'Non exhaustive patterns in function' error when declaring a function within ghci

Asked May 16 '18 at 14:49

Active May 16 '18 at 15:20

Viewed 113 times

I have a haskell file, sum.hs, that reads:

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

This works as expected within ghci by loading the module in.

However, if I try to declare the function within ghci I get an error:

Prelude> sum' [] = 0
Prelude> sum' (x:xs) = x + sum' xs
Prelude> sum' [3,4]
*** Exception: <interactive>:2:1-25: Non-exhaustive patterns in function sum'

Please can someone explain why loading the module in works but declaring the function within ghci results in this error?

edited May 16 '18 at 15:20

asked May 16 '18 at 14:49

laker93

Note that the definition you wrote in the question is incorrect, it should be `xs` rather than `x` in the argument to `sum'` – Cubic May 16 '18 at 14:55
whoops! Copy and paste error - thanks for spotting. – laker93 May 16 '18 at 15:20

'Non exhaustive patterns in function' error when declaring a function within ghci

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