Can't get type signature working for simple recursive function

Question

Here is my code:

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

Yet when I run it through ghci as :l test, I get this error:

[1 of 1] Compiling Main ( test.hs, interpreted )

test.hs:3:7:
    Couldn't match type `a' with `[a]'
      `a' is a rigid type variable bound by
          the type signature for spew :: Num a => [a] -> a at test.hs:2:1
    In the pattern: x : xs
    In the pattern: [x : xs]
    In an equation for `spew': spew [x : xs] = x + spew xs
Failed, modules loaded: none.

Try not to laugh :) it's my first attempt at haskell. Any help or explanations would be awesome.

PS: I know this could be easily done with a fold, but I'm trying to practice writing my own type signatures. Thanks in advance!!

Answer 1

You mean

test :: (Num a) => [a] -> a
test []     = 0
test (x:xs) = x + test xs -- note round brackets

with round brackets.

[x:xs] is a list with one element, itself a list, whereas (x:xs) is a list with a first element x and tail xs.

If you type length (1:[1,1,1]) you'll get 4, but if you type length [1:[1,1,1]] you'll get 1 - the only element is a list.