Prelude> let sH' x = case x of { [] -> Nothing ; (y:_) -> y }
Prelude> :type sH'
sH :: [Maybe a] -> Maybe a
Prelude> :type [1,2,3]
[1,2,3] :: Num t => [t]
Prelude> sH' [1,2,3]

<interactive>:1:9:
    No instance for (Num (Maybe a0))
      arising from the literal `3'
    Possible fix: add an instance declaration for (Num (Maybe a0))
    In the expression: 3
    In the first argument of `sH', namely `[1, 2, 3]'
    In the expression: sH [1, 2, 3]

Prelude> let sH x = case x of { [] -> Nothing ; (y:_) -> Just y }
Prelude> :type sH
sH :: [a] -> Maybe a