|sumof ::||[a] -> b|
"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
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