Next Atypical Types 51

Type Inference

sumof ::[a] -> b
h ::a
t ::[a]

"h must have type a and t must have type [a]."

        sumof []    = 0
        sumof (h:t) = h + sumof t

"We're adding h to the return value of sumof."

"So the return value must be a also."

"And + is only defined for instances of Num, so a is such an instance

"So the return value is really of type (Num a) => a."

     sumof :: (Num a) => [a] -> a

Next Copyright © 1999,2008 Mark Dominus