On Monday 27 September 2010 17:41:24, Martin Tomko wrote:
Dear all
I am sure this is a very beginner problem, but I am not sure how to solve it: I have a myFunct function: myFunct :: Int -> [a] -> a
defined as: myFunct _ [] = error "empty list provided as arg" myFunct a [b] | length [x|x<-[b],fid x == a] == 0 = error "no match" | otherwise = head [x|x<-[b],fid x == a]
The pattern `[b]' matches only lists of length 1. [b] is syntactic sugar for (b : []) So when you pass a longer list to the function, no pattern matches. Presumably you wanted the second equation to treat all nonempty lists, n which case you simply have to replace all three occurrences of `[b]' with a generic variable, e.g. b. Since you test for empty lists first, no empty list ever reaches that equation. Also, don't use length list == 0 to check for empty lists, if list is long, that needs a long time (and possibly a lot of space). To check for empty lists, use null. Or pattern match. That would also improve the style in the second equation: myFunct _ [] = error ".." myFunct a xs = case [x | x <- xs, fid x == a] of [] -> error "no match" (y:_) -> y Cheers, Daniel