Re: Type-classes and call-time choice vs. run-time choice

From: Sergio Antoy <antoy_at_redstar.cs.pdx.edu>
Date: Thu, 27 Aug 2009 10:16:27 -0700 (PDT)

Hi Wolfgang,

There is an interaction between the order of evaluation and the
run-time choice. Incidentally, "run-time" is a bad choice (sic!)
of name, since the call-time choice occurs at run-time as well;
better names are eager choice and lazy choice.

In your example "dbl", if you want the lazy choice, but evaluate
(0 ? 1) a bit early, you will not get the value 1, hence you will
not get the lazy choice.

Sergio

P.S. I'll be traveling starting tomorrow and I will be unable to
regularly access my e-mail for a week or so.

Wolfgang Lux writes:
> A while ago, Sebastian Fischer made me aware of a subtle bug in the
> type-classes branch of MCC (for those who are not aware of it, the
> source is available in the darcs repository at [1]). Consider a type
> class
>
> > class Arb a where
> > arb :: a
>
> This definition introduces a new overloaded function arb with
> no arguments. An instance of this class for type Bool could be
> defined by
>
> > instance Arb Bool where
> > arb = False ? True
>
> Evaluating the goal
> arb::Bool
> in the interpreter yields the expected results, namely False
> and True. Now let us define a few small functions using arb.
>
> > arbP2 = (arb,arb)
> > arbL2 = [arb,arb]
>
> If we evaluate
> arbP2 :: (Bool,Bool)
> we get the four expected results (False,False), (False,True),
> (True,False), and (True,True). But if we evaluate the goal
> arbL2 :: [Bool]
> we observe a surprising result or rather absence of results.
> The interpreter computes only [False,False] and [True,True],
> but neither [False,True] and [True,False] as one would expect
> and would be computed if arb were a normal top-level function.
>
>
> This bug is easily explained by MCC's internal translation of
> type-classes, which uses the common dictionary approach. The
> type-class Arb is replaced by a data type declaration together
> with a top-level function for each type class method, i.e,
>
> > data Arb' a = Arb' ({-arb::-} a)
> > arb' (Arb' f) = f
>
> The instance declaration is transformed into a top-level
> function for each instance method and a function that returns
> the instance dictionary for type Bool
>
> > inst'Arb'Bool = Arb' arb'Bool
> > arb'Bool = False ? True
>
> The overloaded functions arbP2 and arbL2 receive an additional
> argument for each type predicate from their types' contexts, which
> are arbP2 :: (Arb a, Arb b) => (a,b) and arbL2 :: Arb a => [a].
>
> > arbP2' da db = (arb' da, arb' db)
> > arbL2' da = [arb' da, arb' da]
>
> Observe the essential difference between both functions that
> arbL2' receives only a single dictionary, which is used for
> both method calls. Now have a careful look at the definition
> of inst'Arb'Bool again. Since arb'Bool is a function with no
> arguments, Curry's call-time choice semantics means that the
> argument of the Arb constructor is evaluated just once per
> inst'Arb'Bool call and thus -- unintentionally -- is shared
> for both method calls in arbL2'
>
>
> One can work around this bug by using a different transformation
> which forces each argument of a dictionary constructor to become
> a function. While there may indeed be good reasons for favoring
> such an approach, I believe it is a serious shortcoming of Curry
> that the naive dictionary approach cannot be implemented as a
> source to source transformation in Curry (at least for the subset
> that does not use polymorphic methods). What is lacking here is
> the ability to give the user some freedom to specify whether an
> expression should be evaluated with call-time choice or with
> run-time choice. For instance, one could extend the expression
> syntax with
> BasicExpr ::= ... | ({ Expr }) | ...
> where an expression ({ e }) is evaluated with run-time choice.
> E.g., given
> dbl x = x + x
> main = dbl ({ 0 ? 1 }) =:= 1
> the goal main succeeds, which it doesn't with call-time choice.
> However, I'm a bit reluctant to such a change because it destroys
> the property that a variable in a function always refers to a
> single value. Giving up this property apparently has a big impact
> on the correctness of some source code transformations (it almost
> certainly will break some code in MCC).
>
> A more modest proposal, which would be sufficient to cover the
> dictionary transformation, is to restrict run-time choice to
> specially marked arguments of data constructors. Just as Haskell
> allows marking some data constructor arguments strict by
> prefixing the argument type with an exclamation mark, e.g.,
> data Complex = Complex !Float !Float
> we could mark data constructor arguments as being evaluated with
> run-time choice by prefixing the argument types with a question
> mark, e.g.,
> data Arb a = Arb ?({-arb ::-} a)
> This gives a compiler a clear hint on which function arguments are
> evaluated with run-time choice and thus must be handled specially.
>
>
> Thoughts?
>
> Wolfgang
>
>
> [1] http://danae.uni-muenster.de/~lux/curry/darcs/type-classes
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Received on Fri Aug 28 2009 - 09:14:29 CEST

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