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Re^3: Use of wantarray Considered Harmful

by dragonchild (Archbishop)
on Dec 14, 2008 at 02:45 UTC ( #730237=note: print w/ replies, xml ) Need Help??


in reply to Re^2: Use of wantarray Considered Harmful
in thread Use of wantarray Considered Harmful

Whether or not caller and each behave sanely is a separate question. Just because it's in core doesn't mean it's a good idea. Frankly, I think that a subroutine should return one and only one type of entity. It shouldn't return a collection (array, in this case) in one situation and an element (first element, in this case) in another. That leads to serious hard-to-debug issues. This isn't theoretical - it's personal experience speaking.


My criteria for good software:
  1. Does it work?
  2. Can someone else come in, make a change, and be reasonably certain no bugs were introduced?


Comment on Re^3: Use of wantarray Considered Harmful
Re^4: Use of wantarray Considered Harmful
by BrowserUk (Pope) on Dec 14, 2008 at 03:35 UTC
    Frankly, I think that a subroutine should return one and only one type of entity.... That leads to serious hard-to-debug issues. This isn't theoretical - it's personal experience speaking.

    In isolation, those sound like wise words, but essentially what you are saying now, is that you think all uses of Perl's context sensitivity are bad and should be deprecated, because you've had trouble with it occasionally. That argument is as fallacious as these, please read the counter arguments.

    Context is a unique, integral, important and powerful feature of Perl--and unless things have changed markedly in the last short while--it will be at the core of Perl 6 also.

    Speaking Perl without using context, is like speaking English (or French or Dutch or German), without contractions. Possible, but stilted, unnatural, laboured. It's speaking Perl like a foriegner.

    Children are using contractions correctly aged 3 or 4 years. Can it really be so hard to get right?

    Please, please. Is there no one out there with political weight and a descent level of (natural) language skills that'll help me defend my beloved Perl from the one-idiom-fits-all, OO uniformatists?


    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.
      Your right Brow sir Uk

      My level of language skills is in perpetual descent. :) But I'm down with the context. As long as things are well documented I don't mind context sensitivity a bit and it can make hacking easier when applying the same underlying code in different places. E.g.: I find I would rather DBIC return an iterator for use in TT2 and a collection of objects in most CLI stuff so I like the fact that DBIx::Class::ResultSet->search does both. And yes I know about search_rs but it wasn't around in the early days and I like the idiom without just fine.

      Update: I see that my use case in agreement with dragonchild above. Surprise, surprise! ...there is really only one good use of wantarray - to return an array or an iterator.

        Surprise, surprise! ...there is really only one good use of wantarray - to return an array or an iterator.

        Hm. I agree that is a very good use case. And by itself, it justifies having context sensitivity. But once you have context sensitivity, it is certainly not the only good use case.

        Perhaps my most often used built-in examples are localtime; m//g; readline (often as not as the diamond operator). These return a scalar or list.

        Another contention above is: "a subroutine should return one and only one type of entity.". Not only does the "iterator or array" assertion flatly contradict this--an iterator is either a scalar or a coderef; but certainly not an array--but from another view point, a subroutine can either return a list, or a scalar (or nothing). And a scalar is just a single element list. And nothing is the empty list.

        That makes the return always of one type: a list of 0, 1, or many elements. Sounds pretty CS to me.

        Context only becomes a problem when you stop thinking in Perl and start thinking in terms of some other language, or overarching paradymn. Just as Great Circle math becomes a problem if you are a flat earther.


        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.
      I'm not saying that context sensitivity is bad. I'm saying that switching contexts should be lossless. Lossy context switches lead to hard-to-debug issues.

      My criteria for good software:
      1. Does it work?
      2. Can someone else come in, make a change, and be reasonably certain no bugs were introduced?
Re^4: Use of wantarray Considered Harmful
by graff (Chancellor) on Dec 14, 2008 at 04:00 UTC
    Frankly, I think that a subroutine should return one and only one type of entity. It shouldn't return a collection (array, in this case) in one situation and an element (first element, in this case) in another.

    But by virtue of how context works in Perl, this issue is actually beyond the control of the subroutine's designer/coder. A subroutine that returns a list will, by virtue of how context works, return a scalar in scalar context (as per ikegami's table above). If some problem is "hard-to-debug" on this basis, it is because the person doing the debugging doesn't understand context -- a situation that is all too frequent among people using Perl, obviously.

    Given this environment within which the subroutine designer/coder must work, the key point, as explained by BrowserUK and some others in this thread, is to document, clearly and accurately, what the subroutine does in different contexts, even if this amounts to repeating (briefly) the standard description of Perl's normal behavior (because the next "maintainer" might be some novice who will be seeing this information for the first time). Anyone who tries to use (or debug the use of) someone else's subroutine without reading the docs for it will get no sympathy or special indulgence from me.

    As long as context-dependent behavior is properly documented, I don't see a problem with it, and in fact I appreciate the flexibility that it makes available.

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