mikeman has asked for the wisdom of the Perl Monks concerning the following question:

When unit testing, how do I test that a function returns nothing when called in void context?

For example, I can easily test if the following function returns a list or a scalar containing 'value'. But as soon as I test its return value, I am no longer calling the function in void context.

sub context {
    return unless defined wantarray;
    return wantarray ? ('value') : 'value';
}

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Thanks, and sorry if this is a dumb question!

Replies are listed 'Best First'.
Re: Unit testing, context and wantarray
by BrowserUk (Patriarch) on Aug 13, 2011 at 11:05 UTC
    how do I test that a function returns nothing when called in void context?

    Two thoughts:

    1. Why would it matter?

      If a function returned a value in a void context, it is just discarded.

      Not returning anything in a void context is not the point, it is not calculating the return values if they are never going to be used that makes a difference.

    2. Testing something that is obvious by inspection is a waste of time.

      If the first line of the sub is return unless defined wantarray;, the sub will not return anything when called in a void context. There is no need to test that.

    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
    "Science is about questioning the status quo. Questioning authority".
    In the absence of evidence, opinion is indistinguishable from prejudice.
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      Thanks for your reply.

      I think you answer has pointed me in the right direction: I am trying to test for the wrong thing. I should be testing (perhaps via a mock object or similar), that my function does not do anything expensive (e.g., call a database function) to return a result when called in void context. I was erroneously focusing on the return result, rather than the operations of my function. Testing the return value (or lack thereof) would not prove if my function was performing a pointless operation.

      Thanks!

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        As a general thing though, one may often call a method in a void context and expect something expensive to happen (connecting to a remote database for example), especially if you use exception handling to manage errors rather than passing error states back.

        True laziness is hard work
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        I should be testing (perhaps via a mock object or similar), that my function does not do anything expensive (e.g., call a database function) to return a result when called in void context.
        Such as playing nethack? :)

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Re: Unit testing, context and wantarray
by ikegami (Patriarch) on Aug 13, 2011 at 16:48 UTC

    Parens don't create lists*. 

    return wantarray ? ('value') : 'value';

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    is the same as 
    return wantarray ? 'value' : 'value';

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    which is the same as 
    return 'value';

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    * — Except on the LHS of an assignment (indirectly) and inside of a list literal.

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Re: Unit testing, context and wantarray
by happy.barney (Friar) on Aug 13, 2011 at 15:42 UTC
    If your function call other function that should not be called in void context, try to locally redefine it. 
    sub Foo::bar { return 42; }
say '1: ', Foo::bar;

{
    no warnings 'redefine';
    local *Foo::bar = sub { die };

    eval { say '2: ', Foo::bar; };
}

say '3: ', Foo::bar;

__END__

1: 42
3: 42

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