sum :: Num a => [a] -> a
sum [] = 0
sum (n:ns) = n + sum ns

ghci> :load sum

ghci> :load
sum [1 of 1] Compiling Main ( sum.hs, interpreted )

sum.hs:3:18: error: Ambiguous occurrence ‘sum’ It could refer to
either ‘Prelude.sum’, imported from ‘Prelude’ at sum.hs:1:1
(and originally defined in ‘Data.Foldable’) or ‘Main.sum’,
defined at sum.hs:2:1
||
3 | sum (n:ns) = n + sum ns
| ^^^ |
Failed, no modules loaded.

import Prelude hiding (sum)

sum :: Num a => [a] -> a
sum [] = 0
sum (n:ns) = n + sum ns

[1 of 1] Compiling Main             ( sum.hs, interpreted )
Ok, one module loaded.

ghci> sum [1,3,5,7,9,11]
36
ghci> sum [1..10]
55