Ninety-Nine Haskell Problems 8
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八問目．
8 Problem 8 (**) Eliminate consecutive duplicates of list elements. If a list contains repeated elements they should be replaced with a single copy of the element. The order of the elements should not be changed. Example: * (compress '(a a a a b c c a a d e e e e)) (A B C A D E) Example in Haskell: *Main> compress ['a','a','a','a','b','c','c','a','a','d','e','e','e','e'] ['a','b','c','a','d','e']http://www.haskell.org/haskellwiki/99_questions/1_to_10#Problem_8
で，とりあえず，Haskell/Control structuresで，if文について調べて，できたのが次．
compress (x:(y:[])) = if x == y then [x] else [x, y] compress (x:(y:ys)) = if x == y then compress (y:ys) else ([x] ++ compress (y:ys))
これで実行してみると，
Main> compress [1,1, 2] [1,2] Main> compress [1,2] [1,2] Main> compress [1,1,1,2,2,3,4,4,4,5,1,1,2] [1,2,3,4,5,1,2]
おお，なんかうまくいってる模様．
しかし，
Main> compress ['a','a','a','a','b','c','c','a','a','d','e','e','e','e'] "abcade"
駄目でした．
うーん，一回切り離してから，文字を結合してるから，全部繋がって文字列になっちゃうってわけだな．
と，思ったけど，
Main> ['a','b','c','a','d','e'] "abcade"
ってなるから，いいのかな？どういうこっちゃ．
とりあえず，解答．
Solution:
compress :: Eq a => [a] -> [a]
compress = map head . group
We simply group equal values together (group), then take the head of each. Note that (with GHC)
we must give an explicit type to compress otherwise we get:
Ambiguous type variable `a' in the constraint:
`Eq a'
arising from use of `group'
Possible cause: the monomorphism restriction applied to the following:
compress :: [a] -> [a]
Probable fix: give these definition(s) an explicit type signature
or use -fno-monomorphism-restriction
We can circumvent the monomorphism restriction by writing compress this way
(See: section 4.5.4 of the report):
compress xs = map head $ group xs
An alternative solution is
compress [] = []
compress [a] = [a]
compress (x : y : xs) = (if x == y then [] else [x]) ++ compress (y : xs)おう，長いやんけ．
groupが謎なんだけど，とりあえず，mapについて調べる．
map :: (a -> b) -> [a] -> [b] map _ [] = [] map f (x:xs) = f x : map f xshttp://haskell.org/ghc/docs/6.12.2/html/libraries/base-4.2.0.1/src/GHC-Base.html#map
よし，非常にわかりやすい．リストの各要素にfをかけるってことだな．
で，謎なのがgroup．WinHugsで実行してみても，Undefinedとか言われちゃう．
たぶん，WinHugsには含まれてないんだろう．かといって，GHCiでも実行できない．なのにググるとでてくる．もうやだ，よくわからん．
無視！！
で，
compress [] = [] compress [a] = [a] compress (x : y : xs) = (if x == y then [] else [x]) ++ compress (y : xs)
は，少しだけ，僕のつくったのと似てる．でもこっちのほうがわかりやすくていいな．同じ場合は，[]でスルーして，違う場合だけ残すって感じか．しかたないけど，若干くやしい．
おわり．
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