Left hand sides and priorities.

[Kent Karlsson <kent@se.chalmers.cs>]

Original-Via: uk.ac.ukc; Wed, 6 Mar 91 17:38:44 GMT
From: Kent Karlsson <kent@se.chalmers.cs>
Date: Wed, 6 Mar 91 18:29:11 +0100
To: haskell@cs.glasgow.ac.uk
Subject: Left hand sides and priorities.
Original-Sender: kent%se.chalmers.cs@sunic.sunet.se
Sender: haskell-request@cs.glasgow.ac.uk


			LEFT HAND SIDES and PRIORITIES
			==============================

On p. 32 the Haskell report says:

	lhs -> ...
	     |   apat1  varop  apat2
	     | ( apat1  varop  apat2 ) apat3 ... apatk		(k >= 3)

Why must the argument patterns be "apat"s?  In expressions and patterns
the priority (and associativity) of operators is used, why not in "lhs"s?
I.e. why not:

	lhs -> ...
	     |   pat1  varop  pat2
	     | ( pat1  varop  pat2 ) apat3 ... apatk		(k >= 3)


   Another priority issue, why is  -2^2  equal to  4  while  0-2^2  is  -4 ?
I find this rather surprising, as would any neophyte programmer... :-)
I think unary minus should have the *same* priority as binary minus.  From
the point of view of the Haskell report:  change      exp  ->  - aexp 
to     exp  ->  - exp    and state that unary and binary minus have the
same priority.	(I.e. the parenthesis in  -(e op e)  can be dropped
if the priority of  op  is > 6; and in  (-e) op e  if the priority of
right or non-assoc  op  is < 6, or for a left assoc op its priority
is <= 6; and in  e op (-e)  if op has priority < 6.)


				/kent k