Re: from static typing to runtime constraints

From: Michael Hanus <>
Date: Fri, 08 Dec 2006 11:24:08 +0100

Sebastian Hanowski wrote:
> ...
> But the recent extension of Curry with /function patterns/ introduced
> variables which are bound to unevaluated expressions when defined
> functions are applied to them in pattern expressions on the left-hand
> side. One can thus exploit run-time-choice non-determinism for the
> encoding of type constructors.
> Tree (id a) = Leaf ? Node a (Tree a) (Tree a)
> > Node Red (Node Green Leaf Leaf) Leaf =:= Tree Color

This is a nice proposal but, first, I was surprised that
function pattern should provide the opportunity to encode
run-time choice. Since function patterns are interpreted
as an abbrivation of all possible patterns obtained by
narrowing the function pattern, the above definition should
be equivalent to

Tree a = Leaf ? Node a (Tree a) (Tree a)

Thus, run-time choice cannot be encoded in such a way.

I guess you have tried PAKCS and, indeed, PAKCS
has a different behavior. However, this is a bug in the
implementation and not a feature :-(
The current implementation does not implement sharing for
function pattern variables which is now corrected in the
next release to come.

Thus, thanks for your example that showed a bug in PAKCS!
On the negative side, you have to look for another solution
for your problem...

Best regards,


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Received on Fr Dez 08 2006 - 13:06:13 CET

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