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Acme::Don't - The opposite of do


use Acme::Don't;

don't { print "This won't be printed\n" };    # NO-OP


The Acme::Don't module provides a don't command, which is the opposite of Perl's built-in do.

It is used exactly like the do BLOCK function except that, instead of executing the block it controls, it...well...doesn't.

Regardless of the contents of the block, don't returns Nil.

You can even write:

don't {
    # code here
} while condition();

And, yes, in strict analogy to the semantics of Perl's magical do...while, the don't...while block is unconditionally not done once before the test. ;-)

Note that the code in the don't block must be syntactically valid Perl. This is an important feature: you get the accelerated performance of not actually executing the code, without sacrificing the security of compile-time syntax checking.


No opposite

Doesn't (yet) implement the opposite of do STRING. The current workaround is to use:

don't {"filename"};

Double don'ts

The construct:

don't { don't { ... } }

isn't (yet) equivalent to:

do { ... }

because the outer don't prevents the inner don't from being executed, before the inner don't gets the chance to discover that it actually should execute.

This is an issue of semantics. don't... doesn't mean do the opposite of...; it means do nothing with....

In other words, doin nothing about doing nothing does...nothing.

Unless not

You can't (yet) use a:

don't { ... } unless condition();

as a substitute for:

do { ... } if condition();

Again, it's an issue of semantics. don't...unless... doesn't mean do the opposite of...if...; it means do nothing with...if not....


Since Perl 6 doesn't have undef, the closest thing to it (Nil) is being returned by don't instead.


Elizabeth Mattijsen liz@raku.rocks

Source can be located at: https://github.com/lizmat/Acme-don-t . Comments and Pull Requests are welcome.


Copyright 2018, 2019, 2021 Elizabeth Mattijsen

Original author: Damian Conway. Re-imagined from Perl 5 as part of the CPAN Butterfly Plan.

This library is free software; you can redistribute it and/or modify it under the Artistic License 2.0.