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

From: Bernd Brassel <bbr_at_informatik.uni-kiel.de>
Date: Fri, 28 Aug 2009 15:57:58 +0200

Just out of curiosity: What happens in your approach with the
following example:

data ETC2 a = ETC2 a? a?
data ETC2' a = ETC2 a? a

f x = ETC2 x x
f' x = ETC2' x x

plus (ETC2 x y) = x+y
plus' (ETC2' x y) = x+y

What are the values of plus(f (0?1)) and plus'(f' (0?1))


Am Aug 28, 2009 um 3:38 PM schrieb Sebastian Fischer:

>
> On Aug 27, 2009, at 11:55 AM, Wolfgang Lux wrote:
>
>> A while ago, Sebastian Fischer made me aware of a subtle bug in the
>> type-classes branch of MCC [...]
>> This bug is easily explained by MCC's internal translation of
>> type-classes, which uses the common dictionary approach. [...]
>> Curry's call-time choice semantics means that the
>> argument of the [dictionary] constructor [...] -- unintentionally
>> -- is shared [...]
>> I believe it is a serious shortcoming of Curry [...]
>
> Wolfgang goes on to propose two possible Curry extensions:
>
>> For instance, one could extend the expression
>> syntax with
>> BasicExpr ::= ... | ({ Expr }) | ...
>> where an expression ({ e }) is evaluated with run-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.
>
> I share your reluctance because I like the property that variables
> denote values. Hence, I would also prefer your more modest proposal
>
>> to restrict run-time choice to specially marked arguments of data
>> constructors
>
> where the exceptions to this rule are less intrusive.
>
> In general, I would like to see some possibility for selective eval-
> time choice in Curry. There are examples where ETC is preferable
> over CTC like the regular expression matcher Sergio mentions in his
> paper on "Evaluation Strategies for Functional Logic
> Programming" [1] I consider the technique to introduce dummy
> arguments in order to avoid sharing a hack which is only necessary
> because Curry lacks an appropriate feature.
>
> Incidentally, Wolfgangs proposal to add annotations to arguments of
> data constructors (which prevent the annotated arguments from being
> shared if the constructed value is shared) seems to be easily
> expressible in the framework for monadic explicit sharing [2].
>
> I have encoded Wolfgangs dictionary example in this framework [3]
> and tried this other example to challenge my (and your) intuition
> about the proposed extension. What should be the results of test1
> and test2?
>
> data ETC a = ETC ?a
>
> dup :: a -> (a,a)
> dup x = (x,x)
>
> test1 = dup (ETC (False?True))
>
> shareInside :: Bool -> (ETC (Bool,Bool), ETC (Bool,Bool))
> shareInside x = dup (ETC (x,x))
>
> test2 = shareInside (False?True)
>
> The explicit-sharing library computes these results:
>
> test1 ~> (ETC F,ETC F)
> ? (ETC F,ETC T)
> ? (ETC T,ETC F)
> ? (ETC T,ETC T)
>
> test2 ~> (ETC (F,F),ETC (F,F))
> ? (ETC (T,T),ETC (T,T))
>
> So, arguments of constructors annotated using ? can still
> incorporate sharing, especially, the sharing of 'x' in the
> definition of 'shareInside' is not destroyed by wrapping 'x' inside
> ETC.
>
> No I am the third who refers to his own work when commenting on
> Wolfgangs proposal. What are the differences between annotating
>
> - specific arguments of constructors in data-type declarations
> (Wolfgang)
> - all arguments in specific function declarations (Juan)
> - specific (pattern?) variables (Bernd)
>
> With respect to implementing such annotations I observe that I could
> easily implement Wolfgangs approach. Of course, that is no
> convincing argument to prefer it over the others. However, I wonder
> what is special about the data centric approach that lets it stand
> out in this respect.
>
> Cheers,
> Sebastian
>
> [1]: http://web.cecs.pdx.edu/~antoy/homepage/publications/jsc/
> paper.pdf
> [2]: http://sebfisch.github.com/explicit-sharing/
> [3]: http://gist.github.com/176983
>
> --
> Underestimating the novelty of the future is a time-honored tradition.
> (D.G.)
>
>
>
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Received on Fri Aug 28 2009 - 18:15:11 CEST

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