Simon Peyton-Jones wrote:
| The trouble with those parts is that NOWHERE do they discuss how to
| translate a let or where containing more than one binding. If they're
| not to be translated via tupling, then how are they to be translated?
Sorry I wasn't clear. Given
let { p1 = e1; ... ; pn = en } in e0
(P1) For each pattern pi that is of form !qi = ei, transform it to
[EMAIL PROTECTED] = ei
and replace e0 by (xi `seq` e0)
(P2) Now no pattern has a ! at the top. Now apply the existing rules
in
3.12 of the Haskell report.
So step (P1) above adds some seqs, and after that it's all just standard
Haskell 98.
My summary so far:
Good summary.
1) Bang patterns by themselves are quite decent, well-behaved patterns.
2) Rule (P1) is simple to describe. But the ! in a pattern binding is
treated as part of the *binding* rather than part of the *pattern* which
is wart-y.
And as a consequence, it is no longer possible to transform a pair of
bindings into a binding of a pair. In Haskell 98,
p1 = e1
p2 = e2
is always equivalent to
(~p1, ~p2) = (e1,e2)
and you can make this change *locally*, without consideration of the
body of the let in which the bindings appear. With ! bindings (let's use
a different name from ! patterns, because they are not the same thing),
there's no way to rewrite
!p1 = e1
!p2 = e2
as a single tuple binding, because there's nowhere you can put the !
that will have the same effect. Thus we lose a law from the algebra of
bindings, which is part of the reason why this is warty.
John
_______________________________________________
Haskell-prime mailing list
Haskell-prime@haskell.org
http://haskell.org/mailman/listinfo/haskell-prime
