Assertions: Difference between revisions
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=={{header|Lingo}}== |
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Lingo has no assert statement, but the abort command (that exits the full call stack) allows to implement something like it as global function: |
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<lang lingo>-- in a movie script |
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on assert (ok, message) |
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if not ok then |
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if not voidP(message) then _player.alert(message) |
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abort -- exits from current call stack, i.e. also from the caller function |
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end if |
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end |
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-- anywhere in the code |
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on test |
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x = 42 |
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assert(x=42, "Assertion 'x=42' failed") |
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put "this shows up" |
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x = 23 |
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assert(x=42, "Assertion 'x=42' failed") |
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put "this will never show up" |
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end</lang> |
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=={{header|Lisaac}}== |
=={{header|Lisaac}}== |
Revision as of 18:25, 29 September 2016
You are encouraged to solve this task according to the task description, using any language you may know.
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.
Ada
Using pragma Assert: <lang ada>pragma Assert (A = 42, "Oops!");</lang> The behavior of pragma is controlled by pragma Assertion_Policy. Another way is to use the predefined package Ada.Assertions: <lang ada>with Ada.Assertions; use Ada.Assertions; ... Assert (A = 42, "Oops!");</lang> The procedure Assert propagates Assertion_Error when condition is false.
Aime
<lang aime>integer x;
x = 41; if (x != 42) {
error("x is not 42");
}</lang> Executing the program will produce on standard error:
aime: assert: 5: x is not 42
ALGOL 68
The "Revised Report on the Algorithmic Language - ALGOL 68" suggest that ASSERT may be made available by a particular implementation, quote: "Pragmats may ... convey to the implementation some piece of information affecting some aspect of the meaning of the program which is not defined by this Report,..."
Example given[1]:
INT a, b; read((a, b)) PR ASSERT a >= 0 & b > 0 PR;
This works with neither ELLA ALGOL 68 nor ALGOL 68G.
The standard alternative would be to implement the assertions as an exception as per the Exceptions sample code.
In ELLA ALGOL 68 the ASSERT is implemented as an operator in the environment prelude: <lang algol68>OP ASSERT = (VECTOR [] CHAR assertion,BOOL valid) VOID: IF NOT valid THEN type line on terminal(assertion);
terminal error( 661 {invalid assertion } )
FI;</lang> And can be "USEd" as follows: <lang algol68>PROGRAM assertions CONTEXT VOID USE standard,environment BEGIN
INT a := 43; "Oops!" ASSERT ( a = 42 )
END FINISH</lang>
ALGOL W
Assertions were added to the 1972 version of Algol W. If the tested condition is false, the program terminates. In the following, the write does not get executed.
<lang algolw>begin
integer a; a := 43; assert a = 42; write( "this won't appear" )
end.</lang>
Apex
Asserts that the specified condition is true. If it is not, a fatal error is returned that causes code execution to halt. <lang apex> String myStr = 'test; System.assert(myStr == 'something else', 'Assertion Failed Message'); </lang>
Asserts that the first two arguments are the same. If they are not, a fatal error is returned that causes code execution to halt. <lang apex> Integer i = 5; System.assertEquals(6, i, 'Expected 6, received ' + i); </lang>
Asserts that the first two arguments are different. If they are the same, a fatal error is returned that causes code execution to halt. <lang apex> Integer i = 5; System.assertNotEquals(5, i, 'Expected different value than ' + i); </lang>
You can’t catch an assertion failure using a try/catch block even though it is logged as an exception.
AutoHotkey
Exceptions
<lang AHK>a := 42 Assert(a > 10) Assert(a < 42) ; throws exception
Assert(bool){
If !bool throw Exception("Expression false", -1)
}</lang>
Legacy versions
<lang AutoHotkey>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 }</lang>
Axe
<lang axe>A=42??Returnʳ</lang>
BBC BASIC
<lang bbcbasic> PROCassert(a% = 42)
END DEF PROCassert(bool%) IF NOT bool% THEN ERROR 100, "Assertion failed" ENDPROC</lang>
Brat
<lang brat>squish import :assert :assertions
assert_equal 42 42 assert_equal 13 42 #Raises an exception</lang>
C
<lang c>#include <assert.h>
int main(){
int a; /* ...input or change a here */ assert(a == 42); /* aborts program when a is not 42, unless the NDEBUG macro was defined */
return 0;
}</lang> 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: <lang c>assert(a == 42 && "Error message");</lang> This trick only works with messages written directly in the source code (i.e. cannot be a variable or be computed), however, since the assertion message is captured by the macro at compile-time.
C#
<lang csharp>using System.Diagnostics;
Debug.Assert(a == 42);</lang>
C++
<lang cpp>#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
}</lang>
Note that assert does not get a std::
prefix because it's a macro.
Clojure
<lang Clojure> (let [i 42]
(assert (= i 42)))
</lang>
Common Lisp
<lang lisp>(let ((x 42))
(assert (and (integerp x) (= 42 x)) (x)))</lang>
Component Pascal
Works with BlackBox Component Builder <lang oberon2> MODULE Assertions; VAR x: INTEGER; PROCEDURE DoIt*; BEGIN x := 41; ASSERT(x = 42); END DoIt; END Assertions.
Assertions.DoIt </lang> Output:
TRAP 0 Assertions.DoIt [0000001DH] Kernel.Call [00001A7CH] .adr INTEGER 1685454913 .kind INTEGER 0 .n INTEGER 0 .p INTEGER 0 .par ARRAY 256 OF INTEGER elements .r REAL 8.70603013185328E+175 .sig POINTER [64760018H] .size INTEGER 2287288 .sp INTEGER 256 .typ POINTER NIL Meta.Item.ParamCallVal [00002B5EH] .adr INTEGER 1685454913 .data ARRAY 256 OF INTEGER elements
D
<lang d>import std.exception: enforce;
int foo(in bool condition) pure nothrow in {
// Assertions are used in contract programming. assert(condition);
} out(result) {
assert(result > 0);
} body {
if (condition) return 42;
// assert(false) is never stripped from the code, it generates an // error in debug builds, and it becomes a HALT instruction in // -release mode. // // It's used as a mark by the D type system. If you remove this // line the compiles gives an error: // // Error: function assertions.foo no return exp; // or assert(0); at end of function assert(false, "This can't happen.");
}
void main() pure {
int x = foo(true);
// A regular assertion, it throws an error. // Use -release to disable it. // It can be used in nothrow functions. assert(x == 42, "x is not 42");
// This throws an exception and it can't be disabled. // There are some different versions of this lazy function. enforce(x == 42, "x is not 42");
}</lang>
Dart
Dart supplies a class Expect that works similar to the Assert methods of Junit <lang d>main() {
int i=42; int j=41;
Expect.equals(42,i); Expect.equals(42,j);
}</lang>
Delphi
<lang Delphi>Assert(a = 42);</lang>
If an assertion fails, EAssertionFailed exception is raised.
The generation of assertion code can be disabled by compiler directive
<lang Delphi>{$ASSERTIONS OFF}</lang>
Here is a simple console demo app which raises and handles assertion exception:
<lang Delphi>program TestAssert;
{$APPTYPE CONSOLE}
{.$ASSERTIONS OFF} // remove '.' to disable assertions
uses
SysUtils;
var
a: Integer;
begin
try Assert(a = 42); except on E:Exception do Writeln(E.Classname, ': ', E.Message); end; Readln;
end.</lang>
DWScript
Simple assertion, with a custom (optional) message <lang Delphi>Assert(a = 42, 'Not 42!');</lang> 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 <lang Delphi>procedure UniversalAnswer(var a : Integer); require
a = 42;
begin
// code here
ensure
a = 42;
end;</lang>
E
E does not have the specific feature of assertions which may be disabled by a global option. But it does have a utility to throw an exception if a condition is false:
<lang e>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</lang>
EchoLisp
<lang lisp> (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) </lang>
ECL
<lang> ASSERT(a = 42,'A is not 42!',FAIL);</lang>
Eiffel
version 2.4
There are many assertion types in Eiffel, one is the following:
File called main.e: <lang eiffel>class MAIN
creation main feature main is local test: TEST; do create test;
io.read_integer; test.assert(io.last_integer); end
end</lang> Another file called test.e: <lang eiffel>class TEST
feature assert(val: INTEGER) is require val = 42; do print("Thanks for the 42!%N"); end
end</lang>
Elixir
<lang elixir>ExUnit.start
defmodule AssertionTest do
use ExUnit.Case def return_5, do: 5 test "not equal" do assert 42 == return_5 end
end</lang>
- Output:
1) test not equal (AssertionTest) test.exs:8 Assertion with == failed code: 42 == return_5 lhs: 42 rhs: 5 stacktrace: test.exs:9: (test) Finished in 0.1 seconds (0.1s on load, 0.01s on tests) 1 test, 1 failure Randomized with seed 869000
Emacs Lisp
Assertion can be loaded from cl.el: <lang lisp>(require 'cl) (let ((x 41))
(assert (= x 42) t "the answer is not right"))</lang>
Erlang
Erlang doesn't have an assert statement. However, it is single assignment, and its assignment operator won't complain if you reassign the exact same value to an existing variable but will throw an exception otherwise. <lang erlang>1> N = 42. 42 2> N = 43.
- exception error: no match of right hand side value 43
3> N = 42. 42 4> 44 = N.
- exception error: no match of right hand side value 42
5> 42 = N. 42</lang>
As such, the behavior of Erlang's assignment operator is extremely similar to a regular assert in other languages.
Euphoria
<lang euphoria>type fourty_two(integer i)
return i = 42
end type
fourty_two i
i = 41 -- type-check failure</lang>
Factor
Throw an exception if the value on the top of the stack is not equal to 42:
<lang factor>USING: kernel ; 42 assert=</lang>
FBSL
One needs to DECLARE the asserter variable at the top of script.
This implementation evaluates the expression given to the function and displays a message if it evaluates to false. <lang qbasic>#APPTYPE CONSOLE
DECLARE asserter
FUNCTION Assert(expression)
DIM cmd AS STRING = "DIM asserter AS INTEGER = (" & expression & ")" EXECLINE(cmd, 1) IF asserter = 0 THEN PRINT "Assertion: ", expression, " failed"
END FUNCTION
Assert("1<2") Assert("1>2")
PAUSE</lang> Output
Assertion: 1>2 failed Press any key to continue...
Forth
<lang fsharp>variable a
- assert a @ 42 <> throw ;
41 a ! assert</lang>
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. <lang fsharp>let test x =
assert (x = 42)
test 43</lang>
GAP
<lang gap># See section 7.5 of reference manual
- GAP has assertions levels. An assertion is tested if its level
- is less then the global level.
- Set global level
SetAssertionLevel(10);
a := 1; Assert(20, a > 1, "a should be greater than one");
- nothing happens
a := 1; Assert(4, a > 1, "a should be greater than one");
- error
- Show current global level
AssertionLevel();
- 10</lang>
Go
Assertions are a feature consciously omitted from Go. For cases where you want feedback during development, the following code should provide a similar purpose. While it is simply an if statement and a panic, the technique does have some properties typical of assertions. For one, the predicate of an if statement in Go is required to be of boolean type. Specifically, ints are not tacitly tested for zero, pointers are not tested for nil: the expression must be boolean, as the WP article mentions is typical of assertions. Also, it provides a good amount of information should the predicate evaluate to true. First, a value of any type can be passed to the panic, and by default is displayed, followed by a stack trace which includes the location of the panic in the source code—function name, file name, and line number. <lang go>package main
func main() {
x := 43 if x != 42 { panic(42) }
}</lang> Output:
panic: 42 panic PC=0x2b772d1a1048 runtime.panic+0xa7 /pool/go/src/pkg/runtime/proc.c:1032 runtime.panic(0x40e820, 0x2a) main.main+0x48 /pool/test.go:8 main.main() runtime.mainstart+0xf /pool/go/src/pkg/runtime/amd64/asm.s:77 runtime.mainstart() runtime.goexit /pool/go/src/pkg/runtime/proc.c:148 runtime.goexit()
Groovy
<lang groovy>def checkTheAnswer = {
assert it == 42 : "This: " + it + " is not the answer!"
}</lang>
Test program: <lang groovy>println "before 42..." checkTheAnswer(42) println "before 'Hello Universe'..." checkTheAnswer("Hello Universe")</lang>
Output:
before 42... before 'Hello Universe'... java.lang.AssertionError: This: Hello Universe is not the answer!. Expression: (it == 42). Values: it = Hello Universe at ConsoleScript80$_run_closure1.doCall(ConsoleScript80:2) at ConsoleScript80.run(ConsoleScript80:8)
Haskell
<lang haskell>import Control.Exception
main = let a = someValue in
assert (a == 42) -- throws AssertionFailed when a is not 42 somethingElse -- what to return when a is 42</lang>
Icon and Unicon
<lang Icon>... 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. ...</lang>
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:
<lang Icon> $define DEBUG 1 # this allows the assertions to go through
procedure check (a)
if DEBUG then stop (42 = a, " is invalid value for 'a'") write (a)
end
procedure main ()
check (10) check (42) check (12)
end </lang>
This produces the output:
10 42 is invalid value for 'a'
Changing the define to: $define DEBUG &fail
turns off the assertion checking.
J
<lang j> assert n = 42</lang>
Java
<lang java5>public static void main(String[] args){
int a; //...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 non-void expression
}</lang> 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.
Julia
<lang julia>#assert() function takes expression as 1st argument, failed-assertion message as optional 2nd argument julia> assert(x==42,"x is not 42") ERROR: assertion failed: x is not 42
- @assert macro checks the supplied conditional expression, with the expression returned in the failed-assertion message
julia> @assert x==42 ERROR: assertion failed: :((x==42))
- Julia also has type assertions of the form, x::Type which can be appended to a variable for type-checking at any point
julia> x::String ERROR: type: typeassert: expected String, got Int32</lang>
Lasso
<lang lasso>local(a) = 8 fail_if(
#a != 42, error_code_runtimeAssertion, error_msg_runtimeAssertion + ": #a is not 42"
)</lang>
- Output:
-9945 Runtime assertion: #a is not 42
Liberty BASIC
Liberty BASIC has no exceptions or user-defined error messages, but we could break program if condition is not met. We can even make it spell "AssertionFailed". In a way. <lang lb> a=42 call assert a=42 print "passed"
a=41 call assert a=42 print "failed (we never get here)" end
sub assert cond
if cond=0 then 'simulate error, mentioning "AssertionFailed" AssertionFailed(-1)=0 end if
end sub </lang>
- Output:
passed
Stops with error message
- Output:
RuntimeError: Subscript out of range: -1, AssertionFailed()
Lingo
Lingo has no assert statement, but the abort command (that exits the full call stack) allows to implement something like it as global function: <lang lingo>-- in a movie script on assert (ok, message)
if not ok then if not voidP(message) then _player.alert(message) abort -- exits from current call stack, i.e. also from the caller function end if
end
-- anywhere in the code on test
x = 42 assert(x=42, "Assertion 'x=42' failed") put "this shows up" x = 23 assert(x=42, "Assertion 'x=42' failed") put "this will never show up"
end</lang>
Lisaac
<lang Lisaac>? { n = 42 };</lang>
Lua
<lang lua>a = 5 assert (a == 42) assert (a == 42,'\..a..'\' is not the answer to life, the universe, and everything')</lang>
Maple
(Taken from Lua, above.) <lang Maple>a := 5: ASSERT( a = 42 ); ASSERT( a = 42, "a is not the answer to life, the universe, and everything" );</lang>
Mathematica / Wolfram Language
<lang Mathematica>Assert[var===42]</lang>
MATLAB / Octave
<lang MATLAB>assert(x == 42,'x = %d, not 42.',x);</lang>
Sample Output: <lang MATLAB>x = 3; assert(x == 42,'Assertion Failed: x = %d, not 42.',x); ??? Assertion Failed: x = 3, not 42. </lang>
Metafont
Metafont has no really an assert built in, but it can easily created:
<lang metafont>def assert(expr t) = if not (t): errmessage("assertion failed") fi enddef;</lang>
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:
<lang metafont>n := 41; assert(n=42); message "ok";</lang>
Output (failed assertion):
This is METAFONT, Version 2.71828 (Web2C 7.5.5) (./assert.mf ! assertion failed. <to be read again> ; l.4 assert(n=42); ?
Modula-3
ASSERT
is a pragma, that creates a run-time error if it returns FALSE
.
<lang modula3><*ASSERT a = 42*></lang>
Assertions can be ignored in the compiler by using the -a
switch.
Nemerle
A basic assertion uses the assert keyword: <lang Nemerle>assert (foo == 42, $"foo == $foo, not 42.")</lang> 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: <lang Nemerle>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 }</lang> The design by contract macros throw Nemerle.AssertionException's unless another Exception is specified using the otherwise keyword after the requires/ensures statement. For further details on design by contract macros, see here.
Nim
<lang Nim>var a = 42 assert(a == 42)</lang> Assertions may be disabled by compiling with --assertions:off.
Oberon-2
Oxford Oberon-2 <lang oberon2> MODULE Assertions; VAR a: INTEGER; BEGIN a := 40; ASSERT(a = 42); END Assertions. </lang> Output:
Runtime error: assertion failed (0) on line 6 in module Assertions In procedure Assertions.%main called from MAIN
Objective-C
For use within an Objective-C method: <lang objc>NSAssert(a == 42, @"Error message");</lang>
If you want to use formatting arguments, you need to use the assertion macro corresponding to your number of formatting arguments: <lang objc>NSAssert1(a == 42, @"a is not 42, a is actually %d", a); # has 1 formatting arg, so use NSAssert"1"</lang>
Within a regular C function you should use NSCAssert
or NSCAssertN
instead.
To turn off assertions, define the NS_BLOCK_ASSERTIONS macro.
OCaml
<lang ocaml>let a = get_some_value () in
assert (a = 42); (* throws Assert_failure when a is not 42 *) (* evaluate stuff to return here when a is 42 *)</lang>
It is possible to compile with the parameter -noassert
then the compiler won't compile the assertion checks.
Oforth
In Oforth, assertions are handled as tests.
Assertions are checked only if oforth is launched using --a command line. Default value is to not check assertions.
If an assertion is ko (and if oforth is launched using --a), an exception is raised.
<lang Oforth>: testInteger(n, m)
assert: [ n isInteger ] assert: [ n 42 == ]
System.Out "Assertions are ok, parameters are : " << n << ", " << m << cr ;</lang>
- Output:
testInteger(41, 43) [1:interpreter] ExRuntime : Assertion failed into <#testInteger> testInteger(42, 43) Assertions are ok, parameters are : 42, 43
Oz
Oz does not have an assert statement. But if different values are assigned to the same dataflow variable, an exception will be thrown (similar to Erlang).
<lang oz>declare
proc {PrintNumber N} N=42 %% assert {Show N} end
in
{PrintNumber 42} %% ok {PrintNumber 11} %% throws </lang>
Output:
%***************************** failure ************************** %** %** Tell: 11 = 42 %** %** Call Stack: %** procedure 'PrintNumber' in file "Oz<8>", line 3, column 0, PC = 18600220 %**--------------------------------------------------------------
PARI/GP
PARI can use any of the usual C methods for making assertions. GP has no built-in assertions.
<lang C>#include <assert.h>
- include <pari/pari.h>
void test() {
GEN a; // ... input or change a here
assert(equalis(a, 42)); /* Aborts program if a is not 42, unless the NDEBUG macro was defined */
}</lang>
More common is the use of pari_err_BUG
in such cases:
<lang C>if (!equalis(a, 42)) pari_err_BUG("this_function_name (expected a = 42)");</lang>
Pascal
See Delphi
Perl
While not exactly an assertion, a common Perl idiom is to use or die
to throw an exception when a certain statement is false.
<lang perl>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;</lang>
This idiom is typically used during file operations: <lang perl>open my $fh, '<', 'file'
or die "Cannot open file: $!\n"; # $! contains the error message from the last error</lang>
It is not needed whith the "autodie" pragma: <lang perl>use autodie; open my $fh, '<', 'file'; # automatically throws an exception on failure</lang>
Some third-party modules provide other ways of using assertions in Perl: <lang perl>use Carp::Assert; assert($a == 42);</lang>
There is also a number of ways to test assertions in test suites, for example: <lang perl>is $a, 42; ok $a == 42; cmp_ok $a, '==', 42, 'The answer should be 42';
- etc.</lang>
Perl 6
<lang perl6>my $a = (1..100).pick; $a == 42 or die '$a ain\'t 42';</lang>
Note: This example uses an experimental feature, and does not work in the primary Perl 6 compiler, Rakudo. <lang perl6># with a (non-hygienic) macro macro assert ($x) { "$x or die 'assertion failed: $x'" } assert('$a == 42');</lang>
Phix
User defined types allow the value to be automatically tested whenever it changes, and can be disabled using the "without type_check" directive: <lang Phix>type int42(object i)
return i=42
end type
int42 i
i = 41 -- type-check failure</lang> 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: <lang Phix>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!!")</lang> 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: <lang Phix>if i!=42 then ?9/0 end if if i!=42 then crash("i is not 42!!") end if</lang> Again, if the application was run from Edita, on error it automatically jumps to the offending file and line.
PHP
<lang php><?php $a = 5
- ...input or change $a here
assert($a == 42) # when $a is not 42, take appropriate actions,
# which is set by assert_options()
?></lang>
PicoLisp
The 'assert' function, in combination with the tilde read macro, generates code only in debug mode: <lang PicoLisp>... ~(assert (= N 42)) # Exists only in debug mode ...</lang> Other possibilities are either to break into an error handler: <lang PicoLisp>(let N 41
(unless (= N 42) (quit "Incorrect N" N)) ) # 'quit' throws an error
41 -- Incorrect N ?</lang> or to stop at a debug break point, allowing to continue with the program: <lang PicoLisp>(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</lang>
PL/I
<lang> /* 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); </lang>
Prolog
<lang prolog> test(A):-
assertion(A==42).
</lang>
PureBasic
PureBasic does not have a native function for assertion, but allows for the definition of one.
The Macro below will only be included in the code if is compiled in debug mode, if so it will test the condition and if it fails it will inform with the message defined by the programmer, the line where it happened and in which source code file.
<lang PureBasic>Macro Assert(TEST,MSG="Assert: ")
CompilerIf #PB_Compiler_Debugger If Not (TEST) Debug MSG+" Line="+Str(#PB_Compiler_Line)+" in "+#PB_Compiler_File CallDebugger EndIf CompilerEndIf
EndMacro</lang>
A implementation as defined above could be; <lang PureBasic>A=42 Assert(A=42,"Assert that A=42") A=42-1 Assert(A=42)</lang> Where the second test would fail resulting in a message to the programmer with cause (if given by programmer), code line & file.
Python
<lang python>a = 5
- ...input or change a here
assert a == 42 # throws an AssertionError when a is not 42 assert a == 42, "Error message" # throws an AssertionError
# when a is not 42 with "Error message" for the message # the error message can be any expression</lang>
It is possible to turn off assertions by running Python with the -O (optimizations) flag.
R
<lang R>stopifnot(a==42)</lang>
Racket
Racket has higher-order assertions known as contracts that can protect any values including functions and objects. Contracts are typically applied on the imports or exports of a module.
<lang Racket>#lang racket
(define/contract x
(=/c 42) ; make sure x = 42 42)
(define/contract f
(-> number? (or/c 'yes 'no)) ; function contract (lambda (x) (if (= 42 x) 'yes 'no)))
(f 42) ; succeeds (f "foo") ; contract error! </lang>
If typical assertion checking (i.e. error unless some boolean condition holds) is needed, that is also possible:
<lang Racket>#lang racket
(define x 80) (unless (= x 42)
(error "a is not 42")) ; will error
</lang>
REXX
<lang REXX>/* REXX ***************************************************************
- There's no assert feature in Rexx. That's how I'd implement it
- 10.08.2012 Walter Pachl
- /
x.=42 x.2=11 Do i=1 By 1
Call assert x.i,42 End
Exit assert:
Parse Arg assert_have,assert_should_have If assert_have\==assert_should_have Then Do Say 'Assertion fails in line' sigl Say 'expected:' assert_should_have Say ' found:' assert_have Say sourceline(sigl) Say 'Look around' Trace ?R Nop Signal Syntax End Return
Syntax: Say 'program terminated'</lang> Output:
Assertion fails in line 8 expected: 42 found: 11 Call assert x.i,42 Look around Here I enter Say i 2 and then I press just enter program terminated
RLaB
RLaB does not have a special function to deal with assertions. The following workaround will do the trick:
<lang RLaB> // 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!");
} </lang>
Ring
<lang ring> x = 42 assert( x = 42 ) assert( x = 100 ) </lang>
Ruby
This uses test/unit from the standard library.
<lang ruby>require "test/unit/assertions" include Test::Unit::Assertions
n = 5 begin
assert_equal(42, n)
rescue Exception => e
# Ruby 1.8: e is a Test::Unit::AssertionFailedError # Ruby 1.9: e is a MiniTest::Assertion puts e
end</lang>
Output:
<42> expected but was <5>.
Rust
<lang rust> let x = 42; assert!(x == 42); assert_eq!(x, 42); </lang>
Sather
<lang sather>class MAIN is
main is i ::= 41; assert i = 42; -- fatal -- ... end;
end;</lang>
(The current GNU Sather compiler v1.2.3 I am using to test the code seems to ignore the assertion and no fatal error is raised, despite Sather should, see e.g. here).
Scala
These two are the same thing, and are tagged @elidable(ASSERTION)
:
<lang scala>assert(a == 42)
assert(a == 42, "a isn't equal to 42")
assume(a == 42)
assume(a == 42, "a isn't equal to 42")</lang>
The next one does the same thing as above, but it is not tagged. Often used as a pre-condition checker on class constructors. <lang scala>require(a == 42) require(a == 42, "a isn't equal to 42")</lang>
This one checks a value and returns it for further use (here shown being printed). It
uses assert
, which, as explained, gets tagged.
<lang scala>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"))</lang>
Scheme
<lang scheme>(let ((x 42))
(assert (and (integer? x) (= x 42))))</lang>
Sidef
<lang ruby>var num = pick(0..100); assert_eq(num, 42); # dies when "num" is not 42</lang>
- Output:
assert_eq: 26 == 42 is false at assertions.sf line 2.
Slate
<lang slate>load: 'src/lib/assert.slate'. define: #n -> 7. assert: n = 42 &description: 'That is not the Answer.'.</lang> raises an AssertionFailed condition (an Error).
Smalltalk
<lang smalltalk>foo := 41. ... self assert: (foo == 42).</lang>
In TestCase and subclasses, a number of check methods are inherited; among them: <lang smalltalk>self assert: (... somethingMustEvaluateToTrue.. ) self should:[ some code ] raise: someException "ensures that an exception is raised</lang>
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): <lang smalltalk>self assert: (... somethingMustEvaluateToTrue.. ) "implemented in Object"</lang> the implementation in Object raises an AssertionFailedError exception, which usually opens a debugger when in the IDE, but can be caught in deployed apps.
SPARK
Works with SPARK GPL 2010
Assertions are analysed statically, before compilation or execution. They can appear in various places:
- inline in the code, either
<lang ada>-# check X = 42;</lang>
- or
<lang ada>-# assert X = 42;</lang>
- as a precondition on an operation:
<lang ada>procedure P (X : in out Integer); --# derives X from *; --# pre X = 42;</lang>
- or as a postcondition on an operation:
<lang ada>procedure P (X : in out Integer); --# derives X from *; --# post X = 42;</lang> Example: <lang ada>X := 7; --# check X = 42;</lang> produces the following output:
H1: true . -> C1: false .
which is an unprovable theorem that tells you that there is a guaranteed failure.
Swift
<lang swift>var a = 5 //...input or change a here assert(a == 42) // aborts program when a is not 42 assert(a == 42, "Error message") // aborts program
// when a is not 42 with "Error message" for the message // the error message must be a static string</lang>
In release mode assertion checks are turned off.
Tcl
<lang tcl>package require control
set x 5 control::assert {$x == 42}</lang> Produces the output:
assertion failed: $x == 42
UNIX Shell
Assertions are not builtin commands, but we can add a function easily. <lang bash>assert() {
if test ! $1; then $2 && echo "$2" >&2 exit 1 fi
} x=42 assert "$x -eq 42" "that's not the answer" ((x--)) assert "$x -eq 42" "that's not the answer" echo "won't get here"</lang>
Vala
<lang vala>int a = 42; int b = 33; assert (a == 42); assert (b == 42); // will break the program with "assertion failed" error </lang>
VBScript
Definition
<lang vb>sub Assert( boolExpr, strOnFail ) if not boolExpr then Err.Raise vbObjectError + 99999, , strOnFail end if end sub </lang>
Invocation
<lang vb>dim i i = 43 Assert i=42, "There's got to be more to life than this!"</lang>
Output
<lang VBScript>>cscript "C:\foo\assert.vbs" C:\foo\assert.vbs(3, 3) (null): There's got to be more to life than this!</lang>
Visual Basic
VB's Assert
only fires when run from within the IDE. When compiled, all Debug
lines are ignored.
<lang vb>Debug.Assert i = 42</lang>
XPL0
XPL0 does not have an assert command. The equivalent is usually synthesized something like this.
<lang XPL0>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");</lang>
zkl
<lang zkl>n:=42; (n==42) or throw(Exception.AssertionError); n=41; (n==42) or throw(Exception.AssertionError("I wanted 42!"));</lang>
- Output:
Stack trace for VM#1 (): Cmd.__constructor addr:38 args(0) reg(1) startup.__constructor addr:2242 args(0) reg(1) ER startup.__constructor addr:2178 args(0) reg(22) Exception thrown: AssertionError(I wanted 42!)
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