Brian Bloniarz wrote:
I got confused by the GHC documentation recently, I was wondering how it could be improved. From: http://www.haskell.org/ghc/docs/latest/html/users_guide/bang-patterns.html
Seeing the rule pat ::= !pat you'll probably want to avoid patterns like: "!!pat", "! ! pat", or "~ ! ~ pat". 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.
Cheers Christian