect has a built-in "assert" function:

var x = 42
assert(42, x)

This function throws an exception if assertion fails.

E

E does not have the specific feature of assertions which may be disabled by a global option. But it does have a utility to throw an exception if a condition is false:

require(a == 42)          # default message, "Required condition failed"

require(a == 42, "The Answer is Wrong.")   # supplied message

require(a == 42, fn { `Off by ${a - 42}.` })   # computed only on failure

EchoLisp

(assert (integer? 42)) → #t ;; success returns true

;; error and return to top level if not true;
(assert (integer? 'quarante-deux))
⛔ error: assert : assertion failed : (#integer? 'quarante-deux)

;; assertion with message (optional)
(assert (integer? 'quarante-deux) "☝️ expression must evaluate to the integer 42")
💥 error: ☝️ expression must evaluate to the integer 42 : assertion failed : (#integer? 'quarante-deux)

ECL

ASSERT(a = 42,'A is not 42!',FAIL);

Ecstasy

Ecstasy assertions raise an exception on failure. The default assert statement raises an IllegalState, but there are a few varieties:

statement	exception class
`assert`	`IllegalState`
`assert:arg`	`IllegalArgument`
`assert:bounds`	`OutOfBounds`
`assert:TODO`	`NotImplemented`

The above assertions are always evaluated when they are encountered in the code; in other words, assertions are always enabled. There are three additional forms that allow developers to alter this behavior:

statement	exception class
`assert:test`	Evaluate when in "test" mode, but never evaluate in production mode
`assert:once`	Evaluate the assertion only the first time it is encountered
`assert:rnd(`n`)`	Statistically sample, such that the assertion is evaluated 1 out of every `n` times that the assertion is encountered

This example will always evalaute (and fail) the assertion:

module test {
    void run() {
        Int x = 7;
        assert x == 42;
    }
}

Note that the text of the assertion expression and the relevant values are both included in the exception message:

Output:

x$ xec test

2024-04-24 17:29:23.427 Service "test" (id=1) at ^test (CallLaterRequest: native), fiber 4: Unhandled exception: IllegalState: "x == 42": x=7
	at run() (test.x:4)
	at ^test (CallLaterRequest: native)

Eiffel

Works with: SmartEiffel

version 2.4

There are many assertion types in Eiffel, one is the following:

File called main.e:

class MAIN
    creation main
    feature main is
        local
            test: TEST;
        do
            create test;

            io.read_integer;
            test.assert(io.last_integer);
        end
end

Another file called test.e:

class TEST
    feature assert(val: INTEGER) is
        require
            val = 42;
        do
            print("Thanks for the 42!%N");
        end
end

Elixir

ExUnit.start

defmodule AssertionTest do
  use ExUnit.Case
  
  def return_5, do: 5
  
  test "not equal" do
    assert 42 == return_5
  end
end

Output:

  1) test not equal (AssertionTest)
     test.exs:8
     Assertion with == failed
     code: 42 == return_5
     lhs:  42
     rhs:  5
     stacktrace:
       test.exs:9: (test)



Finished in 0.1 seconds (0.1s on load, 0.01s on tests)
1 test, 1 failure

Randomized with seed 869000

Emacs Lisp

(require 'cl-lib)
(let ((x 41))
  (cl-assert (= x 42) t "This shouldn't happen"))

Erlang

Erlang doesn't have an assert statement. However, it is single assignment, and its assignment operator won't complain if you reassign the exact same value to an existing variable but will throw an exception otherwise.

1> N = 42.
42
2> N = 43.
** exception error: no match of right hand side value 43
3> N = 42.
42
4> 44 = N.
** exception error: no match of right hand side value 42
5> 42 = N.
42

As such, the behavior of Erlang's assignment operator is extremely similar to a regular assert in other languages.

Euphoria

type fourty_two(integer i)
    return i = 42
end type

fourty_two i

i = 41 -- type-check failure

F#

F# provides an assert function that is only enabled when the program is compiled with DEBUG defined. When an assertion fails, a dialog box is shown with the option to enter the debugger.

let test x =
  assert (x = 42)

test 43

For additional information about assertions in .NET, see #C# and Visual Basic .NET

Factor

Throw an exception if the value on the top of the stack is not equal to 42:

USING: kernel ;
42 assert=

FBSL

One needs to DECLARE the asserter variable at the top of script.

This implementation evaluates the expression given to the function and displays a message if it evaluates to false.

#APPTYPE CONSOLE

DECLARE asserter

FUNCTION Assert(expression)
    DIM cmd AS STRING = "DIM asserter AS INTEGER = (" & expression & ")"
    EXECLINE(cmd, 1)
    IF asserter = 0 THEN PRINT "Assertion: ", expression, " failed"
END FUNCTION

Assert("1<2")
Assert("1>2")

PAUSE

Output

Assertion: 1>2 failed

Press any key to continue...

Forth

variable a
: assert   a @ 42 <> throw ;

41 a ! assert

FreeBASIC

' FB 1.05.0 Win64
' requires compilation with -g switch

Dim a As Integer = 5
Assert(a = 6)
'The rest of the code will not be executed
Print a
Sleep

Output:

assert.bas(5): assertion failed at __FB_MAINPROC__: a =6

GAP

# See section 7.5 of reference manual

# GAP has assertions levels. An assertion is tested if its level
# is less then the global level.

# Set global level
SetAssertionLevel(10);

a := 1;
Assert(20, a > 1, "a should be greater than one");
# nothing happens

a := 1;
Assert(4, a > 1, "a should be greater than one");
# error

# Show current global level
AssertionLevel();
# 10

Go

Assertions are a feature consciously omitted from Go. For cases where you want feedback during development, the following code should provide a similar purpose. While it is simply an if statement and a panic, the technique does have some properties typical of assertions. For one, the predicate of an if statement in Go is required to be of boolean type. Specifically, ints are not tacitly tested for zero, pointers are not tested for nil: the expression must be boolean, as the WP article mentions is typical of assertions. Also, it provides a good amount of information should the predicate evaluate to true. First, a value of any type can be passed to the panic, and by default is displayed, followed by a stack trace which includes the location of the panic in the source code—function name, file name, and line number.

package main

func main() {
    x := 43
    if x != 42 {
        panic(42)
    }
}

Output:

panic: 42

panic PC=0x2b772d1a1048
runtime.panic+0xa7 /pool/go/src/pkg/runtime/proc.c:1032
        runtime.panic(0x40e820, 0x2a)
main.main+0x48 /pool/test.go:8
        main.main()
runtime.mainstart+0xf /pool/go/src/pkg/runtime/amd64/asm.s:77
        runtime.mainstart()
runtime.goexit /pool/go/src/pkg/runtime/proc.c:148
        runtime.goexit()

Groovy

def checkTheAnswer = {
   assert it == 42 : "This: " + it + " is not the answer!"
}

Test program:

println "before 42..."
checkTheAnswer(42)
println "before 'Hello Universe'..."
checkTheAnswer("Hello Universe")

Output:

before 42...
before 'Hello Universe'...
java.lang.AssertionError: This: Hello Universe is not the answer!. Expression: (it == 42). Values: it = Hello Universe
	at ConsoleScript80$_run_closure1.doCall(ConsoleScript80:2)
	at ConsoleScript80.run(ConsoleScript80:8)

Haskell

import Control.Exception

main = let a = someValue in
         assert (a == 42) -- throws AssertionFailed when a is not 42
                somethingElse -- what to return when a is 42

Icon and Unicon

...
runerr(n,( expression ,"Assertion/error - message."))  # Throw (and possibly trap) an error number n if expression succeeds. 
...
stop(( expression ,"Assertion/stop - message."))       # Terminate program if expression succeeds.
...

There are no 'assertions', which can be turned on/off by the compiler. We can emulate them by prefixing a stop statement with a check on a global variable:

$define DEBUG 1 # this allows the assertions to go through

procedure check (a)
  if DEBUG then stop (42 = a, " is invalid value for 'a'")
  write (a)
end

procedure main ()
  check (10)
  check (42)
  check (12)
end

This produces the output:

10
42 is invalid value for 'a'

Changing the define to: $define DEBUG &fail turns off the assertion checking.

J

   assert n = 42

Java

In Java, the assert keyword is used to place an assertive statement within the program.
A 'false' assertion will stop the program, as opposed to pausing, as with some other languages.
It's worth noting that assertions were created specifically for the development and debugging of the program, and are not intended to be part of the control-flow.
Some JVM implementations will have assertions disabled by default, so you'll have to enable them at the command line, switch '-ea' or '-enableassertions'.
Inversely, to disable them, use '-da', or '-disableassertions'.
For more information see Oracle - Programming With Assertions.

The assert syntax is as follows.

assert valueA == valueB;

It is essentially a boolean expression, which if not met, will throw an AssertionError exception.
It is effectively shorthand for the following code.

if (valueA != valueB)
    throw new AssertionError();

You can also specify a String with the assertion, which will be used as the exception's detail-message, which is displayed with the stack-trace upon error.

assert valueA == valueB : "valueA is not 42";

Exception in thread "main" java.lang.AssertionError: valueA is not 42
	at Example.main(Example.java:5)

You can also specify any other Object or primitive data type as a message.
If it's an object, the toString method will be used as the message.

assert valueA == valueB : valueA;

JavaScript

function check() {
  try {
    if (isNaN(answer)) throw '$answer is not a number';
    if (answer != 42)  throw '$answer is not 42';
  }
  catch(err) {
    console.log(err);
    answer = 42;
  }
  finally { console.log(answer); }
}

console.count('try'); // 1
let answer;
check();

console.count('try'); // 2
answer = 'fourty two';
check();

console.count('try'); // 3
answer = 23;
check();

Output:

try: 1 $answer is not a number 42

try: 2 $answer is not a number 42

try: 3 $answer is not 42 42

jq

Works with: jq

Works with gojq, the Go implementation of jq

The assertion filters defined here are designed so that assertions intended for development or testing could be left in-place, possibly even in production code.

Highlights

The input to every assertion filter is always passed through without alteration.
If assertion checking is turned on, then an assertion violation

simply results in an error message being emitted on STDERR ("standard error").

Assertions are only checked if the environment variable JQ_ASSERT

is set (see below for how to turn assertion checking on or off).

Pinpointing the occurrence of an assertion violation by using $__loc__ is supported,

provided your implementation of jq includes this function.

The assertion checking filters defined here are as follows:

assert(x)
assert(x; msg)
asserteq(x; y; msg)

where:

msg determines the message that is printed as part of an assertion violation warning; it would typically be specified as a string or the $__loc__ object if your jq supports it.

JQ_ASSERT

Here is a table indicating how the JQ_ASSERT environment variable can be set or unset in various contexts:

 SHELL          SET                    UNSET
 sh bash (etc)  export JQ_ASSERT=1     unset JQ_ASSERT
 fish           set -x JQ_ASSERT 1     set -u JQ_ASSERT
 csh, tcsh      setenv JQ_ASSERT 1     unsetenv JQ_ASSERT

 Windows/DOS    SET JQ_ASSERT=1        set JQ_ASSERT=
 Windows:       Start > Control Panel > System > Advanced > Environment Variables

See also the example below for how to set it on a per-invocation basis.

assert.jq

def assert(exp; $msg):
  def m: $msg | if type == "string" then . else [.[]] | join(":") end;
  if env.JQ_ASSERT then
    (exp as $e | if $e then . else . as $in | "assertion violation @ \(m) => \($e)" | debug | $in end)
  else . end;

def assert(exp):
  if env.JQ_ASSERT then
    (exp as $e | if $e then . else . as $in | "assertion violation: \($e)" | debug | $in end)
   else . end;

def asserteq(x;y;$msg):
  def m: $msg | if type == "string" then . else [.[]] | join(":") end;
  def s: (if $msg then m + ": " else "" end) + "\(x) != \(y)";
  if env.JQ_ASSERT then
     if x == y then .
     else . as $in | "assertion violation @ \(s)" | debug | $in
     end
   else . end;

Example

# File: example.jq
# This example assumes the availability of the $__loc__ function
# and that assert.jq is in the same directory as example.jq.

include "assert" {search: "."};

def test:
  "This is an input"
  | 0 as $x
  | assert($x == 42; $__loc__),
    asserteq($x; 42; $__loc__);

test

Invocation JQ_ASSERT=1 jq -n -f example.jq

In the output, the DEBUG line above appears on stderr.

Output:

["DEBUG:","assertion violation @ <top-level>:16 => false"]
"This is an input"
["DEBUG:","assertion violation @ <top-level>:17: 0 != 42"]
"This is an input"

Julia

const x = 5
 
# @assert macro checks the supplied conditional expression, with the expression
# returned in the failed-assertion message
@assert x == 42
# ERROR: LoadError: AssertionError: x == 42
 
# Julia also has type assertions of the form, x::Type which can be appended to
# variable for type-checking at any point
x::String
# ERROR: LoadError: TypeError: in typeassert, expected String, got Int64

Kotlin

Kotlin supports Jva-style assertions. These need to be enabled using java's -ea option for an AssertionError to be thrown when the condition is false. An assertion should generally never fire, and throws an Error. Errors are seldom used in Kotlin (and much less assertions), as they represent catastrophic issues with the program, such as classes failing to load. These are usually only ever raised by the JVM itself, rather than actual user code.

fun main() {
   val a = 42
   assert(a == 43)
}

Output:

Exception in thread "main" java.lang.AssertionError: Assertion failed
        at AssertKt.main(assert.kt:5)

A more Kotlin idiomatic approach to checks are the require (and requireNotNull) to check arguments, and the check (and checkNotNull), and error to check state. The former is mostly meant for checking input, and will throw IllegalArgumentExceptions, whereas the later is meant for state-checking, and will throw IllegalStateExceptions

fun findName(names: Map<String, String>, firstName: String) {
    require(names.isNotEmpty()) { "Please pass a non-empty names map" } // IllegalArgumentException
    val lastName = requireNotNull(names[name]) { "names is expected to contain name" } // IllegalArgumentException

    val fullName = "$firstName $lastName"
    check(fullName.contains(" ")) { "fullname was expected to have a space...?" } // IllegalStateException
    return fullName
}

Lasso

local(a) = 8
fail_if(
    #a != 42,
    error_code_runtimeAssertion,
    error_msg_runtimeAssertion + ": #a is not 42"
)

Output:

-9945 Runtime assertion: #a is not 42

Liberty BASIC

Liberty BASIC has no exceptions or user-defined error messages, but we could break program if condition is not met. We can even make it spell "AssertionFailed". In a way.

a=42
call assert a=42
print "passed"

a=41
call assert a=42
print "failed (we never get here)"
end

sub assert cond
    if cond=0 then 'simulate error, mentioning "AssertionFailed"
        AssertionFailed(-1)=0
    end if
end sub

Output:

passed

Stops with error message

Output:

RuntimeError: Subscript out of range: -1, AssertionFailed()

Lingo

Lingo has no assert statement, but the abort command (that exits the full call stack) allows to implement something like it as global function:

-- in a movie script
on assert (ok, message)
  if not ok then
    if not voidP(message) then _player.alert(message)
    abort -- exits from current call stack, i.e. also from the caller function
  end if
end

-- anywhere in the code
on test
  x = 42
  assert(x=42, "Assertion 'x=42' failed")
  put "this shows up"
  x = 23
  assert(x=42, "Assertion 'x=42' failed")
  put "this will never show up"
end

Lisaac

? { n = 42 };

Lua

a = 5
assert (a == 42)
assert (a == 42,'\''..a..'\' is not the answer to life, the universe, and everything')

M2000 Interpreter

M2000 use Error to produce error, Try variable {} and Try {} to capture errors, and return a number from a function when function called as module (zero means no error).

Trapping error may leave current stack of values with values so if we have above try {} a block of Stack New {} then we get old stack after exit of Stack New {} (this statement hold current stack, attach a new stack of value, and at the exit restore old stack). Another way is to use Flush which clear stack. Statement Flush Error clear all level of error information.

Making own logging for errors

Module Assert {
      \\ This is a global object named Rec
      Global Group Rec {
      Private:
            document doc$="Error List at "+date$(today)+" "+time$(now)+{
            }
      Public:
            lastfilename$="noname.err"
            Module Error(a$) {
                  if a$="" then exit
                  .doc$<="     "+a$+{
                  }
                  flush error
            }
            Module Reset {
                  Clear .doc$
            }
            Module Display {
                  Report  .doc$
            }
            Module SaveIt {
                  .lastfilename$<=replace$("/", "-","Err"+date$(today)+str$(now, "-nn-mm")+".err")
                  Save.Doc .doc$,.lastfilename$
            }
      }
      Module Checkit {
            Function Error1 (x) {
                  if x<10 then  Print "Normal" : exit 
                  =130   ' error code
            }
            Call Error1(5) 
            Try ok {
                  Call Error1(100)
            } 
            If not Ok then Rec.Error Error$ : Flush Error 
            
            Test "breakpoint A"   ' open Control form, show code as executed, press next or close it
            
            Try {
                  Test
                  Report "Run this"
                  Error "Hello"
                  Report "Not run this"
            }
            Rec.Error Error$
            
            Module Error1 (x) {
                  if x<10 then  Print "Normal" : exit 
                  Error "Big Error"
            }
            Try ok {
                   Error1 100
            }
            If Error then Rec.Error Error$
      }
      Checkit
      Rec.Display
      Rec.SaveIt
      win "notepad.exe", dir$+Rec.lastfilename$     
}
Assert

Using Assert Statement

Assert is a statement from 10th version (the previous example run because we can locally alter a statement using a module with same name).

When we run a program by applying a file gsb as argument the escape off statement applied by default. So asserts didn't work for programs. If we open the M2000 environment and then load a program, then the asserts work (or not if we use escape off). So this example can show x, from print x if we have x=10, but can show 0 if we run it with escape off statement.

// escape off   // using escape off interpreter skip assert statements
x=random(0, 1)*10
try {
	assert x=10
	print x
}

Maple

(Taken from Lua, above.)

a := 5:
ASSERT( a = 42 );
ASSERT( a = 42, "a is not the answer to life, the universe, and everything" );

Mathematica / Wolfram Language

Assert[var===42]

MATLAB / Octave

assert(x == 42,'x = %d, not 42.',x);

Sample Output:

x = 3;
assert(x == 42,'Assertion Failed: x = %d, not 42.',x);
??? Assertion Failed: x = 3, not 42.

Metafont

Metafont has no really an assert built in, but it can easily created:

def assert(expr t) = if not (t): errmessage("assertion failed") fi enddef;

This assert macro uses the errmessage built in to show the "error". The errmessage gives the error message and asks the user what to do.

Usage example:

n := 41;
assert(n=42);
message "ok";

Output (failed assertion):

This is METAFONT, Version 2.71828 (Web2C 7.5.5)
(./assert.mf
! assertion failed.
<to be read again> 
                   ;
l.4 assert(n=42);
                 
?

Modula-3

ASSERT is a pragma, that creates a run-time error if it returns FALSE.

<*ASSERT a = 42*>

Assertions can be ignored in the compiler by using the -a switch.

Nanoquery

a = 5
assert (a = 42)

Nemerle

A basic assertion uses the assert keyword:

assert (foo == 42, $"foo == $foo, not 42.")

Assertion violations throw an AssertionException with the line number where the assertion failed and the message provided as the second parameter to assert.

Nemerle also provides macros in the Nemerle.Assertions namespace to support preconditions, postconditions and class invariants:

using Nemerle.Assertions;

class SampleClass
{
	public SomeMethod (input : list[int]) : int
	  requires input.Length > 0                 // requires keyword indicates precondition, 
                                                    // there can be more than one condition per method
	{ ... }

	public AnotherMethod (input : string) : list[char]
	  ensures value.Length > 0                  // ensures keyword indicates postcondition
	{ ... }                                     // value is a special symbol that indicates the method's return value
}

The design by contract macros throw Nemerle.AssertionException's unless another Exception is specified using the otherwise keyword after the requires/ensures statement. For further details on design by contract macros, see here.

NGS

Assertions in NGS follow the general design of the language: return the original value, use patterns.

In the examples below, wherever 42 is a parameter by itself:

better exception with expected and actual value is produced
it is a pattern, numbers match themselves exactly, other patterns that would match: Int, 0..100, Any

a = 42

assert(a==42)
assert(a, 42)
assert(a==42, "Not 42!")
assert(a, 42, "Not 42!")

Nim

In Nim there are two main ways to check assertions.

var a = 42
assert(a == 42, "Not 42!")

This first kind of assertion may be disabled by compiling with --assertions:off or -d:danger.

var a = 42
doAssert(a == 42, "Not 42!")

This second kind of assertion cannot be disabled.

Nutt

module main

demand 5==42

end

Oberon-2

Oxford Oberon-2

MODULE Assertions;
VAR
	a: INTEGER;
BEGIN
	a := 40;
	ASSERT(a = 42);
END Assertions.

Output:

Runtime error: assertion failed (0) on line 6 in module Assertions
In procedure Assertions.%main
   called from MAIN

Objeck

If variable is not equal to 42 a stack trace is generated and the program is halts.

class Test {
  function : Main(args : String[]) ~ Nil {
    if(args->Size() = 1) {
      a := args[0]->ToInt();
      Runtime->Assert(a = 42);
    };
  }
}

Objective-C

For use within an Objective-C method:

NSAssert(a == 42, @"Error message");

If you want to use formatting arguments, you need to use the assertion macro corresponding to your number of formatting arguments:

NSAssert1(a == 42, @"a is not 42, a is actually %d", a); # has 1 formatting arg, so use NSAssert"1"

Within a regular C function you should use NSCAssert or NSCAssertN instead.

To turn off assertions, define the NS_BLOCK_ASSERTIONS macro.

OCaml

let a = get_some_value () in
  assert (a = 42); (* throws Assert_failure when a is not 42 *)
  (* evaluate stuff to return here when a is 42 *)

It is possible to compile with the parameter -noassert then the compiler won't compile the assertion checks.

Oforth

In Oforth, assertions are handled as tests.

Assertions are checked only if oforth is launched using --a command line. Default value is to not check assertions.

If an assertion is ko (and if oforth is launched using --a), an exception is raised.

: testInteger(n, m)
   assert: [ n isInteger ]
   assert: [ n 42 == ]

   System.Out "Assertions are ok, parameters are : " << n << ", " << m << cr ;

Output:

testInteger(41, 43)
[1:interpreter] ExRuntime : Assertion failed into <#testInteger>

testInteger(42, 43)
Assertions are ok, parameters are : 42, 43

Ol

(define i 24)

(assert i ===> 42)
; or
(assert (= i 42))

Output:

Welcome to Otus Lisp 2.1-2486-481fd919
type ',help' to help, ',quit' to end session.
> (define i 24)
;; Defined i
> (assert i ===> 42)
assertion error:
  i must be 42
> (assert (= i 42))
assertion error: 
  (= i 42) is not a true

Oz

Oz does not have an assert statement. But if different values are assigned to the same dataflow variable, an exception will be thrown (similar to Erlang).

declare
  proc {PrintNumber N}
     N=42  %% assert
     {Show N}
  end
in
  {PrintNumber 42} %% ok
  {PrintNumber 11} %% throws

Output:

%***************************** failure **************************
%**
%** Tell: 11 = 42
%**
%** Call Stack:
%** procedure 'PrintNumber' in file "Oz<8>", line 3, column 0, PC = 18600220
%**--------------------------------------------------------------

PARI/GP

PARI can use any of the usual C methods for making assertions. GP has no built-in assertions.

Translation of: C

#include <assert.h>
#include <pari/pari.h>

void
test()
{
  GEN a;
  // ... input or change a here

  assert(equalis(a, 42)); /* Aborts program if a is not 42, unless the NDEBUG macro was defined */
}

More common is the use of pari_err_BUG in such cases:

if (!equalis(a, 42)) pari_err_BUG("this_function_name (expected a = 42)");

Pascal

See Delphi

PascalABC.NET

Assert procedure works only in Debug mode. In Release all Assert calls are ignored. It is impossible to handle assertion exception.

begin
  var a := 3;
  Assert(a = 3);
  Assert(a = 4, 'Bad assert!');
end.

Perl

While not exactly an assertion, a common Perl idiom is to use or die to throw an exception when a certain statement is false.

print "Give me a number: ";
chomp(my $a = <>);

$a == 42 or die "Error message\n";

# Alternatives
die "Error message\n" unless $a == 42;
die "Error message\n" if not $a == 42;
die "Error message\n" if $a != 42;

This idiom is typically used during file operations:

open my $fh, '<', 'file'
    or die "Cannot open file: $!\n"; # $! contains the error message from the last error

It is not needed whith the "autodie" pragma:

use autodie;
open my $fh, '<', 'file'; # automatically throws an exception on failure

Some third-party modules provide other ways of using assertions in Perl:

use Carp::Assert;
assert($a == 42);

There is also a number of ways to test assertions in test suites, for example:

is $a, 42;
ok $a == 42;
cmp_ok $a, '==', 42, 'The answer should be 42';
# etc.

Phix

User defined types allow the value to be automatically tested whenever it changes, and can be disabled using the "without type_check" directive:

type int42(object i)
    return i=42
end type
 
int42 i
 
i = 41 -- type-check failure (desktop/Phix only)

When a type check occurs, program execution halts and if the program was run from the editor, it automatically jumps to the offending source file and line.

Note that, under "without type_check", the run-time reserves the right to continue to perform limited type checking, for example were the type declared as int42(integer i) then ensuring that i is an integer may allow subsequent optimisations to be applied, and therefore, despite the compiler directive, integer() could still be enforced even though "=42" would not.

You can also use constants to reduce code output on release versions:

global constant DEBUG = 0  -- (or any other identifier name can be used)
global procedure check(integer flag, string msg) -- see also assert() below
    if DEBUG then
        if not flag then
            IupMessage("check failed",msg) 	-- or
            puts(1,msg)                     -- and/or
            crash(msg)      -- crash/ex.err report, or
            trace(1)        -- start debugging
        end if
    end if
end function

check(i=42,"i is not 42!!")

Note that while the body of check() and the call to it are suppressed, the calculation of the expression (i=42) may still generate code; sometimes further improvements to the compiler may be possible, sometimes the calls may need "if DEBUG" around them. Also note that, as things stand, the constants 42 and "i is not 42!!" will be created in the executable file whatever DEBUG is set to, though again there is nothing to prevent the compiler from being enhanced to avoid emitting such unnecessary values, one day.

Lastly, I find the following trivial idioms to be spectacularly effective in Phix, the first line terminates with a divide by zero, whereas the second and third produce a slightly more user-friendly, and therefore potentially less developer-friendly message:

if i!=42 then ?9/0 end if
if i!=42 then crash("i is not 42!!") end if
assert(i=42,"i is not 42!!")

The assert statment is really just shorthand for the crash statement above it, except that the error message is optional (defaults to "assertion failure"). Again, if the application was run from Edita/Edix, on error it automatically jumps to the offending file and line.

PHP

<?php
$a = 5
#...input or change $a here
assert($a == 42) # when $a is not 42, take appropriate actions,
                 # which is set by assert_options()
?>

Picat

Picat does not have a built-in assertion feature; a simple implementation is shown below. Note that predicates and functions must be handled differently:

predicates: using call/n
functions: using apply/n

The predicate/function that is tested but be "escaped" by $ in order to not be evaluated before the test.

go =>

  %
  % Test predicates
  % 
  S = "ablewasiereisawelba",
  assert("test1",$is_palindrome(S)),
  assert_failure("test2",$not is_palindrome(S)),
  
  assert("test3",$member(1,1..10)), 
  assert_failure("test4",$member(1,1..10)), % bad test
  nl,

  % 
  % Test functions
  % 
  assert("test5",$2+2,4),
  assert_failure("test6",$2+2, 5),

  assert("test7",$to_lowercase("PICAT"),"picat"),
  assert_failure("test8",$sort([3,2,1]),[1,3,2]),
  nl.

is_palindrome(S) => 
  S == S.reverse().

% Test a predicate with call/1 
assert(Name, A) =>
  println(Name ++ ": " ++ cond(call(A), "ok", "not ok")).
assert_failure(Name, A) => 
  println(Name ++ ": " ++ cond(not call(A), "ok", "not ok")).

% Test a function with apply/1
assert(Name, A, Expected) => 
  println(Name ++ ": " ++ cond(apply(A) == Expected, "ok", "not ok")).
assert_failure(Name, A, Expected) => 
  println(Name ++ ": " ++ cond(apply(A) != Expected , "ok", "not ok")).

Output:

test1: ok
test2: ok
test3: ok
test4: not ok

test5: ok
test6: ok
test7: ok
test8: ok

PicoLisp

The 'assert' function, in combination with the tilde read macro, generates code only in debug mode:

...
~(assert (= N 42))  # Exists only in debug mode
...

Other possibilities are either to break into an error handler:

(let N 41
   (unless (= N 42) (quit "Incorrect N" N)) )  # 'quit' throws an error
41 -- Incorrect N
?

or to stop at a debug break point, allowing to continue with the program:

(let N 41
   (unless (= N 42) (! setq N 42)) )   # '!' is a breakpoint
(setq N 42)                            # Manually fix the value
!                                      # Hit ENTER to leave the breakpoint
-> 42

PL/I

/* PL/I does not have an assert function as such, */
/* but it is something that can be implemented in */
/* any of several ways.  A straight-forward way   */
/* raises a user-defined interrupt. */

on condition (assert_failure) snap
   put skip list ('Assert failure');
....
if a ^= b then signal condition(assert_failure);

/* Another way is to use the preprocessor, thus: */
%assert: procedure (a, b) returns (character);
   return ('if ' || a || '^=' || b ||
      ' then signal condition(assert_failure);');
%end assert;
%activate assert;

assert(a, 42);

Prolog

Works with: SWI Prolog

test(A):-
    assertion(A==42).

PureBasic

PureBasic does not have a native function for assertion, but allows for the definition of one.

The Macro below will only be included in the code if is compiled in debug mode, if so it will test the condition and if it fails it will inform with the message defined by the programmer, the line where it happened and in which source code file.

Macro Assert(TEST,MSG="Assert: ")
  CompilerIf #PB_Compiler_Debugger
    If Not (TEST)
      Debug MSG+" Line="+Str(#PB_Compiler_Line)+" in "+#PB_Compiler_File
      CallDebugger
    EndIf
  CompilerEndIf
EndMacro

A implementation as defined above could be;

A=42
Assert(A=42,"Assert that A=42")
A=42-1
Assert(A=42)

Where the second test would fail resulting in a message to the programmer with cause (if given by programmer), code line & file.

Python

a = 5
#...input or change a here
assert a == 42 # throws an AssertionError when a is not 42
assert a == 42, "Error message" # throws an AssertionError
       # when a is not 42 with "Error message" for the message
       # the error message can be any expression

It is possible to turn off assertions by running Python with the -O (optimizations) flag.

QB64

$ASSERTS:CONSOLE
 
DO
    a = INT(RND * 10)
    b$ = myFunc$(a)
    PRINT a, , b$
    _LIMIT 3
LOOP UNTIL _KEYHIT
 
FUNCTION myFunc$ (value AS SINGLE)
    _ASSERT value > 0, "Value cannot be zero"
    _ASSERT value <= 10, "Value cannot exceed 10"
 
    IF value > 1 THEN plural$ = "s"
    myFunc$ = STRING$(value, "*") + STR$(value) + " star" + plural$ + " :-)"
END FUNCTION

R

stopifnot(a==42)

Racket

Racket has higher-order assertions known as contracts that can protect any values including functions and objects. Contracts are typically applied on the imports or exports of a module.

#lang racket

(define/contract x
  (=/c 42) ; make sure x = 42
  42)

(define/contract f
  (-> number? (or/c 'yes 'no)) ; function contract
  (lambda (x)
    (if (= 42 x) 'yes 'no)))

(f 42)    ; succeeds
(f "foo") ; contract error!

If typical assertion checking (i.e. error unless some boolean condition holds) is needed, that is also possible:

#lang racket

(define x 80)
(unless (= x 42)
  (error "a is not 42")) ; will error

Raku

(formerly Perl 6)

my $a = (1..100).pick;
$a == 42 or die '$a ain\'t 42';

REXX

version 1

/* REXX ***************************************************************
* There's no assert feature in Rexx. That's how I'd implement it
* 10.08.2012 Walter Pachl
**********************************************************************/
x.=42
x.2=11
Do i=1 By 1
  Call assert x.i,42
  End
Exit
assert:
  Parse Arg assert_have,assert_should_have
  If assert_have\==assert_should_have Then Do
    Say 'Assertion fails in line' sigl
    Say 'expected:' assert_should_have
    Say '   found:' assert_have
    Say sourceline(sigl)
    Say 'Look around'
    Trace ?R
    Nop
    Signal Syntax
    End
  Return
Syntax: Say 'program terminated'

Output:

Assertion fails in line 8
expected: 42
   found: 11
  Call assert x.i,42
Look around
    Here I enter Say i
2
    and then I press just enter
program terminated

version 2

Programming note: this simple version will work better when the ASSERT be on one line,
and be without other REXX statements.

The clauses don't require parentheses, but are used here for clarity.

The ASSERT function could be programmed to exit as one of many
possible actions. Here, it just returns to the next REXX statement after the call assert.

/*REXX program implements a simple  ASSERT  function;  the expression can be compound.  */
  a =  1                                         /*assign a value to the   A   variable.*/
  b =  -2                                        /*   "   "   "    "  "    B       "    */
gee =  7.00                                      /*   "   "   "    "  "   GEE      "    */
zee = 26                                         /*   "   "   "    "  "   ZEE      "    */

call assert (a = 1)
call assert (b > 0)
call assert (gee = 7)
call assert (zee = a  &  zee>b)
exit                                             /*stick a fork in it,  we're all done. */
/*──────────────────────────────────────────────────────────────────────────────────────*/
assert: if arg(1)=1  then return;    parse value sourceline(sigl)  with x;  say
        say '===== ASSERT failure in REXX line' sigl", the statement is:";  say '====='  x
        say;       return

output when using the internal defaults:

===== ASSERT failure in REXX line 8, the statement is:
===== call assert (b > 0)


===== ASSERT failure in REXX line 10, the statement is:
===== call assert (zee = a  &  zee>b)

Ring

x = 42
assert( x = 42 )
assert( x = 100 )

RLaB

RLaB does not have a special function to deal with assertions. The following workaround will do the trick:

// test if 'a' is 42, and if not stop the execution of the code and print 
// some error message
if (a != 42)
{
  stop("a is not 42 as expected, therefore I stop until this issue is resolved!");
}

RPL

There is no build-in assertion feature in RPL, but it can be easily programmed

≪ IF SWAP THEN ABORT ELSE DROP END ≫ 'ASSRT' STO  ( condition message -- message )

≪ 43 → a
   ≪ a 42 == "Not good" ASSRT
     "This won't happen"
≫ ≫ EVAL

Output:

1: "Not good"

Ruby

This uses test/unit from the standard library.

require "test/unit/assertions"
include Test::Unit::Assertions

n = 5
begin
  assert_equal(42, n)
rescue Exception => e
  # Ruby 1.8: e is a Test::Unit::AssertionFailedError
  # Ruby 1.9: e is a MiniTest::Assertion
  puts e
end

Output:

<42> expected but was
<5>.

Rust

let x = 42;
assert!(x == 42);
assert_eq!(x, 42);

Sather

class MAIN is
  main is
    i ::= 41;
    assert i = 42; -- fatal
    -- ...
  end;
end;

(The current GNU Sather compiler v1.2.3 I am using to test the code seems to ignore the assertion and no fatal error is raised, despite Sather should, see e.g. here).

Scala

These two are the same thing, and are tagged @elidable(ASSERTION):

assert(a == 42)
assert(a == 42, "a isn't equal to 42")
assume(a == 42)
assume(a == 42, "a isn't equal to 42")

The next one does the same thing as above, but it is not tagged. Often used as a pre-condition checker on class constructors.

require(a == 42)
require(a == 42, "a isn't equal to 42")

This one checks a value and returns it for further use (here shown being printed). It uses assert, which, as explained, gets tagged.

println(a.ensuring(a == 42))
println(a.ensuring(a == 42, "a isn't equal to 42"))
println(a.ensuring(_ == 42))
println(a.ensuring(_ == 42, "a isn't equal to 42"))

Scheme

Works with: Scheme version R

^{6}

RS

Translation of: Common Lisp

(let ((x 42))
  (assert (and (integer? x) (= x 42))))

SETL

assert( n = 42 );

Sidef

var num = pick(0..100);
assert_eq(num, 42);         # dies when "num" is not 42

Output:

assert_eq: 26 == 42 is false at assertions.sf line 2.

Slate

load: 'src/lib/assert.slate'.
define: #n -> 7.
assert: n = 42 &description: 'That is not the Answer.'.

raises an AssertionFailed condition (an Error).

Smalltalk

foo := 41.
...
self assert: (foo == 42).

In TestCase and subclasses, a number of check methods are inherited; among them:

self assert: (... somethingMustEvaluateToTrue.. )
self should:[ some code ] raise: someException "ensures that an exception is raised

Works with: Smalltalk/X

Object also implements assert:; these are evaluated dynamically, but can be disabled via a flag setting. Also the compiler can be instructed to ignore them for production code (which is not normally done; disabled instead by default):

self assert: (... somethingMustEvaluateToTrue.. ) "implemented in Object"

the implementation in Object raises an AssertionFailedError exception, which usually opens a debugger when in the IDE, but can be caught in deployed apps.

SPARK

Works with SPARK GPL 2010

Assertions are analysed statically, before compilation or execution. They can appear in various places:

inline in the code, either

-# check X = 42;

or

-# assert X = 42;

as a precondition on an operation:

procedure P (X : in out Integer);
--# derives X from *;
--# pre  X = 42;

or as a postcondition on an operation:

procedure P (X : in out Integer);
--# derives X from *;
--# post X = 42;

Example:

X := 7;
--# check X = 42;

produces the following output:

H1:    true .
       ->
C1:    false .

which is an unprovable theorem that tells you that there is a guaranteed failure.

Standard ML

Using exceptions:

fun assert cond =
  if cond then () else raise Fail "assert"

val () = assert (x = 42)

Stata

Assertions in Stata are limited to checking a property on the observations of a dataset. See assert in Stata help.

For instance, if a dataset contains two variables x, y, z, one can check if x<y for all data lines for which z>0, with:

assert x<y if z>0

There is another command, confirm, that can be used to check existence and type of program arguments or files. For instance, to check that the file titanium.dta exists:

confirm file titanium.dta

If the file does not exist, an error is thrown with return code 601.

It's also possible to use error to throw an error if some condition is satisfied. However, this command can only print predefined error messages: it takes the error number as an argument. For instance:

if (`n'==42) error 3
* Will print "no dataset in use"

To print a more sensible message, one would do instead:

if (`n'==42) {
	display as error "The correct answer is not 42."
	exit 54
}

Then, if capture is used to trap the error, the return code (here 54) can be retrieved in _rc.

Swift

var a = 5
//...input or change a here
assert(a == 42) // aborts program when a is not 42
assert(a == 42, "Error message") // aborts program
       // when a is not 42 with "Error message" for the message
       // the error message must be a static string

In release mode assertion checks are turned off.

Tcl

Library: Tcllib (Package: control)

package require control

set x 5
control::assert {$x == 42}

Produces the output:

assertion failed: $x == 42

UNIX Shell

Works with: bash

Assertions are not builtin commands, but we can add a function easily.

assert() {
    if test ! $1; then
        [[ $2 ]] && echo "$2" >&2
        exit 1
    fi
}
x=42
assert "$x -eq 42" "that's not the answer"
((x--))
assert "$x -eq 42" "that's not the answer"
echo "won't get here"

Vala

int a = 42;
int b = 33;
assert (a == 42);
assert (b == 42); // will break the program with "assertion failed" error

VBA

Sub test()
    Dim a As Integer
    a = 41
    Debug.Assert a = 42
End Sub

Output:

When run in the development area executing halts and highlights with yellow background the debug.assert line.

VBScript

Definition

sub Assert( boolExpr, strOnFail )
	if not boolExpr then
		Err.Raise vbObjectError + 99999, , strOnFail
	end if
end sub

Invocation

dim i
i = 43
Assert i=42, "There's got to be more to life than this!"

Output

>cscript "C:\foo\assert.vbs"
C:\foo\assert.vbs(3, 3) (null): There's got to be more to life than this!

Visual Basic

VB's Assert only fires when run from within the IDE. When compiled, all Debug lines are ignored.

Debug.Assert i = 42

Visual Basic .NET

See #C# and Visual Basic .NET.

V (Vlang)

fn main(){
    x := 43
    assert x == 43 // Fine
    assert x > 42 // Fine
    assert x == 42 // Fails
}

Output:

../rosetta/assert.v:5: FAIL: fn main.main: assert x == 42
   left value: x = 43
  right value: 42
V panic: Assertion failed...
v hash: e42dc8e
C:~/assert.12166200709334891880.tmp.c:6356: at _v_panic: Backtrace
C:~/assert.12166200709334891880.tmp.c:11581: by main__main
C:~/assert.12166200709334891880.tmp.c:11946: by wmain
0044d2a8 : by ???
0044d40b : by ???
7ffc4cd07034 : by ???

Wren

Wren does not have assertions as such though we can write something similar.

var assertEnabled = true

var assert = Fn.new { |cond|
    if (assertEnabled && !cond) Fiber.abort("Assertion failure")
}

var x = 42
assert.call(x == 42)  // fine
assertEnabled = false
assert.call(x > 42)   // no error
assertEnabled = true
assert.call(x > 42)   // error

Output:

$ wren assertion.wren
Assertion failure
[./assertion line 4] in new(_) block argument
[./assertion line 12] in (script)

Library: Wren-debug

The above module also provides limited support for assertions.

import "./debug" for Debug

var x = 42
Debug.assert("x == 42", 4, x == 42) // fine
Debug.off
Debug.assert("x > 42", 6, x > 42)   // no error
Debug.on
Debug.assert("x > 42", 8, x > 42)   // error

Output:

ASSERTION on line 8 labelled 'x > 42' failed. Aborting fiber.
Assertion failure.
[./debug line 100] in assert(_,_,_)
[./assert line 8] in (script)

XPL0

XPL0 does not have an assert command. The equivalent is usually synthesized something like this.

proc Fatal(Str);        \Display error message and terminate program
char Str;
[\return;                uncomment this if "assertions" are to be disabled
SetVid(3);              \set normal text display if program uses graphics
Text(0, Str);           \display error message
ChOut(0, 7);            \sound the bell
exit 1;                 \terminate the program; pass optional error code to DOS
];

if X#42 then Fatal("X#42");

Yabasic

sub assert(a)
	if not a then
		error "Assertion failed"
	end if
end sub

assert(myVar = 42)

Zig

const assert = @import("std").debug.assert;

pub fn main() void {
    const n: i64 = 43;
    assert(n == 42); // On failure, an `unreachable` is reached
}

Zig's assert gives a stack trace for debugging on failure.

Z80 Assembly

There is no hardware support for error handling, but the programmer can do this the same way they would create any other if statement:

ld a,(&C005)   ;load A from memory (this is an arbitrary memory location designated as the home of our variable)
cp 42
jp nz,ErrorHandler

zkl

n:=42; (n==42) or throw(Exception.AssertionError);
n=41;  (n==42) or throw(Exception.AssertionError("I wanted 42!"));

Output:

Stack trace for VM#1 ():
   Cmd.__constructor addr:38  args(0) reg(1) 
   startup.__constructor addr:2242  args(0) reg(1) ER
   startup.__constructor addr:2178  args(0) reg(22) 
Exception thrown: AssertionError(I wanted 42!)

zonnon

module Assertions;
var
	a: integer;
begin
	a := 40;
	assert(a = 42,100)
end Assertions.