Re: Definable operators (was: Problems with Hardware, Languages, and Compilers)

Nick Maclaren <nmm1@cus.cam.ac.uk> wrote:
>> [ re: user-defined infix operators ]
>
>Algol 68 did, and was by no means the first. But Haskell has made a
>very old mistake in being too general - consider the problems about
>parsing a mixture of left- and right-associative operators of the same
>priority.


Haskell treats this condition (correctly, IMO) as a parse error. (I
think ML does the same thing).


> Even worse, consider varying commutativity and distributivity.


Distributivity and commutativity are semantic properties, not
syntactic ones; I don't see why this should affect parsing. Haskell
does allow you to specify an operator as non-associative -- so that (X
`op` Y `op` Z) will be flagged as an error without further
parenthesization -- which is sometimes useful for operators that
aren't commutative.


> The 1960s experience was that allowing user- defined operators
> (including redefinition) was fine, as was allowing user-defined
> precedences for textually new ones, but beyond that lies madness.


I think the 1990s experience has been somewhat different :-) Judicious
use of user-defined infix operators has been very beneficial to
clarity of exposition in most of the Haskell code I've written and
read.


--Joe English


    jenglish@crl.com
--

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