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Re: Re (tilly) 4: Paradigm Shift - when to use goto

by TheDamian (Priest)
on Nov 18, 2001 at 03:21 UTC ( #126056=note: print w/ replies, xml ) Need Help??


in reply to Re (tilly) 4: Paradigm Shift - when to use goto
in thread Paradigm Shift - Don't use strict

I concur with everything tilly said above. To summarize, a goto is not considered harmful when you're:

  • replacing the current subroutine via a goto &othersub (rare),
  • autogenerating code that will never be seen or maintained directly (rarer),
  • implementing a flow-of-control that Perl doesn't support natively (rarest).

For me, the key point in tilly's reply is that virtually all other "legitimate" uses of a goto in other languages are made redundant in Perl by the availability of named loops.

Whereas in C/C++ you might reasonably have to write:

for (i=1; i<10; i++) { for (j=1; j<10; j++) { for (k=1; k<10; k++) { /* Process data[i][j][k] here */ if (data[i][j][k] < threshold) goto EOLOOPS; } } } EOLOOPS:
Perl has a much cleaner way to escape from a deep nesting:
LOOPS: for $i (1..10) { for $j (1..10) { for $k (1..10) { # Process $data[$i][$j][$k] here last LOOPS if $data[$i][$j][$k] < $threshold; } } }
Long ago, before I discovered the Way of the Camel, I used to rely on a couple of moderately evil preprocessor commands to give myself named loops in C/C++ too:
#define named(name) goto name; name##_break: if (0) name: #define break(name) goto name##_break; /* Which then allows you to write... */ named (LOOPS) for (i=1; i<10; i++) { for (j=1; j<10; j++) { for (j=1; j<10; j++) { /* Process data[i][j][k] here */ if (data[i][j][k] < threshold) break(LOOPS); } } }
Exploring how those #defines work -- and why they don't interfere with the semantics of normal (unnamed) break statements -- is left as an exercise for those of you who still follow the Dark Path. ;-)


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Re^2: Re (tilly) 4: Paradigm Shift - when to use goto
by JadeNB (Chaplain) on Nov 04, 2009 at 04:36 UTC

    To shed my vast light on the discussion, 8 years later: Is this

    replacing the current subroutine via a goto &othersub (rare),
    really true? I thought that goto was the only way to get proper tail-call recursion, which means that those of us with functional backgrounds who aren't so fond of iteration would be using it a lot.

    Incidentally, nothingmuch has recently offered Sub::Call::Tail for those who recoil from the syntactic ugliness of the goto-based set-up. Given your contributions in the past, I imagine you have your own way of twisting tail-calling gotos into shape when the occasion warrants. :-)

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