On Fri, 11 Feb 2005, Remi Turk wrote:
1) It's talking about the compiler having difficulty with some warnings when using guards.
http://www.haskell.org//pipermail/haskell-cafe/2005-January/008290.html
f x | odd x = ... | even x = ...
GHC does complain. I would also call it Bad Code, but if it's what you mean, _this_ example should be in the wiki.
Yes, your example is better.
2) foo xs | length xs == 1 = bar (head xs) As already said in "Don't ask for the length of a list, if you don't need it", this usage of length is bad in itself, and doesn't really help the argument against patterns IMO.
I have seen it similarly in the example I give below at that page. So I found it worth noting that some guards can nicely be replaced by simple patterns. More examples are welcome. May be we should replace it by foo xs | not (null xs) = bar (head xs) vs. foo (x:_) = bar x This example might be useful, too: foo x | x == 0 = blub x /= 0 = bla vs. foo 0 = blub foo _ = bla
3) the pattern guards extension. I have two objections against this one. First, I don't think it's a good idea to talk about a non-standard extension like pattern guards in a wiki about newbie-problems.
It was given to me as a good example why Guards are invaluable: http://www.haskell.org//pipermail/haskell-cafe/2005-January/008320.html
P.P.S. Does a piece about "Avoid explicit lambda's" stand any chance of not being removed? (Basically about "\x y -> x + y" vs "(+)", and "when it gets more complicated it probably deserves a name.")
Nice!