[Haskell-beginners] Are these solution the Haskell way ?

Wed May 13 07:08:59 UTC 2015

Think about what the possible values of "xs" might be, and trace through
the next call to "plusplus xs ys".

It would help if I didn't name the variables stupidly... here is a slightly
better version:

plusplus [] ys = ys
plusplus (x:xs) ys = x : plusplus xs ys

If it helps, try tracing through the steps required to evaluate "plusplus
[1, 2] [3, 4]" manually (using a pen and paper, not on the computer)

2015-05-13 15:55 GMT+09:00 Roelof Wobben <r.wobben at home.nl>:

>  Thanks,
>
> So the answer is x and the rest of xs and ys.
> How do x then get added to ys.
>
> Roelof
>
> Daniel P. Wright schreef op 13-5-2015 om 8:52:
>
> Ah, that's just a small syntactic issue -- in Haskell, operators are infix
> (go between the arguments) by default, but named functions are not.  So you
> would have to write it:
>
>  plusplus [] xs = xs
> plusplus (x:xs) ys = x : plusplus xs ys
>
>
> 2015-05-13 15:39 GMT+09:00 Roelof Wobben <r.wobben at home.nl>:
>
>>  Thanks,
>>
>> If I re-implement it like this :
>>
>> plusplus :: [a] -> [a] -> [a]
>> plusplus [] (xs) = xs
>>  plusplus (x:xs) yx = x : xs plusplus yx
>>
>> main = print $ ["a","b"] plusplus ["c","d"]
>>
>>
>> I see this error appear :
>>
>> src/Main.hs at 3:26-3:40
>> Couldn't match expected type ‘([a0] -> [a0] -> [a0]) -> [a] -> [a]’ with
>> actual type
>> [a]
>>  Relevant bindings include yx :: [a] (bound at
>> /home/app/isolation-runner-work/projects/112825/session.207/src/src/Main.hs:3:17)
>> xs :: [a] (bound at
>> /home/app/isolation-runner-work/projects/112825/session.207/src/src/Main.hs:3:13)
>> x :: a (bound at
>> /home/app/isolation-runner-work/projects/112825/session.207/src/src/Main.hs:3:11)
>> plusplus :: [a] -> [a] -> [a] (bound at
>> /home/app/isolation-runner-work/projects/112825/session.207/src/src/Main.hs:2:1)
>> The function
>> xs
>>  is applied to two arguments, but its type
>> [a]
>>  has none …
>>
>> So for me not a aha moment.  I was hoping I would get it
>>
>> Roelof
>>
>> Daniel P. Wright schreef op 13-5-2015 om 8:27:
>>
>> Hi Roelof,
>>
>>  If you don't consider it cheating (and I suggest you shouldn't, having
>> had a stab at the answers), you will find great enlightenment looking at
>> how these functions are *actually* implemented in the wild.  Did you know
>> that there is a "source" link for each function on Hackage?  By clicking on
>> that, for your first example, you can compare the actual implementation of
>> (++) with your "plusplus" function:
>>
>>
>> http://hackage.haskell.org/package/base-4.8.0.0/docs/src/GHC-Base.html#%2B%2B
>>
>>  (By the way, it's that function in particular that gave me one of my
>> first "Aha!" moments in Haskell... it's quite beautiful in its simplicity).
>>
>>  One thing with viewing the source on Hackage is that sometimes it can
>> be a little more confusing than it needs to be for the sake of efficiency.
>> A really good source for good, readable examples of Prelude functions in
>> Haskell is the Haskell Report:
>>
>>  https://www.haskell.org/onlinereport/standard-prelude.html
>>
>>  (In this case, though, the implementation is the same).
>>
>>  Having a shot at defining these library functions yourself, as you have
>> done, and then comparing your version with the "official" version in the
>> prelude is a great way to learn good style!
>>
>>  -Dani.
>>
>> 2015-05-13 15:10 GMT+09:00 Roelof Wobben <r.wobben at home.nl>:
>>
>>> Hello,
>>>
>>> For practising pattern matching and recursion I did recreate some
>>> commands of Data,list.
>>>
>>> My re-implementation of ++ :
>>>
>>> plusplus :: [a] -> [a] -> [a]
>>> plusplus [] [] = [] ;
>>> plusplus [] (xs) = xs
>>> plusplus (xs) [] = xs
>>> plusplus (xs) yx = plusplus' (reverse xs) yx
>>>
>>> plusplus' :: [a] -> [a] -> [a]
>>> plusplus' [] (xs) = xs
>>> plusplus' (x:xs) yx = plusplus' xs (x:yx)
>>>
>>> main = print $ plusplus ["a","b"] ["c","d"]
>>>
>>> my re-implementation of init :
>>>
>>> import Data.Maybe
>>>
>>> -- | The main entry point.
>>> init' :: [a] -> Maybe [a]
>>> init' [] = Nothing
>>> init' [x] = Just []
>>> init' (x:xs) = Just (x:fromMaybe xs (init' xs))
>>>
>>> main = print . init' $ [1,3]
>>>
>>>
>>> my re-implementation of last :
>>>
>>> -- | The main entry point.
>>> last' :: [a] -> Maybe a
>>> last' [] = Nothing
>>> last' [x] = Just x
>>> last' (_:xs) = last' xs
>>>
>>> main = print . last' $ []
>>>
>>> Now I wonder if these solutions are the haskell way ? if not so, how can
>>> I improve them ,
>>>
>>> Roelof
>>>
>>>
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>>
>>
>>
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