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Scope/Function names and labels

From Rosetta Code
Task
Scope/Function names and labels
You are encouraged to solve this task according to the task description, using any language you may know.
Task

Explain or demonstrate the levels of visibility of function names and labels within the language.


See also
  • Variables for levels of scope relating to visibility of program variables
  • Scope modifiers for general scope modification facilities



ALGOL 68[edit]

Algol 68 follows the traditional block structure scoping rules introduced by Algol 60.
A label, function (PROC), variable etc. declared in an outer block is only visible within that block and all inner blocks, unless another label, function, etc. is declared with the same name in an inner block.
In addition, functions etc. declared between IF and THEN are in scope in the THEN and ELSE parts, but declarations in the THEN part and not visible in the ELSE part (and vice versa), so the following is invalid:

IF PROC x = ...;
...
THEN
# can call x here #
PROC y = ...;
...
GO TO l1 # invalid!! #
ELSE
# can call x here, but not y #
...
l1: ...
FI

Similarly, declarations between WHILE and DO are in scope between DO and OD and those declared between CASE and IN are visible in the IN and OUT parts of a CASE.
Note that labels cannot be defined between IF and THEN, between WHILE and DO and between CASE and IN.

AWK[edit]

In awk, function names are always global and can be referenced in sections of code appearing before the definition:

# This program outputs a greeting
BEGIN {
sayhello() # Call the function defined below
exit
}
 
function sayhello {
print "Hello World!" # Outputs a message to the terminal
}

Note that the awk extraction and reporting language is data driven and does not support arbitary line labels.

Axe[edit]

Function names (which are also labels) are global in Axe. They can be accessed from anywhere in any source file that gets compiled together.

BASIC[edit]

Line numbers are used instead of labels. These are also immediately accessible as soon as the line is entered, even if the program is not run:

GOTO 50: REM THIS WILL WORK IMMEDIATELY

The visibility of functions depends on the implementation. Most versions of basic will allow a function to be referenced from a point in the code prior to its definition. However, other implementations may require the function definition to be run, before it can be used:

10 DEF FN S(A)=A*A
20 PRINT FN S(2): REM THIS WILL WORK
30 PRINT FN C(2): REM CALLING A FUNCTION PRIOR TO DEFINITION MAY NOT WORK
40 GOSUB 9000
50 PRINT FN C(2): REM THIS WILL WORK
60 END
9000 DEF FN C(A)=A*A*A
9999 RETURN

bc[edit]

Functions have global scope and must be defined before usage. A defined function can be replaced with a newer definition later in the file. Recursive calls are possible because function are considered as defined within their own body.

There are no labels in bc.

f(1)              /* First output line */ 
define f(x) {
return(x)
}
f(3) /* Second output line */
 
define f(x) {
return(x - 1)
}
f(3) /* Third output line */
Output:
Runtime error (func=(main), adr=3): Function f not defined.
3
2

C[edit]

Demonstrating function scope as well as goto in a C program invariably leads to code like the one below. The Wikipedia article is a good starting point.

/* Abhishek Ghosh, 8th November 2013, Rotterdam */
#include <stdio.h>
 
#define sqr(x) ((x) * (x))
#define greet printf("Hello There!\n")
 
int twice(int x)
{
return 2 * x;
}
 
int main(void)
{
int x;
 
printf("This will demonstrate function and label scopes.\n");
printf("All output is happening through printf(), a function declared in the header stdio.h, which is external to this program.\n");
printf("Enter a number: ");
if (scanf("%d", &x) != 1)
return 0;
 
switch (x % 2) {
default:
printf("Case labels in switch statements have scope local to the switch block.\n");
case 0:
printf("You entered an even number.\n");
printf("Its square is %d, which was computed by a macro. It has global scope within the translation unit.\n", sqr(x));
break;
case 1:
printf("You entered an odd number.\n");
goto sayhello;
jumpin:
printf("2 times %d is %d, which was computed by a function defined in this file. It has global scope within the translation unit.\n", x, twice(x));
printf("Since you jumped in, you will now be greeted, again!\n");
sayhello:
greet;
if (x == -1)
goto scram;
break;
}
 
printf("We now come to goto, it's extremely powerful but it's also prone to misuse. Its use is discouraged and it wasn't even adopted by Java and later languages.\n");
 
if (x != -1) {
x = -1; /* To break goto infinite loop. */
goto jumpin;
}
 
scram:
printf("If you are trying to figure out what happened, you now understand goto.\n");
return 0;
}
Output:
Example run:
This will demonstrate function and label scopes.
All output is happening throung printf(), a function declared in the header stdio.h, which is external to this program.
Enter a number: 5

You entered an odd number.
Hello There!
We now come to goto, it's extremely powerful but it's also prone to misuse. Its use is discouraged and it wasn't even adopted by Java and later languages.
2 times -1 is -2, which was computed by a function defined in this file. It has global scope within the translation unit.
Since you jumped in, you will now be greeted, again!
Hello There!
If you are trying to figure out what happened, you now understand goto.

Eiffel[edit]

Functions (routines which return a result), procedures (routines which have no result) and variables are considered features of a class and follow the same visibility rules. Eiffel allows for information hiding, where features can be selectively hidden from particular classes.

All features are assigned (at compile-time) to a particular scope defined by the most-recently preceding feature clause. Various feature clauses are given below, from least restrictive (most visible) to most restrictive (most hidden).

--assume A, B and C to be valid classes
class X
feature -- alias for "feature {ANY}"
-- ANY is the class at the top of the class hierarchy and all classes inherit from it
-- features following this clause are given "global" scope: these features are visible to every class
 
feature {A, B, C, X}
-- features following this clause are only visible to the specified classes (and their descendants)
-- classes not in this set do not even know of the existence of these features
 
feature {A, B, C}
-- similar to above, except other instances of X cannot access these features
 
feature {X}
-- features following this clause are only visible to instances of X (and its descendants)
 
feature {NONE}
-- NONE is the class at the bottom of the class hierarchy and inherits from every class
-- features following this clause are only visible to this particular instance of X
 
end


Erlang[edit]

The scope of an Erlang function is limited to the module where it is declared. It is possible to modify function scope by exporting a function from a module. There are no labels.

 
-module( a_module ).
 
-export( [exported_function/0] ).
 
exported_function() -> 1 + local_function().
 
local_function() -> 2.
 

Go[edit]

Go is block scoped and has both functions and labels.

Functions can only be declared at the "top level" in a source file, that is, not nested or declared inside of anything else. This gives them "package scope," making them visible within a package (which can consist of multiple source files.) A They are not visible outside their package unless "exported." They are exported if the function name begins with an upper case letter, specifically Unicode class "Lu."

A "function literal" is different than a declaration. It is an expression that returns a function value, which can be assigned or passed around like any other value.

A function definition, either a top-level declaration or a function literal, represents a function block.

package main
 
import (
"fmt"
"runtime"
 
"ex"
)
 
func main() {
// func nested() { ... not allowed here
 
// this is okay, variable f declared and assigned a function literal.
f := func() {
// this mess prints the name of the function to show that it's an
// anonymous function defined in package main
pc, _, _, _ := runtime.Caller(0)
fmt.Println(runtime.FuncForPC(pc).Name(), "here!")
}
 
ex.X(f) // function value passed to exported function
 
// ex.x() non-exported function not visible here
}
package ex
 
import (
"fmt"
"runtime"
)
 
// X is exported.
func X(x func()) {
pc, _, _, _ := runtime.Caller(0)
fmt.Println(runtime.FuncForPC(pc).Name(), "calling argument x...")
x()
}
 
func x() { // not exported, x not upper case.
panic("top level x")
}
Output:
ex.X calling argument x...
main.func·001 here!

Labels can only be declared in function blocks. The scope is the function block where the label is declared excluding any nested function blocks.

package main
 
import "fmt"
 
func main() {
// labels loop and y both in scope of main
loop:
for false {
continue loop
}
goto y
y:
y := 0 // variable namespace is separate from label namespace
 
func() {
// goto loop ...loop not visible from this literal
 
// label y in outer scope not visible so it's okay to define a label y
// here too.
y:
for {
break y
}
y++ // regular lexical scoping applies to variables.
}()
fmt.Println(y)
}
 
// end: // labels not allowed outside function blocks
Output:
1

haskell[edit]

Functions are considered global in haskell and can be referenced in the sections of code appearing before the function definition. The following code illustrates the same. add2 was declared after add3 and used in add3. The variables x,y and z are local to the function.

 
add3 :: Int -> Int-> Int-> Int
add3 x y z = add2 x y + z
 
add2 :: Int -> Int -> Int
add2 x y = x + y
 
main :: putStrLn(show (add3 5 6 5))
 
Output:
ghci>main
16

In the function below the functions g and h are local to the function getSquaredSum. They cannot be called from anywhere outside the function.

 
getSquaredSum :: Int-> Int-> Int
getSquaredSum x y = g x + h y
where
g a = a*a
h b = b*b
 
Output:
 
ghci> getSquaredSum 3 4
25
ghci>h 4
<interactive>: 115:1:error: 
Variable not in scope: h :: Integer -> t

Icon and Unicon[edit]

In both languages, function names (including procedure names - functions are language builtins while procedures are written in the language) have global scope. In Unicon, class methods are locally visible within the class but must be referenced through the class instance externally.

There are no labels in either language.

J[edit]

NAMES

J is scoped lexically but with non-nested block scope. Nesting can be emulated, where that is required. But deep nesting can be difficult to understand and debug so the coder is penalized (with a bit of extra work) for implementing deep nesting.

Specifically, J provides two scopes for user defined names:

Local scope, names defined using =. will have local scope if a block scope exists.

   a=. 1

Locale scope, names defined using =: will have locale scope (and the base locale is used by default).

   b=: 2

Names may include a locale qualifier. A locative is a qualified name. Locale qualifiers contain two _ characters, are a suffix on what would be the unqualified name. Locale qualifiers come in two forms: absolute and relative. Relative locale qualifiers use another name in the current locale to identify the target locale. Absolute locatives place the locale name between the two _ characters, while relative locatives name the locale reference following the pair of _ characters.

   c_thingy_=: 3
d=: <'test'
e__d=: 4
b + e_test_
6

If a local definition exists for a name, it is an error to use use =: to assign that name (use a locative instead of the unqualified name if you really need to do this).

verb define ''
f=. 6
g=: 7
g=. 8
g=: 9
)
|domain error
| g =:9

Meanwhile, there is a global current locale (default: base) but each verb's definition is evaluated with the current locale being the locale where that verb was defined.

Name resolution first checks for local definitions for a name, and if none are found the current locale is checked, and if nothing is found there the current locale's search path is used to check other locales. By default the z locale is included as the last element of this path. (And, since the z locale contains the system defined words, removing it from a locale's path will break most things in that locale.)

The current locale during the execution of a verb is the locale where that verb was defined. But the current locale can be changed when the current named verb finishes using 18!:4.

If you want to emulate static nested scope (or dynamic scope) you need to arrange for the desired behavior using the locale path mechanism.

LABELS

Labels are available in explicit definitions, and are names beginning with label_ and ending with a .. You may use them with a goto which is a name beginning with goto_ and ending with a .. Use of labels is restricted to the scope where they are defined, and they cannot be used to enter control structures (except in the sense of the normal entry point).

<label j>example=:3 :0

 if. y do.
   echo 0
   goto_a.
   echo 1
 else.
   echo 2
   goto_a.
   echo 3
 end.
 echo 4
 label_a.
 echo 5

)

  example 1

0 5

  example 0

2 5</lang>

jq[edit]

jq is scoped lexically.

A function can only be called within its definition or following it, it being understood that functions can in effect be passed by name as parameters to other functions.

A further restriction is that inner functions are invisible outside their enclosing function, an inner function being one which is defined within the body of another.

A function that is not defined within an inner function will be called a top-level function. The lowest-level function in which an inner function is defined will be called its enclosing function.

If more than one outer function has the same name, then the last definition effectively overwrites the first, at least as far as subsequent invocations are concerned.

A similar rule applies to two inner functions defined within the same enclosing function. Otherwise, inner functions of the same name can co-exist. In particular, a function named NAME may define an inner function of the same name. For example:

def NAME:
def NAME: 2;
1, NAME; # this calls the inner function, not the outer function
 
NAME # => 1, 2

Mutually Defined Functions The "declare-before-use" rule means that two top-level functions cannot be defined in terms of each other. Consider the following example:

def F(x): if x == 0 then M(x) else 1 end;  # NOT POSSIBLE
def M(x): if x == 1 then F(x) else 2 end;

There are several possible workarounds using inner functions. For example, if both F and M must be top-level functions, we could define them as follows:

def F(x):
def M(x): if x == 1 then F(x) else 2 end;
if x == 0 then M(x) else 1 end;
 
def M(x): if x == 1 then F(x) else 2 end;

If F and M are not required to be top-level functions, then both F and M could be defined as inner functions of the same enclosing function.

Oforth[edit]

Functions are global and must be defined before use. Methods are global and must be declared before use. They can be used before a method implementation.

Perl 6[edit]

First a little hand-wavey exposition. The lines are rather blurry in Perl 6 between subroutines, methods, operators and functions. Methods are associated with an object and are inheritable. Subroutines and operators are not. Other than that though there is a lot of overlap. "A function" doesn't really have a specific definition, but is more of a generic term used when talking about code reference type of things.

Methods don't have a separate scope from the object they are attached to. If the object is in scope, the method will be.

A subroutine is really just another type of object. It has a code reference and has ROUTINE semantics attached to it. The same holds for operators. Operators are really just subroutines with a funny calling convention. That being the case, scoping for subroutines very closely follows scoping rules for any other Perl 6 variable type.

In general, subroutines are "my" variables by default (if you don't specify, the "my" is implicit), meaning scoping is lexical to the enclosing block and flows inward. A subroutine defined within a block will be visible to everything inside that block, even other blocks within that block. However, any inner block can define its own subroutine with the same name and that will be used in preference to the routine from an outer block. That implies you can easily override / redefine core functions from the Perl 6 setting. The setting is the collection of built in functions supplied by Perl 6, typically and somewhat incongruously referred to as "CORE" even though technically it is the outermost scope. ( SKIN? BARK? CRUST? ... oooo! EXOSKELETON! :-) )

Alternately, subroutines may be declared as an "our" variable making it a package global, visible anywhere in the packages' namespace. That is somewhat discouraged though as it pollutes the namespace and reduces the granularity of control.

There are several ways to modify the relative scope of a subroutine (any item in the symbol table really) by adding modifiers to the name.

CALLER    # Contextual symbols in the immediate caller's lexical scope
OUTER     # Symbols in the next outer lexical scope
UNIT      # Symbols in the outermost lexical scope of compilation unit
SETTING   # Lexical symbols in the unit's DSL (usually CORE)
PARENT    # Symbols in this package's parent package (or lexical scope)
# call a routine before it has been defined
say log(); # prints: outer
 
# define a subroutine that overrides a CORE function
sub log { 'outer' };
{
# redefine the subroutine in this block
sub log { 'inner' };
{
# redefine the subroutine yet again
sub log { 'way down inside' };
 
# call it within this block
say log(); # prints: way down inside
 
# call it from the block one level out
say &OUTER::log(); # prints: inner
 
# call it from the block two levels out
say &OUTER::OUTER::log(); # prints: outer
 
# call it from the outermost block
say &UNIT::log(); # prints: outer
 
# call a subroutine that is post declared in outermost scope
outersub()
}
 
{
# subroutine in an inner block that doesn't redefine it
# uses definition from nearest enclosing block
say log(); # prints: inner
}
# call it within this block
say log(); # prints: inner
 
# call it from the block one level out
say &OUTER::log(); # prints: outer
}
 
sub outersub{
# call subroutine within this block - gets outer sub
say log(); # prints: outer
 
# call subroutine from the scope of the callers block
say &CALLER::log(); # prints: way down inside
 
# call subroutine from the outer scope of the callers block
say &CALLER::OUTER::log(); # prints: inner
 
# call the original overridden CORE routine
say &CORE::log(e); # prints: 1 ( natural log of e )
}

Labels are less interesting and are typically useful only for control flow in looping constructs. They generally follow the same scoping rules as "my" variables. There is nearly always easier ways to do control flow though, so they aren't heavily used.

PL/I[edit]

 
Functions are normally internal to a program. If they are at the nesting level
immediately within the program, they are accessible from anywhere in the program.
 
Functions can also be encapsuled in a package, and the function name exported.
 
Functions can be compiled separately, and then linked with a program
in which case they are globally accessible.
 

PowerShell[edit]

A function exists in the scope in which it was created.

If a function is part of a script, the function is available to statements within that script. By default, a function in a script is not available at the command prompt.

You can specify the scope of a function. For example, the function is added to the global scope in the following example:

 
function global:Get-DependentService
{
Get-Service | Where-Object {$_.DependentServices}
}
 

When a function is in the global scope, you can use the function in scripts, in functions, and at the command line.

Functions normally create a scope. The items created in a function, such as variables, exist only in the function scope.

For more information about scope in Windows PowerShell, see about_Scopes

 
Get-Help about_Scopes
 

Python[edit]

In Python; our chief creator of new scopes is a function definition ... functions and classes... classes and functions... Our two creators are functions and classes... and files... Our three creators are ... I'll come in again.

Some (rather dry) rules are:

  1. All names, (of functions, classes, as well as variables), are scoped in the same way.
  2. A names scope is its closest enclosing file, function, or class.
  3. Names belong to the scope where they are assigned-to (or bound)
Cf.
  • Ka-Ping Yee has a tutorial on the above here.
  • This Python Enhancement Proposal: PEP 3104 introduces the non-local keyword of Python 3.
  • And of course, this!

Racket[edit]

Racket inherits the strict lexical-scopedness of Scheme, so function bindings (like any other bindings) are visible only within their scope. For example

 
(define (foo x)
(define (bar y) (+ x y))
(bar 2))
(foo 1) ; => 3
(bar 1) ; => error
 

but that applies only to the *bindings* -- the actual function values (like other values) can be passed around freely:

 
(define (foo x)
(define (bar y) (+ x y))
bar)
(foo 1)  ; => #<procedure:bar>
((foo 1) 2) ; => 3
 

But it should be noted that Racket is flexible enough to make it possible to implement other kinds of scope.

REXX[edit]

In REXX, labels (which are also the name of in-stream procedures (or subroutines or functions) are identified by:

(optional blanks)
a REXX symbol
(optional blanks)
a colon (:)
(optional blanks)
(optional semicolon (;)
(optional blanks)


(all of the above are normally contained on one line (record), but may (syntactically) be continued by the normal REXX rules for continuation.

Any label can be referenced from anywhere in the REXX program   (global scope).

Multiple labels (with the same name) are not considered an error in the REXX language;   the first label found (topmost) is used.

/*REXX program  demonstrates  the  use of    labels    and  also a   CALL   statement.  */
blarney = -0 /*just a blarney & balderdash statement*/
signal do_add /*transfer program control to a label.*/
ttt = sinD(30) /*this REXX statement is never executed*/
/* [↓] Note the case doesn't matter. */
DO_Add: /*coming here from the SIGNAL statement*/
 
say 'calling the sub: add.2.args'
call add.2.args 1, 7 /*pass two arguments: 1 and a 7 */
say 'sum =' result /*display the result from the function.*/
exit /*stick a fork in it, we're all done. */
/*──────────────────────────────────────────────────────────────────────────────────────*/
add.2.args: procedure; parse arg x,y; return x+y /*first come, first served ···*/
add.2.args: say 'Whoa Nelly!! Has the universe run amok?' /*didactic, but never executed*/
add.2.args: return arg(1) + arg(2) /*concise, " " " */

output

calling the sub:  add.2.args
sum = 8

Ruby[edit]

'def' starts the definition of a method, and 'end' ends it - no cute little curly braces.

 
def welcome(name)
puts "hello #{name}"
end
puts "What is your name?"
$name = STDIN.gets
welcome($name)
return

output

What is your name?
xyz
hello xyz

Sidef[edit]

In Sidef, the same rule which is applied to variable scoping, is applied to functions and classes as well, which means that a function defined inside another function is not visible outside the current scope.

# Nested functions
func outer {
func inner {}; # not visible outside
}
 
# Nested classes
class Outer {
class Inner {}; # not visisble outside
}

Tcl[edit]

There are a number of different symbol types in Tcl, all of which are handled independently. Each namespace contains a mapping from (simple) command names to command implementations; when a command is looked up, the search is done by looking in the current namespace, then in the namespaces on that namespace's path (which is usually empty), and finally in the global namespace. There are no local commands (unlike with variables, though a lambda expression in a variable can act very similarly to a command). Commands only have a mapping after they have been created; the proc “declaration” is just a command that creates a procedure at the point where it is called.

doFoo 1 2 3;    # Will produce an error
 
proc doFoo {a b c} {
puts [expr {$a + $b*$c}]
}
doFoo 1 2 3; # Will now print 7 (and will continue to do so until doFoo is renamed or deleted

Tcl does not support labels, either outside or inside procedures. (Other mechanisms are used for jumps and state machines.)

UNIX Shell[edit]

There is no lookahead in the shell, so functions cannot be called until their definition has been run:

#!/bin/sh
multiply 3 4 # This will not work
echo $? # A bogus value was returned because multiply definition has not yet been run.
 
multiply() {
return `expr $1 \* $2` # The backslash is required to suppress interpolation
}
 
multiply 3 4 # Ok. It works now.
echo $? # This gives 12

The shell does not support the use of arbitrary line labels.

zkl[edit]

zkl has no labels.

Functions have two modifiers, public (the default) and private. Private is the same as public inside the compilation unit, outside that unit/file, a private function can only be accessed via reflection.

class C{ fcn [private] f{} }

ZX Spectrum Basic[edit]

Functions are visible as soon as they are entered, even if the program is not run. Line numbers are used instead of labels. These are also immediately accessible as soon as the line is entered, even if the program is not run:

9000 REM The function is immediately visible and usable
9010 DEF FN s(x)=x*x
 
PRINT FN s(5): REM This will work immediately
GO TO 50: REM This will work immediately