将[]环绕列表的每个顶级元素

Question

wrap [1,2,3] should output [[1],[2],[3]]
wrap [[1],[2],[3]] should output [ [[1]], [[2]], [[3]] ]

my implementation is:

wrap [] = []
wrap (x:xs) = [x] :  [wrap xs]

and haskell output an error: Occurs check: cannot construct the infinite type: t ~ [t]

Expected type: [t] -> [t] Actual type: [t] -> [[t]]

Answer 1

[wrap xs] wraps the entire result of wrap xs . You don't want to wrap the entire result. You only want to wrap each individual element of the remainder of the list and that is exactly what wrap xs already does. Thus, wrap is:

wrap :: [a] -> [[a]]
wrap [] = []
wrap (x:xs) = [x] : wrap xs

The error is telling you that you are trying to use a [t] where GHC expects a t . This is exactly what we said above, that you are wrapping something too many times.