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Roelof Wobben

12 May 2015 12 May '15

6:32 a.m.

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Alexey Shmalko

12 May 12 May

6:37 a.m.

It's because first branch of last' returns a list instead of element. last' [] = [] The actual type is last' :: [a] -> [a] On Tue, May 12, 2015 at 9:33 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,

Roelof

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Mike Meyer

6:39 a.m.

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.

Roelof Wobben

6:42 a.m.

Alexey Shmalko

6:46 a.m.

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.

_______________________________________________ Beginners mailing listBeginners@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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Roelof Wobben

6:48 a.m.

Sumit Sahrawat, Maths & Computing, IIT (BHU)

6:51 a.m.

Make a dummy main function, main = return () And load this file in ghci to use you definition of last. The other option is to not make this module the Main module. Then also, you can load it in ghci. On 12 May 2015 at 12:18, 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.

_______________________________________________ Beginners mailing listBeginners@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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-- Regards Sumit Sahrawat

Alexey Shmalko

6:51 a.m.

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.

_______________________________________________ Beginners mailing listBeginners@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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Kim-Ee Yeoh

9:39 a.m.

On Tue, May 12, 2015 at 1:42 PM, Roelof Wobben wrote: -- | The main entry point.

...

last' :: [a] -> a last' [x] = x last' (x:xs) = last xs

Notice that the last line does no recursion: it invokes the Prelude's definition of last. -- Kim-Ee

Nicolas SCHOEMAEKER

9:45 a.m.

Hi In your definition, you seem to have forgotten the case when your list is empty. [x] won't bind on []. Regards, Nicolas SCHOEMAEKER (This email was composed from a mobile phone, please excuse any resulting mistakes) Web, Cloud Computing & WebRTC developer Haskell Programmer Artificial Intelligence & Robotics enthusiast On May 12, 2015, 11:40 AM, at 11:40 AM, Kim-Ee Yeoh wrote:

...

On Tue, May 12, 2015 at 1:42 PM, Roelof Wobben wrote:

-- | The main entry point.

...
last' :: [a] -> a last' [x] = x last' (x:xs) = last xs

Notice that the last line does no recursion: it invokes the Prelude's definition of last.

-- Kim-Ee

------------------------------------------------------------------------

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Roelof Wobben

12:36 p.m.

Sumit Sahrawat, Maths & Computing, IIT (BHU)

12:39 p.m.

The type of (.) (.) :: (b -> c) -> (a -> b) -> a -> c Means that "last' [1,2,3]" should be a function, which it isn't You can instead try, main = (print . last') [1,2,3] Or equivalently, main = print . last' $ [1,2,3] On 12 May 2015 at 18:06, Roelof Wobben wrote:

...

Thanks all.

I have now this :

-- | The main entry point. last' :: [a] -> Maybe a last' [] = Nothing last' [x] = Just x last' (_:xs) = last' xs

main = print . last' [1,2,3]

but I see this message about main :

* GHC threw an exception : Couldn't match expected type ‘GHC.Types.IO http://GHC.Types.IO a0’ with actual type ‘a1 -> GHC.Types.IO http://GHC.Types.IO ()’ I work wih the fpcomplete and they demand a main in the code . Roelof *

Nicolas SCHOEMAEKER schreef op 12-5-2015 om 11:45:

Hi

In your definition, you seem to have forgotten the case when your list is empty. [x] won't bind on [].

Regards, Nicolas SCHOEMAEKER

(*This email was composed from a mobile phone, please** excuse** any resulting mistakes*)

Web, Cloud Computing & WebRTC developer Haskell Programmer Artificial Intelligence & Robotics enthusiast On May 12, 2015, at 11:40 AM, Kim-Ee Yeoh wrote:

...
On Tue, May 12, 2015 at 1:42 PM, Roelof Wobben wrote:

-- | The main entry point.

...
last' :: [a] -> a last' [x] = x last' (x:xs) = last xs

Notice that the last line does no recursion: it invokes the Prelude's definition of last.

-- Kim-Ee

------------------------------

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-- Regards Sumit Sahrawat

Roelof Wobben

12:48 p.m.

Sumit Sahrawat, Maths & Computing, IIT (BHU)

6:47 a.m.

I recommending first trying out the `last' function in ghci. Prelude> last [] *** Exception: Prelude.last: empty list Prelude> last [1,2] 2 Instead of using last [] = [] You can use last [] = error "Empty list" Where error is defined in a way such that it can take on any type. In ghci Prelude> :type error error :: [Char] -> a On 12 May 2015 at 12:09, Mike Meyer wrote:

...

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.

_______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

-- Regards Sumit Sahrawat

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Alexey Shmalko
Kim-Ee Yeoh
Mike Meyer
Nicolas SCHOEMAEKER
Roelof Wobben
Sumit Sahrawat, Maths & Computing, IIT (BHU)