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From: Sergio Antoy <antoy_at_cs.pdx.edu>

Date: Tue, 02 Nov 2004 08:58:14 -0800

Michael writes:

*> I'd like to propose to relax two restrictions in the definition
*

*> of Curry which seem to me unnecessarily strong.
*

*>
*

*> The first one is related to the introduction of free variables.
*

*> The current language definition (Curry Report, C.3) requires
*

*> that in an expression of the form
*

*>
*

*> let x free in e
*

*>
*

*> e must be of type "Success" (1).
*

*>
*

*> On the one hand, this seems justified by the fact that existential
*

*> quantification is reasonable for constraints only.
*

I agree with your proposal, because I somewhat disagree with the

justification. The justification would hold for a construct of

the kind

exists x such that e

In this case, e should be a constraint and, noticeably, the

expression should be boolean.

*> On the other hand, this demands for some nasty transformations
*

*> if one needs local variables in a non-constraint scope. For
*

*> instance, the (currently illegal) definition
*

*>
*

*> f x = \y -> let z free in g x y z -- (2)
*

*>
*

*> (where (g x y z) is not of type Success) can be transformed
*

*> in the valid Curry program
*

*>
*

*> f x = g' x
*

*> g' x y = g x y z where z free
*

*>
*

*> The transformation can be avoided by introducing a
*

*> "generate-free-variable" function (actually, this is the
*

*> solution which I often use):
*

*>
*

*> f x = \y -> g x y any
*

*> any = z where z free
*

*>
*

*> Both solutions are less readable than (2). Furthermore,
*

*> the alternatives demonstrate that restriction (1) is not
*

*> really necessary. In particular, if one defines a type
*

*> isomorphic to Success, the restriction becomes questionable.
*

*>
*

*> Thus, I propose to drop the restriction (2).
*

*> This would also make the application of let and where
*

*> "more equivalent".
*

I agree.

*> The second restriction concerns the sequential conjunction
*

*> of constraints, which is currently defined as
*

*>
*

*> (&>) :: Success -> Success -> Success
*

*> c &> x | c = x
*

*>
*

*> in the prelude.
*

*>
*

*> We have a number of applications where we want to put constraints
*

*> during a functional evaluation. Until now, we have defined for this
*

*> purpose a "guard" function that takes a constraint and an expression as
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*> input and is the identity on the second argument provided that the
*

*> constraint is satisfied. Thus, it is defined as
*

*>
*

*> guard :: Success -> a -> a
*

*> guard c x | c = x
*

*>
*

*> so that you can use it as in (guard (x=:=1) (2+x)).
*

*> Now, you see that the definition of the guard function is
*

*> identical to (&>) apart from the different types.
*

*> Since such a guard function is quite useful, I propose
*

*> to generalize the type of (&>) to
*

*>
*

*> (&>) :: Success -> a -> a
*

*>
*

*> so that it is a general function to establish new constraints
*

*> during arbitrary computations.
*

*>
*

*>
*

*> Since both proposals have no influence on existing programs but
*

*> allow more valid Curry programs, I see no problem in them.
*

*> However, maybe somebody sees some problem?
*

I agree with this too. Wouldn't the definition of &> be

simpler as follows

(&>) :: Success -> a -> a

Success &> x = x

Sergio

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Received on Mi Nov 03 2004 - 08:47:40 CET

Date: Tue, 02 Nov 2004 08:58:14 -0800

Michael writes:

I agree with your proposal, because I somewhat disagree with the

justification. The justification would hold for a construct of

the kind

exists x such that e

In this case, e should be a constraint and, noticeably, the

expression should be boolean.

I agree.

I agree with this too. Wouldn't the definition of &> be

simpler as follows

(&>) :: Success -> a -> a

Success &> x = x

Sergio

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curry mailing list

curry_at_lists.RWTH-Aachen.DE

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Received on Mi Nov 03 2004 - 08:47:40 CET

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