First, what type would such a function have?
Certainly not [a]->[b], because that type say that it can take a list of any
type and turn it into a list of any other type, e.g.,
[Int]->[Bool].
On Thu, Dec 30, 2010 at 4:05 AM, william murphy
Hi All,
I've spent a lot of time trying to write a version of concat, which concatenates lists of any "depth": So: concat'' [[[1,2],[3,4]],[[5]]] would return: [1,2,3,4,5]
The code is: concat'' :: [a] -> [b] concat'' ((y:ys):xs) = (concat'' (y:ys)) ++ (concat'' xs) concat'' [] = [] concat'' (x:xs) = (x:xs)
And the inevitable error is: test.hs:298:12: Couldn't match expected type `a' against inferred type `[t]' `a' is a rigid type variable bound by the type signature for `concat''' at test.hs:297:13 In the pattern: y : ys In the pattern: (y : ys) : xs In the definition of `concat''': concat'' ((y : ys) : xs) = (concat'' (y : ys)) ++ (concat'' xs)
test.hs:300:24: Couldn't match expected type `b' against inferred type `[t]' `b' is a rigid type variable bound by the type signature for `concat''' at test.hs:297:20 In the first argument of `(:)', namely `x' In the expression: (x : xs) In the definition of `concat''': concat'' (x : xs) = (x : xs) Failed, modules loaded: none.
Any help or advice would be appreciated. Will
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