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Roles and functional programming

by zby (Vicar)
on Nov 26, 2008 at 15:06 UTC ( #726126=perlquestion: print w/ replies, xml ) Need Help??
zby has asked for the wisdom of the Perl Monks concerning the following question:

After reading Traits: Composable Units of Behaviour I have the impression that, due to their statelessness, Traits (and Roles) are very much like Functors in some functional languages (SML). So like map and anonymous subs bring some first-order functional programming into Perl, Roles bring higher order functional programming constructs allowing us to refactor the stateless logic into separate, more easily composable units.

Update: Thank you for all the answers! To explain what was the purpose of my post - I had this vague intuition (and since my studies 15 years ago I have not touched SML so it really just a remote glitter through the fog) - and I wanted to check how it matches other people understanding of Traits.

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Re: Roles and functional programming
by JadeNB (Chaplain) on Nov 26, 2008 at 15:41 UTC
    This is an interesting suggestion (one the aptness of which my lack of familiarity with functional programming prevents me from judging), but there doesn't seem to be a question or direction for discussion. I'd be very interested if you could give examples of idioms from functional programming and their translations into the language of roles.
Re: Roles and functional programming
by Bloodnok (Vicar) on Nov 26, 2008 at 15:50 UTC
    As a self-acclaimed unexpert in the field, is it just me, or does the paper, amogst other things, present a more formal recognition/definition of the notion of abstract/pure virtual classes ?

    .oO(...sits back, once more, to await flames:-)

    A user level that continues to overstate my experience :-))

      Except that roles/traits compose and abstract/virtual classes still just inherit. The composition is the real win with traits/roles, without it they would be nothing more then ... uhm... abstract/virtual classes ;)

      -stvn
        I think I must be missing the point stvn, I thought a class (in whatever language) was the composition of methods and/or data to model a behaviour.

        A user level that continues to overstate my experience :-))
Re: Roles and functional programming
by BrowserUk (Pope) on Nov 26, 2008 at 16:22 UTC

    Like others, I'm a little confused by the purpose of your post, but one word of warning that may upset your assertion: Perl 6 roles aren't stateless

    From Synopsis 12:

    Roles may have attributes:
    role Pet { has $.collar = Collar.new(Tag.new); method id () { return $.collar.tag } method lose_collar () { undefine $.collar } }

    Of course, whatever state they have becomes a part of the instance into which they are composed (Can roles have Class attributes?), which in a way is a little similar to the way Functors acquire transient access to the values they are applied to, but still that state persists.

    Given my previously declared (and sometimes comprehensively proven!) lack of Haskell-fu, I'm loath to draw conclusions, but I think that somewhat queers the pitch as far as comparisons between Functors and Roles are concerned.


    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.
      Roles don't -have- attributes. Roles can -provide- attributes to the classes they are composed into. If you have a Cat and Dog, each of which does Pet, then if you have a $cat instance and a $dog instance, the $.collar attribute of each is separate. They aren't shared in any sense. Therefore, Perl6 roles -are- stateless in that the role itself doesn't have any state. (Which makes sense given that you cannot instantiate a role.)

      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?
        Roles don't -have- attributes.

        Hm. The quote was directly from Synopsis 12, so as far as the wording is concerned, you need to argue with the authors.

        As far as the semantics are concerned, I did go on to say: "Of course, whatever state they have becomes a part of the instance into which they are composed", so I think we're in tune there.

        But, as I also said, and regardless of whether the state is has'd by the Role, or provided by it, that state exists and is presumably usually affected by the Role and persists beyond the invocation of whatever methods the role also provides, so there is no sense in which it can reasonably be described as stateless.

        If my limited understanding of ML's Functors means that they are like (my also limited understanding of) Haskell's Typeclasses, then the term composed in the sense you are using it: "composed into"; and that same term in the sense the OP used it: "more easily composable units."; are entirely different things.

        With the former composing both operations and state, and the latter performing operations on values only (hence stateless). Quite different beasts. The best discussion I have located upon the difference is this.


        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.
Re: Roles and functional programming
by stvn (Monsignor) on Nov 26, 2008 at 21:19 UTC
    Traits (and Roles) are very much like Functors in functional programming.

    If you mean Functors in the SML/OCaml sense, then yes. If you mean Functors like in Haskell, then not really, they are closer to Typeclasses in Haskell.

    -stvn
      Yes - I had SML in mind. I've updated the root node. Thanks.

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