# Re: Proposal: Lazy matching for local patterns

From: Wolfgang Lux <lux_at_wi.uni-muenster.de>
Date: Wed, 24 Oct 2001 13:21:46 +0200

Hello Mario,

> I agree with Wolgang that a less strict lifting algorithm would be better,
> in order to achieve succesful computations in those cases where Haskell does
> so. However, I have a comment about the alternative translation which
> Wolgang proposes to eliminate local pattern declarations. It seems to me
> that the translation does not work in the case of recursive local definitions
> .
> Consider for example the definition
>
> f x = let {xs = x:xs} in head (tail xs)
>
> In Haskell, (f 0) evaluates to 0, while the translation proposed by Wolgang
> cannot be applied because free (x:xs) is not a subset of free (f x).
>
> Maybe the translation is not meant to be used for recursive local definitions
> .

you are right. The proposal does not work for recursive definitions.
However, this is the same as in the current report (for the reason
Micheal already noted). In principal one could try to extend the
proposal to transform local recursive bindings into mutually recursive
functions, e.g. your example could be lifted into the following set of
equations:

f x = f'' x (f_1 x)
f'' xs = head (tail xs)
f_1 x = x : f_1 x

But it is not clear to me whether the result preserves the intended
semantics. Consider the example

coin = 0
coin = 1
coins = coin : coins

The value of coins is an infinite list where each element is
instantiated independently to 0 or 1. However, what about the
declaration

f = let coins = coin : coins in coins

Should the result still be an infinite list where each element is
instantiated independently or should be an infinite list where
either all elements are 0 or all elements are 1?

The former case can be handled be modified translation rules but I
do not see how to translate the definition of f into a set mutually
recursive (top-level) equations with the same semantics for the second
case.

Regards
Wolfgang

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Wolfgang Lux				  Phone: +49-251-83-38263
Institut fuer Wirtschaftinformatik	    FAX: +49-251-83-38259
Universitaet Muenster		      Email: wlux_at_uni-muenster.de
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Received on Mi Okt 24 2001 - 13:36:22 CEST

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