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Steffen Müller > Sub-Assert-1.23 > Sub::Assert



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Module Version: 1.23   Source  


Sub::Assert - Subroutine pre- and postconditions, etc.


  use Sub::Assert;
  sub squareroot {
      my $x = shift;
      return $x**0.5;
         pre     => {
            # named assertion:
           'parameter larger than one' => '$PARAM[0] >= 1',
         post    => '$VOID or $RETURN <= $PARAM[0]', # unnamed assertion
         sub     => 'squareroot',
         context => 'novoid',
         action  => 'carp';
  print squareroot(2), "\n";  # prints 1.41421 and so on
  print squareroot(-1), "\n"; # warns
                              # "Precondition 1 for main::squareroot failed."
  squareroot(2);              # warns
                              # "main::squareroot called in void context."
  sub faultysqrt {
      my $x = shift;
      return $x**2;

         pre    => '$PARAM[0] >= 1',
         post   => '$RETURN <= $PARAM[0]',
         sub    => 'faultysqrt';
  print faultysqrt(2), "\n";  # dies with 
                              # "Postcondition 1 for main::squareroot failed."


The Sub::Assert module implements subroutine pre- and postconditions. Furthermore, it allows restricting the subroutine's calling context.

There's one big gotcha with this: It's slow. For every call to subroutines you use assert() with, you pay for the error checking with an extra subroutine call, some memory and some additional code that's executed.

Fortunately, there's a workaround for mature software which does not require you to edit a lot of your code. Instead of use()ing Sub::Assert, you simply use Sub::Assert::Nothing and leave the assertions intact. While you still suffer the calls to assert() once, you won't pay the run-time penalty usually associated with subroutine pre- and postconditions. Of course, you lose the benefits, too, but as stated previously, this is a workaround in case you want the verification at development time, but prefer speed in production without refactoring your code.


The assert subroutine takes a key/value list of named parameters.


The only required parameter is the 'sub' parameter that specifies which subroutine (in the current package) to replace with the assertion wrapper. The 'sub' parameter may either be a string in which case the current packages subroutine of that name is replaced, or it may be a subroutine reference. In the latter case, assert() returns the assertion wrapper as a subroutine reference.


This parameter specifies one or more preconditions that the data passed to the transformed subroutine must match. The preconditions may either be a string in case there's only one, unnamed precondition, an array (reference) of strings in case there's many unnamed preconditions, or a hash reference of name/condition pairs for named preconditions.

There are several special variables in the scope in which these preconditions are evaluated. Most importantly, @PARAM will hold the list of arguments as passed to the subroutine. Furthermore, there is the scalar $SUBROUTINEREF which holds the reference to the subroutine that does the actual work. I am mentioning this variable because I don't want you to muck with it.


This parameter specifies one or more postconditions that the data returned from the subroutine must match. Syntax is identical to that of the preconditions except that there are more special vars:

In scalar context, $RETURN holds the return value of the subroutine and $RETURN[0] does, too. $VOID is undefined.

In list context, @RETURN holds all return values of the subroutine and $RETURN holds the first. $VOID is undefined.

In void context, $RETURN is undefined and @RETURN is empty. $VOID, however, is true.

Note the behaviour in void context. May be a bug or a feature. I'd appreciate feedback and suggestions on how to solve is more elegantly.


Optionally, you may restrict the calling context of the subroutine. The context parameter may be any of the following and defaults to no restrictions ('any'):


This means that there is no restriction on the calling context of the subroutine. Please refer to the documentation of the 'post' parameter for a gotcha with void context.


This means that the assertion wrapper will throw an error of the calling context of the subroutine is not scalar context.


This means that the assertion wrapper will throw an error of the calling context of the subroutine is not list context.


This means that the assertion wrapper will throw an error of the calling context of the subroutine is not void context. Please refer to the documentation of the 'post' parameter for a gotcha with void context.


This restricts the calling context to any but void context.


By default, the assertion wrapper croaks when encountering an error. You may override this behaviour by supplying an action parameter. This parameter is to be the name of a function to handle the error. This function will then be passed the error string. Please note that the immediate predecessor of the subroutine on the call stack is the code evaluation. Thus, for a helpful error message, you'd want to use 'carp' and 'croak' instead of the analogeous 'warn' and 'die'. Your own error handling functions need to be aware of this, too. Please refer to the documentation of the Carp module and the caller() function. Examples:

  action => 'carp',
  action => 'my_function_that_handles_the_error',
  action => '$anon_sub->',  # works only in the lexical scope of $anon_sub!


Exports the 'assert' subroutine to the caller's namespace.


Steffen Mueller <>


Copyright (C) 2003-2009 Steffen Mueller

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.



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