[Haskell-beginners] why is this type wrong

[Alexey Shmalko]

I believe, you don't need main at all. Just load your source with ghci and
call your functions from there.

On Tue, May 12, 2015 at 9:48 AM Roelof Wobben <r.wobben at home.nl> wrote:

>  Thanks
>
> Can I just say Main =
> I will google to look how to make a safeLast.
>
> Roelof
>
>
>
> Alexey Shmalko schreef op 12-5-2015 om 8:46:
>
> 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.
>>
>>
>> _______________________________________________
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>>
>>
>>  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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>
>
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