2015-05-13 15:55 GMT+09:00 Roelof Wobben <r.wobben@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@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@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@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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