[Haskell-beginners] why is this type wrong

[Mike Meyer]

On May 12, 2015 1:32 AM, "Roelof Wobben" <r.wobben at home.nl> wrote:
>
> Hello,
>
> To practice recursion I try to make some functions of Data list myself on
the recursive way.
>
> First I will try last.
>
> So I did this :
>
> -- | Main entry point to the application.
> module Main where
>
> -- | The main entry point.
> last' :: [a] -> a
> last' [] = []
> last' (x:xs) = last xs
>
>
> but now I see this error message :
>
> src/Main.hs at 6:12-6:14
> Couldn't match expected type
> a
> with actual type
> [t0]
> a
> is a rigid type variable bound by the type signature for last' :: [a] ->
a at
/home/app/isolation-runner-work/projects/112712/session.207/src/src/Main.hs:5:10
Relevant bindings include last' :: [a] -> a (bound at
/home/app/isolation-runner-work/projects/112712/session.207/src/src/Main.hs:6:1)
…
>
> I my oponion I have said that the input is a array and the output a
string,

Except for saying list instead of array, I'd agree with that. But when you
write last' [] = [], the output is [], which does not have the expected
type of strong. So the actual type is a list.

You need to return a string.
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