varying number of arguments restriction
Wed, 31 Oct 2001 13:51:45 +1300
Ashley Yakeley wrote:
> At 2001-10-30 11:01, Hal Daume wrote:
> >obviously i can rewrite:
> >foo  = ""
> >foo s = (snd . head) s
> >but this is uglier.
> I'm not sure. I actually prefer it written out so that the number of
> arguments in the cases matches (as GHC enforces).
It's defined in the Report, not a GHC idiosyncracy. As to why, I don't
really remember, but I suspect it had to do with a desire by some
members of the Haskell Committee to require that the patterns in all
clauses of a function binding were disjoint, so that reasoning about
programs could deal with each clause independently. This was not
adopted, and the alternative top-to-bottom, left-to-right, semantics
were, but there was still a feeling that good style demanded
In that style, the second clause could be written
foo ((x,y):xs) = y
I don't know whether this is still true, but it used to be argued that
this was likely to be more efficient because compilers could produce
really good pattern-matching code.