Question

mifun s = foldr op 0 s
          where op x r = head x + r 

Is there a way to make ghci tell me?

Answer 1

try :t mifun (short for :type mifun)

which gives

*Main> :t mifun
mifun :: (Num b) => [[b]] -> b

So, for a b an instance of num, mifun takes a list of lists of b and outputs a single b (which in this case is the sum of the first elements of the lists).