Assertions in design by contract

From Rosetta Code
Assertions in design by contract is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.

According to   Wikipedia;   assertions can function as a form of documentation:   they can describe the state the code expects to find before it runs (its preconditions), and the state the code expects to result in when it is finished running (postconditions);   they can also specify invariants of a class.


Show in the program language of your choice an example of the use of assertions as a form of documentation.

6502 Assembly

The BRK opcode can be used to trap errors. It causes a jump to the IRQ vector, but it skips one extra byte upon returning. That byte can be used to hold your error code.

The hard part is telling the difference between a BRK and a normal interrupt request. While the 6502 technically has a break flag, the way you read it is different from the way you would typically read the flags.

You cannot read the break flag this way:

php  ;NV-BDIZC (N=negative V=overflow B=break D=decimal I=Interrupt Z=Zero C=Carry)
and #%00010000
BNE BreakSet

This is because the processor flags register doesn't actually contain the true status of the break flag. The only way to read the break flag is to read the flags value that was pushed onto the stack by the hardware itself. Fortunately, this is always at the top of the stack just after an interrupt. Unfortunately, we can't read the flags without clobbering at least one of our registers, something we can't afford to do during an interrupt of any kind. So we'll need to account for the registers we're pushing onto the stack when searching for the flags.

tempPC_Lo equ $20 ;an arbitrary zero page address set aside for debugging
tempPC_Hi equ $21 ;this must be one byte higher than the previous address.

LDA #$7F
ADC #$01
BVC continue
byte $02  ;put your desired error code here
;rest of program

PHA          ;push all. 
TSX          ;loads the stack pointer into X.
LDA $0104,X  ;read the break flag. Normally this would be at offset $0101,X, but since we pushed the three registers we had to add 3.
AND #$10
BNE debug    ;if the break flag is set, you got here because of a BRK command.
;else, this was a normal IRQ.

;rest of program

;we need the pushed PC, minus 1.
LDA $0105,X  ;get the low program counter byte
SBC #1
STA tempPC_Lo
LDA $0106,X
SBC #0
STA tempPC_Hi
LDY #0
LDA (tempPC),y  ;get the error code that was stored immediately after the BRK
;now do whatever you want with that info, such as display a relevant error message to the screen etc.

;rest of program

org $FFFA

68000 Assembly

Error handlers are referred to as traps on the 68000, and they can be triggered automatically (such as attempting to divide by zero) and by command (such as for if overflow has occurred.)

Precondition example:

    DIVU D3,D2  ;this will cause a jump to the "divide by zero trap" if D3 = 0.

Postcondition examples:

    ADD.L D4,D5
    TRAPV    ;no overflow is expected, so if it occurs call the relevant trap.
    LSL.W #8,D2  ;shift D2 left 8 bits. 
    bcc continue ;if carry clear, we're good.
        trap 9   ;otherwise call trap 9, which has been defined (in this example only) to handle unexpected carries after a bit shift.

continue:        ;the program resumes normally after this point


Ada 2012 introduced aspect specifications to the language. Aspect specifications may be used to specify characteristics about data types, procedures, functions, and tasks. Frequently used aspect specifications for procedures and functions include the ability to specify preconditions and post-conditions. Aspect specifications are written as part of a procedure or function specification such as:

type Nums_Array is array (Integer range <>) of Integer;

procedure Sort(Arr : in out Nums_Array) with
    Pre => Arr'Length > 1,
    Post => (for all I in Arr'First .. Arr'Last -1 => Arr(I) <= Arr(I + 1));

The precondition above requires the array to be sorted to have more than one data element.

The post condition describes the state of the array after being sorted, stating that the array must consist of only increasing values or duplicate values.

Post conditions can also reference parameter changes made during the operation of the procedure such as the following procedure specifications for an unbounded queue:

   procedure Enqueue (Item : in out Queue; Value : Element_Type) with
      Post => Item.Size = Item'Old.Size + 1;
   procedure Dequeue (Item : in out Queue; Value : out Element_Type) with
      Pre  => not Item.Is_Empty,
      Post => Item.Size = Item'Old.Size - 1;

Since this is an unbounded queue there is no size constraint on the Enqueue procedure. The Dequeue procedure can only function properly if the queue is not empty.

The post-condition for Enqueue simply states that the size of the queue after enqueuing an element is one more than the size of the queue before enqueuing the element. Similarly the post-condition for Dequeue states that after the dequeuing an element the size of the queue is one less than the size of the queue before dequeuing the element.

Type invariants can be specified using aspect clauses such as:

subtype Evens is Integer range 0..Integer'Last with
    Dynamic_Predicate => Evens mod 2 = 0;

type Data is array (Natural range <>) of Integer;
subtype Limits is Data with
    Dynamic_Predicate => (for all I in Limits'First..Limits'Last - 1 => Limits(I) < Limits(I + 1));

type Days is (Mon, Tue, Wed, Thu, Fri, Sat, Sun);
subtype Alternates is Days with
    Static_Predicate => Alternates in Mon | Wed | Fri | Sun;

The Dynamic_Predicate used for subtype Evens above specifies that each value of the subtype must be an even number.

The Dynamic_Predicate used for subtype Limits specifies that the array must contain data in increasing value with no duplicates.

The Static_Predicate used for subtype Alternates specifies a type which is a discontinuous subtype of Days.


Translation of: D

Algol W has assertions. Although pre and post conditions are not built in to the language, assertions can be used to simulate them.

    long real procedure averageOfAbsolutes( integer array values( * )
                                          ; integer value valuesLength
                                          ) ;
        long real av;
        % Pre-condition. %
        assert( valuesLength > 0 );
        for i := 1 until valuesLength do av := av + abs( values( i ) );
        av := av / valuesLength;
        % Post-condition. %
        assert( av >= 0 );
    end averageOfAbsolutes ;
        integer array v( 1 :: 2 );
        v( 1 ) := 1; v( 2 ) := 3;
        r_format := "A"; r_w := 6; r_d := 2; % set output formatting %
        write( averageOfAbsolutes( v, 2 ) )


D has exceptions, errors and asserts. In Phobos there is also an enforce(). D has pre-conditions and post conditions, and struct/class invariants. Class method contracts should work correctly in object oriented code with inheritance.

import std.stdio, std.algorithm, std.math;

double averageOfAbsolutes(in int[] values) pure nothrow @safe @nogc
in {
    // Pre-condition.
    assert(values.length > 0);
} out(result) {
    // Post-condition.
    assert(result >= 0);
} body {
    return!abs.sum / double(values.length);

struct Foo {
    int x;
    void inc() { x++; }
    invariant {
        // Struct invariant.
        assert(x >= 0);

void main() {
    [1, 3].averageOfAbsolutes.writeln;
    Foo f;;


Translation of: D
  acc: INTEGER
  average_of_absolutes (values: ARRAY[INTEGER]): INTEGER
      non_empty_values: values.count > 0
      acc := 0
      values.do_all(agent abs_sum)
      Result := acc // values.count
      non_neg_result: Result >= 0


Fortran offers no formal "assert" protocol, but there would be nothing to stop a programmer devising a subroutine such as
         IF (CONDITION) RETURN      !All is well.
         WRITE (6,*) MESSAGE
         STOP "Oops. Confusion!"
And then scattering through the source file lines such as
      CALL AFFIRM(SSQ.GE.0,"Sum of squares can't be negative.")   !Perhaps two passes should be used.

And this could be combined with the arrangements described in Stack_traces#Fortran to provide further interesting information.

As written, the scheme involves an unconditional invocation of a subroutine with parameters. That overhead would be reduced by something like
      IF (SSQ.LT.0) CALL CROAK("Sum of squares can't be negative.")   !Perhaps two passes should be used.

Some compilers allowed a D in column one to signify that this was a debugging statement, and a compiler option could specify that all such statements were to be treated as comments and not compiled. Probably not a good idea for statements performing checks. The code that is run with intent to produce results should be the same code that you have tested... A variation on this theme involves such debugging output being written to a file, then after modifying and recompiling, the new version's execution proceeds only while it produces the same debugging output. The reference output could be considered a (voluminous) contract, but for this to work a special testing environment is required and is not at all a Fortran standard.


FreeBASIC provides three assertions. The #assert preprocessor directive will cause compilation to halt with an error message if its argument evaluates to zero:

#assert SCREENX >= 320

The macro assert will halt at runtime with an error message if its argument evaluates to zero:

'compile with the -g flag
assert( Pi < 3 )

Finally, assertwarn is like assert but only prints an error message and continues running:

'compile with the -g flag
dim as integer a = 2
assertwarn( a+a=5 )
print "Ha, no."

All three show the line number of the failed assertion and the expression that failed, making these nicely self-documenting.

assert.bas(3): assertion failed at __FB_MAINPROC__: a+a=5
Ha, no.


The Go FAQ states:

Why does Go not have assertions?
Go doesn't provide assertions. They are undeniably convenient, but our experience has been that programmers use them as a crutch to avoid thinking about proper error handling and reporting. Proper error handling means that servers continue operation after non-fatal errors instead of crashing. Proper error reporting means that errors are direct and to the point, saving the programmer from interpreting a large crash trace. Precise errors are particularly important when the programmer seeing the errors is not familiar with the code.
We understand that this is a point of contention. There are many things in the Go language and libraries that differ from modern practices, simply because we feel it's sometimes worth trying a different approach.

The "contract" in "design by contract" should be embodied in the API and error return values (if any) not in assertions that are typically only compiled when debugging.

If someone disagrees and they really want to use an "assert" they can simply roll their own:

func assert(t bool, s string) {
	if !t {
	assert(c == 0, "some text here")

(And if the assert function was defined in a file with build constraints and a stub in a file with the opposite constraint then they could be effectively be enabled/disabled at compile time. That's probably a bad idea.)


J can load scripts expecting any non-assigned noun result to be all 1's. If the file tautology_script.ijs contains

NB. demonstrate properties of arithmetic
'A B C' =: 3 ?@$ 0   NB. A B and C are random floating point numbers in range [0, 1).
((A + B) + C) -: (A + (B + C))  NB. addition associates
(A + B) -: (B + A)              NB. addition commutes
(A * B) -: (B * A)              NB. scalar multiplication commutes
(A * (B + C)) -: ((A * B) + (A * C)) NB. distributive property

we could load it into a session as 0!:3<'tautology_script.ijs' with result of 1, because the expressions match (-:). Were a sentence to fail the result would be 0, as for example replacing multiplication with matrix product and A B and C with square matrices of same size.

In next example the assertion both tests substitute when the script loads and shows how to use substitute. Infinity (_) replaces the zeros in the y argument, and the x argument is the vector zero infinity.

substitute =: 4 : '[&.((y~:{.x)&(#!.({:x)))y'
assert _ 1 1 _ 2 -: 0 _ substitute 0 1 1 0 2

Pre-condition adverbs with example:

Positive =: adverb define
 'non-positive' assert *./ , y > 0
 u y
Integral =: adverb define
 'non-integral' assert (-: <.) y
 u y
display =: smoutput :[:
display_positive_integers =: display Integral Positive

   display 1 3 8
1 3 8
   display_positive_integers 1 3 8
1 3 8
   display 1x1 _1p1 
2.71828 _3.14159
   display_positive_integers 1x1 _1p1 
|non-positive: assert
|   'non-positive'    assert*./,y>0

As a post-condition, and this is contrived because a better definition of exact_factorial would be !@:x: ,

IntegralResult =: adverb define
 RESULT =. u y
 'use extended precision!' assert (<datatype RESULT) e. ;:'extended integer'
 RESULT =. x u y
 'use extended precision!' assert (<datatype RESULT) e. ;:'extended integer'

exact_factorial =: !IntegralResult

   exact_factorial 50x
   exact_factorial 50
|use extended precision!: assert
|   'use extended precision!'    assert(<datatype RESULT)e.;:'extended integer'

One could assert an invariant in quicksort such that following the split the maximum of the small group is less than the minimum of the large group:

assert (>./LEFT) < (<./RIGHT)


The -ea or -enableassertions option must be passed to the VM when running the application for this to work.
Example taken from Perceptron task.

int feedForward(double[] inputs) {
    assert inputs.length == weights.length : "weights and input length mismatch";

    double sum = 0;
    for (int i = 0; i < weights.length; i++) {
        sum += inputs[i] * weights[i];
    return activate(sum);


Library: Jq/assert.jq
Works with: jq

jq does not currently support assertions natively though they (and design by contract) can be simulated as here, using a jq module.

The "assert.jq" module allows assertion checking to be turned on or off; in addition, execution can be continued or terminated after an assertion violation is detected.

In the following, it is assumed the "debug" mode of assertion checking has been used, e.g. via the invocation: jq --arg assert debug

This mode allows execution to continue after an assertion violation has been detected.

include "rc-assert" {search: "."};  # or use the -L command-line option

def averageOfAbsolutes:
  . as $values
  # pre-condition
  | assert(type == "array" and length > 0 and all(type=="number");
           "input to averageOfAbsolutes should be a non-empty array of numbers.")
  | (map(length) | add/length) as $result
  # post-condition
  | assert($result >= 0; 
           $__loc__ + { msg: "Average of absolute values should be non-negative."} )
  | $result;

[1, 3], ["hello"]
| averageOfAbsolutes

Invocation: jq -n --arg assert debug -f assertions.jq

["DEBUG:","assertion violation @ input to averageOfAbsolutes should be a non-empty array of numbers. => false"]


The @assert macro is used for assertions in Julia.

function volumesphere(r)
    @assert(r > 0, "Sphere radius must be positive")
    return π * r^3 * 4.0 / 3.0


// version 1.1.2
// requires -ea JVM option

fun main(args: Array<String>) {
    assert(args.size > 0)  { "At least one command line argument must be passed to the program" }
    println("The following command line arguments have been passed:")
    for (arg in args) println(arg)

When run without passing command line arguments:

Exception in thread "main" java.lang.AssertionError: At least one command line argument must be passed to the program

When run passing 1 and 2 as command line arguments:

The following command line arguments have been passed:


Nim provides two kind of assertions: assertions which can be deactivated by compiling without checks and assertions which cannot be deactivated.

The first kind takes the form: assert boolean_expression with a default message or assert boolean_expression, message when with want a specific message. For the second kind, assert is simply replaced with doAssert.

Assertions may be used anywhere, either as preconditions, post-conditions or invariants.

Here is an example:

import math

func isqrt(n: int): int =
  assert n >= 0, "argument of “isqrt” cannot be negative"

If the assertion is not true, the program terminates in error with the exception AssertionDefect:

Error: unhandled exception: test.nim(2, 10) `n >= 0` argument of “isqrt” cannot be negative [AssertionDefect]

Note also that, in this case, rather than using an assertion, we could have simply specified that “n” must be a natural:

import math

func isqrt(n: Natural): int =

If the argument is negative, we get the following error:

Error: unhandled exception: value out of range: -1 notin 0 .. 9223372036854775807 [RangeDefect]


Translation of: Raku
# 20201201 added Perl programming solution

use strict;
use warnings;

package MessageMultiplier;

use Class::Contract;
use Test::More tests => 2;
use Test::Exception;

contract {

   attr 'multiplier' => 'SCALAR';
   attr 'message'    => 'SCALAR';

   ctor 'new';
      impl { ( ${self->multiplier}, ${self->message} ) = @_ };

   method 'execute';
      pre  { ${self->multiplier} > 1 and length ${self->message} > 0 };
      impl { print ${self->message} x ${self->multiplier} , "\n" };

dies_ok { MessageMultiplier->new(1,'B')->execute };
dies_ok { MessageMultiplier->new(3, '')->execute };
ok 1
ok 2


User defined types can be used to directly implement design by contract, and disabled by "without type_check".

type hour(object x)
    return integer(x) and x>=0 and x<=23
end type
hour h
h = 1   -- fine
h = 26  -- bad (desktop/Phix only)
C:\Program Files (x86)\Phix\test.exw:6
type check failure, h is 26

Type check failures can also be caught and processed just like any other exception.

You can also (since 0.8.2) use standard assert statemnents, eg

integer fn = -1 -- (try also 1)
assert(fn!=-1,"cannot open config file")


The function and it's contract are a "translation" of "D".

The Racket Guide introduces contracts here [1]

The Racket Reference defines contracts here [2]

Note that the examples catch contract blame exceptions -- which, if uncaught are enough to halt a program; making them quite assertive.

This example is extremely surface-scratching.

#lang racket
(require racket/contract)

;; This is the contract we will use.
;; "->"                   It is a function
;; (cons/c real?          That takes a list of at least one real (cons/c x (listof x)) means that an x
;;  (listof real?))       must occur before the rest of the list
;; (or/c zero? positive?) returns a non-negative number (for which there is no simpler contract that I
;;                        know of
(define average-of-absolutes/c
  (-> (cons/c real? (listof real?)) (or/c zero? positive?)))

;; this does what it's meant to
(define/contract (average-of-absolutes num-list)
  (/ (apply + (map abs num-list)) (length num-list)))

;; this will return a non-positive real (which will break the contract)
(define/contract (average-of-absolutes:bad num-list)
  (- (/ (apply + (map abs num-list)) (length num-list))))

(define (show-blame-error blame value message)
  (string-append   "Contract Violation!\n"
                   (format "Guilty Party: ~a\n" (blame-positive blame))
                   (format "Innocent Party: ~a\n" (blame-negative blame))
                   (format "Contracted Value Name: ~a\n" (blame-value blame))
                   (format "Contract Location: ~s\n" (blame-source blame))
                   (format "Contract Name: ~a\n" (blame-contract blame))
                   (format "Offending Value: ~s\n" value)
                   (format "Offense: ~a\n" message)))
(current-blame-format show-blame-error)

(module+ test
  ;; a wrapper to demonstrate blame
  (define-syntax-rule (show-contract-failure body ...)
    (with-handlers [(exn:fail:contract:blame?
                     (lambda (e) (printf "~a~%" (exn-message e))))]
                    (begin body ...)))
  (show-contract-failure (average-of-absolutes '(1 2 3)))
  (show-contract-failure (average-of-absolutes '(-1 -2 3)))
  ;; blame here is assigned to this test script: WE're providing the wrong arguments
  (show-contract-failure (average-of-absolutes 42))
  (show-contract-failure (average-of-absolutes '()))

  ;; blame here is assigned to the function implementation: which is returning a -ve value
  (show-contract-failure (average-of-absolutes:bad '(1 2 3)))
  (show-contract-failure (average-of-absolutes:bad '(-1 -2 3)))
  ;; blame here is assigned to this test script: since WE're providing the wrong arguments, so
  ;; the bad function doesn't have a chance to generate an invalid reply
  (show-contract-failure (average-of-absolutes:bad 42))
  (show-contract-failure (average-of-absolutes:bad '())))
Contract Violation!
Guilty Party: ...\Assertions_in_design_by_contract.rkt
Innocent Party: (function average-of-absolutes)
Contracted Value Name: average-of-absolutes
Contract Location: #(struct:srcloc "...\\Assertions_in_design_by_contract.rkt" 28 18 1037 20)
Contract Name: (-> (cons/c real? (listof real?)) (or/c zero? positive?))
Offending Value: 42
Offense: expected: pair?
  given: 42

Contract Violation!
Guilty Party: ...\Assertions_in_design_by_contract.rkt
Innocent Party: (function average-of-absolutes)
Contracted Value Name: average-of-absolutes
Contract Location: #(struct:srcloc "...\\Assertions_in_design_by_contract.rkt" 28 18 1037 20)
Contract Name: (-> (cons/c real? (listof real?)) (or/c zero? positive?))
Offending Value: ()
Offense: expected: pair?
  given: ()

Contract Violation!
Guilty Party: (function average-of-absolutes:bad)
Innocent Party: ...\Assertions_in_design_by_contract.rkt
Contracted Value Name: average-of-absolutes:bad
Contract Location: ...\\Assertions_in_design_by_contract.rkt" 33 18 1238 24)
Contract Name: (-> (cons/c real? (listof real?)) (or/c zero? positive?))
Offending Value: -2
Offense: promised: (or/c zero? positive?)
  produced: -2

Contract Violation!
Guilty Party: (function average-of-absolutes:bad)
Innocent Party: ...\Assertions_in_design_by_contract.rkt
Contracted Value Name: average-of-absolutes:bad
Contract Location: #(struct:srcloc "...\\Assertions_in_design_by_contract.rkt" 33 18 1238 24)
Contract Name: (-> (cons/c real? (listof real?)) (or/c zero? positive?))
Offending Value: -2
Offense: promised: (or/c zero? positive?)
  produced: -2

Contract Violation!
Guilty Party: ...\Assertions_in_design_by_contract.rkt
Innocent Party: (function average-of-absolutes:bad)
Contracted Value Name: average-of-absolutes:bad
Contract Location: #(struct:srcloc "...\\Assertions_in_design_by_contract.rkt" 33 18 1238 24)
Contract Name: (-> (cons/c real? (listof real?)) (or/c zero? positive?))
Offending Value: 42
Offense: expected: pair?
  given: 42

Contract Violation!
Guilty Party: ...\Assertions_in_design_by_contract.rkt
Innocent Party: (function average-of-absolutes:bad)
Contracted Value Name: average-of-absolutes:bad
Contract Location: #(struct:srcloc "...\\Assertions_in_design_by_contract.rkt" 33 18 1238 24)
Contract Name: (-> (cons/c real? (listof real?)) (or/c zero? positive?))
Offending Value: ()
Offense: expected: pair?
  given: ()


(formerly Perl 6)

Works with: Rakudo version 2018.05

Most of theses entries seem to have missed the point. This isn't about how to implement or use assertions, it is about how the language uses design-by-contract to help the programmer get correct results.

Raku doesn't have an assert routine in CORE. You could easily add one; several modules geared toward unit testing supply various "assertion" routines, though I'm not aware of any actually specifically named "assert()".

Raku core has subroutine signatures, multi-dispatch and exceptions to implement design-by-contract.

Subroutine signature allow the programmer to make the parameters supplied to a routine be checked to make sure they are the of correct quantity, type, and value and can constrain the returned value(s) to a particular type. See the below snippet for a brief demonstration. Raku does some static analysis to trap errors at compile time, but traps many errors at run time since it is often impossible to tell if a variable is of the correct type before the program is run.

When Raku encounters a design-by-contract violation, it will fail with an error message telling where the problem occurred, what it expected and what it actually got. Some failures are trappable and resumeable. Failures may be trapped by adding a CATCH { } block in the current scope. Failures will transfer execution to the CATCH block where the failure may be analysed and further action taken. Depending on the failure type, execution may be resumable. Some failures cause execution to halt unavoidably.

In this snippet, the routine repeat takes one Integer that must be greater than 1, a String and returns a String. (Note that as written, it is incorrect since it actually returns a boolean.)

sub repeat ( Int $repeat where * > 1, Str $message, --> Str ) {
    say $message x $repeat;
    True # wrong return type

repeat( 2, 'A' ); # parameters ok, return type check error

repeat( 4, 2 ); # wrong second parameter type

repeat( 'B', 3 ); # wrong first (and second) parameter type

repeat( 1, 'C' ); # constraint check fail

repeat( ); # wrong number of parameters

    default {
        say "Error trapped: $_";
Error trapped: Type check failed for return value; expected Str but got Bool (Bool::True)
Error trapped: Type check failed in binding to parameter '$message'; expected Str but got Int (2)
Error trapped: Type check failed in binding to parameter '$repeat'; expected Int but got Str ("B")
Error trapped: Constraint type check failed in binding to parameter '$repeat'; expected anonymous constraint to be met but got Int (1)
Error trapped: Too few positionals passed; expected 2 arguments but got 0
This exception is not resumable
  in block  at contract.p6 line 19
  in block <unit> at contract.p6 line 14


This is just a simple method of   assertion;   more informative messages could be added in the   assertion   routine.
A   return   statement could've been used instead of an   exit   statement to continue processing.

/*REXX program demonstrates a  method  on how to use  assertions in design  by contract.*/
parse arg top .                                  /*obtain optional argument from the CL.*/
if top=='' | top==","  then top= 100             /*Not specified?  Then use the default.*/
_= left('', 9)                                   /*_:  contains 9 blanks for SAY passing*/
                        w= length(top) + 1       /*W:  is used for aligning the output. */
      do #=1  for 666                            /*repeat for a devilish number of times*/
      a= random(1, top)                          /*generate a random number  (1 ──► TOP)*/
      b= random(1,   a)                          /*    "    "    "      "    (1 ──►   A)*/
      c= a - b                                   /*compute difference between  A and B. */
      say _'a='right(a, w)_"b="right(b, w)_'c='right(c, w)_"sum="right(sumABC(a, b, c), w)
      call assert  date('Weekday') \== "Monday"  /*shouldn't execute this pgm on Monday.*/
      call assert  time('H')<9  |  time("H")>15  /*    ··· and not during banking hours.*/
      call assert  c>0                           /*The value of   C   must be positive. */
      end   /*#*/
exit 0                                           /*stick a fork in it,  we're all done. */
assert: if arg(1)  then return 1;       say      /*if true,  then assertion has passed. */
        say 'assertion failed on line ' sigL " with: "  subword(space(sourceline(sigl)),2)
        say;   say '# index= '  #
        say 'ASSERT is exiting.';       exit 13
sumABC: procedure; parse arg x,y,z; return x+y+z /*Sum three arguments.  Real easy work.*/
output   when using the default input:
         a=  12         b=   5         c=   7         sum=  24
         a=  88         b=  77         c=  11         sum= 176
         a=  35         b=  26         c=   9         sum=  70
         a=  33         b=   9         c=  24         sum=  66
         a=  90         b=  31         c=  59         sum= 180
         a=  64         b=   9         c=  55         sum= 128
         a=  82         b=  18         c=  64         sum= 164
         a=   6         b=   4         c=   2         sum=  12
         a=  85         b=  72         c=  13         sum= 170
         a=  74         b=  49         c=  25         sum= 148
         a=  31         b=  14         c=  17         sum=  62
         a=  58         b=  48         c=  10         sum= 116
         a=  64         b=  59         c=   5         sum= 128
         a=  53         b=   1         c=  52         sum= 106
         a=  54         b=  22         c=  32         sum= 108
         a=  36         b=   9         c=  27         sum=  72
         a=   2         b=   2         c=   0         sum=   4

assertion failed on line  13  with:  assert c>0 /*The value of C must be positive. */

# index=  17
ASSERT is exiting.


require 'contracts'
include Contracts

Contract Num => Num
def double(x)
  x * 2

puts double("oops")
./contracts.rb:34:in `failure_callback': Contract violation: (RuntimeError)
    Expected: Contracts::Num,
    Actual: "oops"
    Value guarded in: Object::double
    With Contract: Contracts::Num, Contracts::Num
    At: main.rb:6 


Scala provides runtime assertions like Java: they are designed to be used by static analysis tools however the default compiler doesn’t perform such analyses by default. The Scala assertions (assume, require, assert, ensuring) are Predef library methods that are enabled by default and can be disabled using the -Xdisable-assertions runtime flag, unlike Java where assertions are disabled by default and enabled with a runtime flag. It is considered poor form to rely on assertions to validate arguments, because they can be disabled. An appropriate informative runtime exception (e.g. NullPointerException or IllegalArgumentException) should be thrown instead.

object AssertionsInDesignByContract extends App {
     * @param ints a non-empty array of integers
     * @return the mean magnitude of the array's elements.
    def averageOfMagnitudes(ints: Array[Int]) = {
        // assertions for static analysis / runtime protoyping:
        assume(ints != null, "array must not be null")
        require(ints.length > 0, "array must be non-empty")
        // runtime exceptions when assertions are disabled:
        if (ints.length < 1) throw new IllegalArgumentException("Cannot find the average of an empty array")
        // note, the above line can implicitly throw a NullPointerException too
        val abs =
        val mean = Math.round(abs.sum.toDouble / ints.length)
        assert(Math.abs(mean) >= abs.min && Math.abs(mean) <= abs.max, "magnitude must be within range")
    } ensuring(_ >= 0, "result must be non-negative (possible overflow)")

    println(averageOfMagnitudes(Array(1))) // 1
    println(averageOfMagnitudes(Array(1,3))) // 2
    println(averageOfMagnitudes(null)) // java.lang.AssertionError: assumption failed: array must not be null
    println(averageOfMagnitudes(Array())) // java.lang.IllegalArgumentException: requirement failed: array must be non-empty
    println(averageOfMagnitudes(Array(Integer.MAX_VALUE, Integer.MAX_VALUE))) // java.lang.AssertionError: assertion failed: magnitude must be within range
    println(averageOfMagnitudes(Array(Integer.MAX_VALUE, 1))) // java.lang.AssertionError: assertion failed: result must be non-negative (possible overflow)


# Custom assertions; names stolen from Eiffel keywords
proc require {expression message args} {
    if {![uplevel 1 [list expr $expression]]} {
	set msg [uplevel 1 [list format $message] $args]
	return -level 2 -code error "PRECONDITION FAILED: $msg"
proc ensure {expression {message ""} args} {
    if {![uplevel 1 [list expr $expression]]} {
	set msg [uplevel 1 [list format $message] $args]
	return -level 2 -code error "POSTCONDITION FAILED: $msg"

proc connect_to_server {server port} {
    require {$server ne ""} "server address must not be empty"
    require {[string is integer -strict $port]} "port must be numeric"
    require {$port > 0 && $port < 65536} "port must be valid for client"

    set sock [socket $server $port]

    # Will never fail: Tcl *actually* throws an error on connection
    # failure instead, but the principle still holds.
    ensure {$sock ne ""} "a socket should have been created"

    return $sock

This can be usefully mixed with Tcl 8.6's try … finally … built-in command.

Example output from above code:
% connect_to_server "" ""
PRECONDITION FAILED: server address must not be empty
% connect_to_server localhost 123456
PRECONDITION FAILED: port must be valid for client


Translation of: D
Library: Wren-assert

Wren doesn't support assertions natively though they (and design by contract) can be simulated using a library.

import "./assert" for Assert
import "./math" for Nums

// Assert.disabled = true

var averageOfAbsolutes = { |values|
    // pre-condition
    Assert.ok(values.count > 0, "Values list must be non-empty.")

    var result = Nums.mean( { |v| v.abs })
    // post-condition
    Assert.ok(result >= 0, "Average of absolute values should be non-negative.")
    return result

class Foo {
    construct new(x) {
        _x = x

    x { _x }
    x=(v) {
        _x = v

    inc {
        // no need to check invariance here
        _x = _x + 1

    checkInvariant_() {
        Assert.ok(_x >= 0, "Field 'x' must be non-negative.")

System.print([1, 3]))
var f =

With assertions enabled:

Field 'x' must be non-negative.
[./assert line 225] in fail(_)
[./assert line 71] in ok(_,_)
[./assertions_in_DBC line 34] in checkInvariant_()
[./assertions_in_DBC line 19] in init new(_)
[./assertions_in_DBC line 20] in 
[./assertions_in_DBC line 39] in (script)

or with assertions disabled (un-comment line 4):


Z80 Assembly

There are no built-in assertions, error handlers, etc. at the hardware level. The closest you can get is a function conditionally returning early. For example, this function for multiplication checks if the second factor equals zero or 1 before attempting to multiply.

;returns A = C * A
or a       ;compares A to zero
ret z      ;returns with the product A = C * 0 = 0.
dec a      ;sets zero flag if A equaled 1 prior to decrement, effectively comparing A to 1.
jr z, C_Times_One
ld b,a
ld a,c
add c
djnz loop_smallMultiply
ret   ;returns A = C * A

ld a,c
ret  ;returns A = C


zkl has exceptions. The _assert_ keyword just wraps the AssertionError exception. _assert_ takes an expression and optional message. There are no pre/post conditionals.

fcn f(a){
   _assert_((z:=g())==5,"I wanted 5, got "+z)

Running the code throws an exception with file and line number:

VM#1 caught this unhandled exception:
   AssertionError : assert(foo.zkl:14): I wanted 5, got hoho