[Haskell-beginners] why is this type wrong

[Alexey Shmalko]

last' [] = error "last' on empty list"

Otherwise, you should wrap the result in Maybe to make your function work
as safeLast.

Oh... GHC's exception seems to say you haven't defined `main` in your
module.

On Tue, May 12, 2015 at 9:42 AM Roelof Wobben <r.wobben at home.nl> wrote:

>  Mike Meyer schreef op 12-5-2015 om 8:39:
>
> 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.
>
>
> _______________________________________________
> Beginners mailing listBeginners at haskell.orghttp://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
> Thanks,
>
> I did change it to this :
>
> -- | Main entry point to the application.
> module Main where
>
> -- | The main entry point.
> last' :: [a] -> a
> last' [x] = x
> last' (x:xs) = last xs
>
> So I have to look at another way to say if there is a empty list then
> there is no answer.
> And when I run it  i see this error message :
>
> GHC threw an exception : Not in scope: ‘Main.main’
>
>
> Roelof
>
>
>
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