⚠️ Warning: This is a draft ⚠️

This means it might contain formatting issues, incorrect code, conceptual problems, or other severe issues.

If you want to help to improve and eventually enable this page, please fork RosettaGit's repository and open a merge request on GitHub.

{{task|Basic language learning}}

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'''.


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.


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


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[http://www.xs4all.nl/~jmvdveer/report_4.html#92]: 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#ALGOL_68|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 } )

And can be "USEd" as follows:

USE standard,environment
  INT a := 43;
  "Oops!" ASSERT ( a = 42 )


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.

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


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.'''


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".

	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"

# 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



{{works with|AutoHotkey_L}}

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

    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




' 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

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

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

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

    RETURN i


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


      PROCassert(a% = 42)

      DEF PROCassert(bool%)
      IF NOT bool% THEN ERROR 100, "Assertion failed"


squish import :assert :assertions

assert_equal 42 42
assert_equal 13 42  #Raises an exception


#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.

=={{header|C sharp}} and {{header|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;


        // 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


        ' 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

{{out|note=for .NET Core debug builds when outside of a debugger}}

After Trace.Assert
Assertion Failed
Debug assertion failed

   at Program.Main() in FILENAME:line 21

{{out}} 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.

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())



#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.


(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;
	x := 41;
	ASSERT(x = 42);
END Assertions.




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


import std.exception: enforce;

int foo(in bool condition) pure nothrow
in {
    // Assertions are used in contract programming.
} 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 supplies a class Expect that works similar to the Assert methods of Junit

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



Assert(a = 42);

If an assertion fails, EAssertionFailed exception is raised.

The generation of assertion code can be disabled by compiler directive


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

program TestAssert;


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


  a: Integer;

    Assert(a = 42);
    on E:Exception do
      Writeln(E.Classname, ': ', E.Message);


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);
   a = 42;
   // code here
   a = 42;


Dyalect has a built-in "assert" function:

var x = 42
assert(42, x)

This function throws an exception if assertion fails.


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


(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)


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


{{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
            test: TEST;
            create test;


Another file called test.e:

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



defmodule AssertionTest do
  use ExUnit.Case

  def return_5, do: 5

  test "not equal" do
    assert 42 == return_5


  1) test not equal (AssertionTest)
     Assertion with == failed
     code: 42 == return_5
     lhs:  42
     rhs:  5
       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

Assertion can be loaded from cl.el:

(require 'cl)
(let ((x 41))
  (assert (= x 42) t "the answer is not right"))


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.

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

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


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

fourty_two i

i = 41 -- type-check failure


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

USING: kernel ;
42 assert=


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.


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"




Assertion: 1>2 failed

Press any key to continue...


variable a
: assert   a @ 42 <> throw ;

41 a ! assert


' 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


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

=={{header|F_Sharp|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]]


# 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

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
# 10


Assertions are a feature [http://golang.org/doc/go_faq.html#assertions 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

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
runtime.mainstart+0xf /pool/go/src/pkg/runtime/amd64/asm.s:77
runtime.goexit /pool/go/src/pkg/runtime/proc.c:148


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

Test program:

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


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)


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

=={{header|Icon}} and {{header|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)

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

This produces the output:

42 is invalid value for 'a'

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


## Java

public class Assertions {

    public static void main(String[] args) {
        int a = 13;

        // ... some real code 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 non-void expression.

Note: assertion checking is disabled by default when you run your program with the java command. You must provide the -ea (short for -enableassertions) flag in order to enable them.


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
# ERROR: LoadError: TypeError: in typeassert, expected String, got Int64


Assertions need to be enabled using java's -ea option for an AssertionError to be thrown when the condition is false.

// version 1.0.6 (assert.kt)

fun main(args: Array<String>) {
   val a = 42
   assert(a == 43)


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


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


-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.

call assert a=42
print "passed"

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

sub assert cond
    if cond=0 then 'simulate error, mentioning "AssertionFailed"
    end if
end sub



Stops with error message {{out}}

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


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

-- 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"


? { n = 42 };


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.

Module Assert {
      \\ This is a global object named Rec
      Global Group Rec {
            document doc$="Error List at "+date$(today)+" "+time$(now)+{
            Module Error(a$) {
                  if a$="" then exit
                  .doc$<="     "+a$+{
                  flush error
            Module Reset {
                  Clear .doc$
            Module Display {
                  Report  .doc$
            Module SaveIt {
                  .lastfilename$<=replace$("/", "-","Err"+date$(today)+time$(now)+".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 {
                  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$
      win "notepad.exe", dir$+Rec.lastfilename$


(Taken from Lua, above.)

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

=={{header|Mathematica}} / {{header|Wolfram Language}}==


=={{header|MATLAB}} / {{header|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 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;
message "ok";

Output (failed assertion):

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


=={{header|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.


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 [http://nemerle.org/wiki/index.php?title=Design_by_contract_macros here].


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

Assertions may be disabled by compiling with --assertions:off.

=={{header|Oberon-2}}== Oxford Oberon-2

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


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


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);

=={{header|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 NSCAssert''N'' instead.

To turn off assertions, define the NS_BLOCK_ASSERTIONS macro.


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.


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 ;


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

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


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).

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


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


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

#include <pari/pari.h>

  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)");


See [[Assertions#Delphi | Delphi]]


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.

Perl 6

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

{{works with|pugs}} ''Note: This example uses an experimental feature, and does not work in the primary Perl 6 compiler, Rakudo.''

# with a (non-hygienic) macro
macro assert ($x) { "$x or die 'assertion failed: $x'" }
assert('$a == 42');


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

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 assert(integer flag, string msg)
    if DEBUG then
        if not flag then
            {} = message_box(msg,"failed assertion",MB_OK)  -- or
            puts(1,msg)                                     -- , and/or
            crash(msg)      -- crash/ex.err report          -- or
            trace(1)        -- start debugging
        end if
    end if
end function

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

Note that while the body of assert() 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 asserts 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 produces 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

Again, if the application was run from Edita, on error it automatically jumps to the offending file and line.


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


The '[http://software-lab.de/doc/refA.html#assert 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 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}}




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

A implementation as defined above could be;

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

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


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.




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

(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


/* REXX ***************************************************************
* There's no assert feature in Rexx. That's how I'd implement it
* 10.08.2012 Walter Pachl
Do i=1 By 1
  Call assert x.i,42
  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
    Signal Syntax
Syntax: Say 'program terminated'


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


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!");


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


This uses test/unit from the standard library.

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

n = 5
  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


<42> expected but was


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


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

(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. [http://www.gnu.org/software/sather/docs-1.2/tutorial/safety2208.html here]).


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"))


{{Works with|Scheme|R^6RS}}

{{trans|Common Lisp}}

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


assert( n = 42 );


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


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


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

raises an AssertionFailed condition (an Error).


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.


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;


-# 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;


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.


Assertions in Stata are limited to checking a property on the observations of a dataset. See '''[http://www.stata.com/help.cgi?assert assert]''' in Stata help.

For instance, if a dataset contains two variables x, y, z, one can check if x0, with:

There is another command, '''[http://www.stata.com/help.cgi?confirm 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:

```stata>confirm file titanium.dta</lang

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

It's also possible to use '''[http://www.stata.com/help.cgi?error 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 '''[http://www.stata.com/help.cgi?capture capture]''' is used to trap the error, the return code (here 54) can be retrieved in '''[http://www.stata.com/help.cgi?_variables _rc]'''.


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.



package require control

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

Produces the output:

assertion failed: $x == 42

{{omit from|gnuplot}} {{omit from|NSIS}}

UNIX Shell

{{works with|bash}} Assertions are not builtin commands, but we can add a function easily.

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


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


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

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



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


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


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.

## Visual Basic .NET

See [[#C# and Visual Basic .NET]].

## 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");


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

assert(myVar = 42)


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

pub fn main() void {
    assert(1 == 0); // On failure, an `unreachable` is reached

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


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


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!)


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