The following code fails to compile:
split :: Char -> String -> [String]
split x ys = split' x ys []
where split' _ [] acc = acc
split' x (z:zs) acc
| x == z = acc : split' x zs []
| otherwise = split' x zs (z:acc)
Compile-time error:
[1 of 1] Compiling Main ( Split.hs, interpreted )
Split.hs:5:36:
Occurs check: cannot construct the infinite type: a0 = [a0]
In the first argument of `(:)', namely `acc'
In the expression: acc : split' x zs []
In an equation for split':
split' x (z : zs) acc
| x == z = acc : split' x zs []
| otherwise = split' x zs (z : acc)
Failed, modules loaded: none.
I read through this helpful post, but I'm not seeing my mistake.
The posted code defined split' returning acc in one case and acc : <<something>> in another case. The compiler message states that, if a0 is the type of acc, since both cases must return the same type it should be a0 = [a0]. Since no type can satisfy this equation (e.g. String is not [String]), the compiler complains with a type error.
A fix could then be to return a list containing acc is both cases. That is:
split :: Char -> String -> [String]
split x ys = split' x ys []
where split' _ [] acc = [acc] -- changed
split' x (z:zs) acc
| x == z = acc : split' x zs []
| otherwise = split' x zs (z:acc)
As a style note, since x is kept the same in each recursive call, there is no actual need for that parameter.
split :: Char -> String -> [String]
split x ys = split' ys []
where split' [] acc = [acc]
split' (z:zs) acc
| x == z = acc : split' zs []
| otherwise = split' zs (z:acc)
The code is now type safe, but it still contains a bug:
> split 'a' "123a1234a"
["321","4321",""]
This is because the code "pops" characters from the string and "pushes" them in acc, so they end up being reversed. The fix is then easy:
split :: Char -> String -> [String]
split x ys = split' ys []
where split' :: String -> String -> [String]
split' [] acc = [reverse acc]
split' (z:zs) acc
| x == z = reverse acc : split' zs []
| otherwise = split' zs (z:acc)
In this last revision I also added a type signature for the worker function, to document its code.
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