I'm fairly new to Haskell and this week I found this particular function in a couple of lecture slides. I'm trying to understand why the following function doesn't need to include a parameter:
-- Return all final segments of the argument, longest first
-- (horrible runtime complexity, used here for illustration purposes only)
tails :: [a] -> [[a]]
tails = reverse . map reverse . inits . reverse
If I would call it like tails "thisisastring" then this would be a valid argument. Isn't it necessary to provide a parameter, for example tails xs = .... All the other functions I seen before were in that fashion.
The parameter is implicit. Or to put it differently, reverse . map reverse . inits . reverse evaluates to a function of type [a] -> [[a]].
Consider a simpler example:
double_impl x = x * 2
double = double_impl
The type of double here is the same type as double_impl, i.e. it takes one parameter of typeclass Num:
main = do
print $ double_impl 5
print $ double 5
-- Out: 10
-- Out: 10