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%2...
(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
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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