Haskell pattern matching on List

Question

I know we can use below pattern matching in Haskell:

sum :: (Num a) => [a] -> a  
sum [] = 0  
sum (x:xs) = x + sum xs

But why can’t we use [x] ++ xs?

sum :: (Num a) => [a] -> a  
sum [] = 0  
sum ([x] ++ xs) = x + sum xs

Thomas M. DuBuisson · Accepted Answer · 2016-11-13 20:17:32Z

5

You can pattern match using constructors and literals but not functions.

answered Nov 13, 2016 at 20:17

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3 Comments

Is : a constructor?

@4castle Yes. [1,2,3,4] is basically just sugar for 1:2:3:4:[].

Indeed, pattern matching using functions would be nonsense in the general case: what would (\(x++y) -> y++x) "abcde" evaluate to?