No instance for (Num a) arising from a use of ‘+’ Haskell

Question

I can't figure out why this won't work:

final' :: [a] -> a
final' lst = foldl(\accum x -> accum - accum + x) 0 lst

I always get the error No instance for (Num a) arising from a use of '+'

Answer 1

The problem has nothing to do with the function itself, but with the signature you attach to it yourself:

final' :: [a] -> a

Here you basically say that your final' function will work for any a . So I could - if I wanted - add String s together, as well as IO () instances, or anything else. But now Haskell inspects your function, and notices that you perform an addition (+) :: Num a => a -> a -> a , with as right operand x , which has type a . Like the signature for (+) already says, both operands should have the same type, and that type should be an instance of Num .

You can solve the problem by making the signature more restrictive:

final' ::  [a] -> a
final' lst = foldl(\accum x -> accum - accum + x) 0 lst

In fact we can also generalize a part of the signature, and let it work for any Foldable :

final' :: Num a =>  a -> a
final' lst = foldl(\accum x -> accum - accum + x) 0 lst

We can however get rid of the accum , since subtracting a number from itself, will usually result in zero (except for rounding issues, etc.):

final' :: (Num a, Foldable f) => f a -> a
final' = foldl  0

Now we got rid of (+) (and (-) ), but still need to use Num . The reason is that you use 0 as initial accumulator, and in case of an empty list, we thus will return 0 , and 0 :: Num n => n .