good idea. done | -----Original Message----- | From: glasgow-haskell-users-bounces@haskell.org [mailto:glasgow-haskell-users- | bounces@haskell.org] On Behalf Of Brian Bloniarz | Sent: 27 February 2009 03:56 | To: glasgow-haskell-users@haskell.org | Subject: Suggestion for bang patterns documentation | | | 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 | | > 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. | | 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. | | The first time I read this, I took the first sentence to be a unqualified truth | and ended up thinking that !(x,y) was equivalent to (x,y) everywhere. Stuff | that comes later actually clarifies this, but I missed it. | | What about making the distinction clear upfront? Something like: | > A bang in an eager pattern match only really has an effect if it precedes a | variable | > or wild-card pattern: | > f3 !(x,y) = [x,y] | > f4 (x,y) = [x,y] | > Because f4 _|_ will force the evaluation of the pattern match anyway, f3 and f4 | > are identical; the bang does nothing. | | It also might be a good idea to immediately follow this with the let/where usage: | | > A bang can also preceed a let/where binding to make the pattern match strict. For | example: | > let ![x,y] = e in b | > is a strict pattern... | (in the existing docs, let comes a bit later): | | Just a thought. Hopefully someone can come up with a better way of | wording what I'm getting at. | | Thanks, | -Brian | | _________________________________________________________________ | Windows Live(tm) Hotmail(r)...more than just e-mail. | http://windowslive.com/howitworks?ocid=TXT_TAGLM_WL_t2_hm_justgotbetter_howi... | 22009_______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users@haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users