Suggestion for bang patterns documentation
Christian.Maeder at dfki.de
Fri Feb 27 05:53:22 EST 2009
Brian Bloniarz wrote:
> I got confused by the GHC documentation recently, I was wondering how
> it could be improved. From:
Seeing the rule
pat ::= !pat
you'll probably want to avoid patterns like: "!!pat", "! ! pat", or "~ !
Even the current http://www.haskell.org/onlinelibrary/exps.html#sect3.17.1
apat -> ~ apat
allows "~ ~x". (Note the space!) So maybe a separate non-terminal "bpat"
should be used with:
bpat -> [~|!] apat
(and bpat used within pat). You may also want to exclude "v@ ~(...)" in
favor of "~v@(...)".
>> A bang only really has an effect if it precedes a variable or wild-card pattern:
>> f3 !(x,y) = [x,y]
>> f4 (x,y) = [x,y]
>> Here, f3 and f4 are identical; putting a bang before a pattern that
>> forces evaluation anyway does nothing.
Maybe the duality (if it is one) should be added that an irrefutable
pattern above would make a difference but not within the let below.
> The first sentence is true, but only in settings where the pattern is being
> evaluated eagerly -- the bang in:
>> f3 a = let !(x,y) = a in [1,x,y]
>> f4 a = let (x,y) = a in [1,x,y]
> has an effect.
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