Re: [Haskell-beginners] why is this type wrong

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 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 wrote:

...

Mike Meyer schreef op 12-5-2015 om 8:39:

On May 12, 2015 1:32 AM, "Roelof Wobben" 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@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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