Guards in pattern bindings

[Simon L Peyton Jones <simonpj@cs.glasgow.ac.uk>]

To: haskell@cs.glasgow.ac.uk
Cc: simonpj@cs.glasgow.ac.uk
Subject: Guards in pattern bindings
Date: Wed, 06 Mar 91 12:38:49 +0000
From: Simon L Peyton Jones <simonpj@cs.glasgow.ac.uk>
Sender: haskell-request@cs.glasgow.ac.uk


I sent a message proposing a clarification to the question of
which variables are in scope in a guard for a pattern binding.
Following a number of responses I now think I was wrong-headed.

The Report already states (Section 4.4.2, page 33) that

	p | g1 = e1
	  | g2 = e2

(p is a pattern, gi are guards, ei are expressions) is equivalent to

	p = if g1 then e1 else
	    if g2 then e2 else
	    error ""

So my example

	y | y>2  = 3
	  | y<=2 = 1

is semantically equivalent to

	y = if y>2 then 3 else 1

which is of course bottom.

So there is a perfectly well-behaved meaning for guarded pattern bindings,
and I withdraw my proposal.  Sorry to consume your bandwidth unnecessarily.

Simon