Re: Puzzle

From: Michael Hanus <hanus_at_informatik.rwth-aachen.de>
Date: Tue, 1 Dec 1998 15:35:37 +0100

Dear Wolfgang,

thanks for your interesting example. It shows that one has
to be careful if there is too much syntactic sugar.
My initial motivation for the remark "a defining equation f = g
between functions will be interpreted in Curry as syntactic sugar
for the corresponding defining equation f x = g x on base types"
was to express that the semantics of Curry is not based on
higher-order rewriting which might complicate things.
Thus, I assumed that this kind of desugaring is done
after lambda lifting. It might be interesting to note
that Haskell has also surprisingly restrictions.
For instance, the program

 f x = 1
 f x = 2

is a valid Haskell program, whereas the version

 f = \x->1
 f = \x->2

seems to be invalid (although Hugs accepts currently both).

Your remark reminds me that I wanted to suggest a further
restriction for local pattern definitions in Curry which solves
these problems:

  All variables occurring in lhs patterns in some let/where
  should occur at most once in a lhs of a local pattern declaration.

This restriction would forbid programs like

f x = coin where coin = 1
                  coin = 2

and also your functions f1,f2,f3.

Do you agree or is this too restrictive?

Michael
Received on Di Dez 01 1998 - 15:39:00 CET