1樓：Belleve
fix$(
<$>)<$>(:
)<*>
((<$>((:
[])<$>
))(=<<
)<$>(*
)<$>
(>>=)(+
)($))$1
2樓：Lee
對我來說 Aeson + Generics 就是黑魔法之一：
例子來自 What I Wish I Knew When Learning Haskell 2.3 ( Stephen Diehl )
import
Data.Text
import
Data.Aeson
import
GHC.Generics
import
qualified
Data.ByteString.Lazy
asBL
data
Refs
=Refs
deriving
(Show
,Generic
,FromJSON
,ToJSON
)data
Data
=Data
deriving
(Show
,Generic
,FromJSON
,ToJSON
)main
::IO
()main=do
contents
<-BL.
readFile
"example.json"
letJust
dat=
decode
contents
$name
datprint$a
(refs
dat)BL.
putStrLn
$encode
dat測試的 example.json 如下：}
3樓：Skitter
神奇的let：
> let 2 = 1 in 2
2一行求冪集：
> filterM (const [True, False]) [1, 2, 3, 4]
[[1,2,3,4],[1,2,3],[1,2,4],[1,2],[1,3,4],[1,3],[1,4],[1],[2,3,4],[2,3],[2,4],[2],[3,4],[3],[4],]
型別安全的強制型別轉換：
class Cast t u where cast :: t -> u
instance Cast Int Int where cast = id
instance Cast Int Double where cast = fromIntegral
instance (Cast u1 t1, Cast t2 u2) => Cast (t1 -> t2) (u1 -> u2) where cast f = \x -> cast $ f $ cast x
instance (Cast t1 u1, Cast t2 u2) => Cast (t1, t2) (u1, u2) where cast (x, y) = (cast x, cast y)
test :: Double -> (Double, Double) -> Int
test x (y, z) = round $ x * y + x * z
> cast test (5 :: Int) (1 :: Int, 2 :: Int) :: Double
15.0
真○OO虛函式，誰說type class只能靜態過載的？
data Exists f where Ex :: f t -> Exists f
data ShowableObject t where ShowableObject :: Show t => t -> ShowableObject t
mkShowableObject :: Show t => t -> Exists ShowableObject
mkShowableObject = Ex . ShowableObject
instance Show (Exists ShowableObject) where show (Ex (ShowableObject x)) = show x
test = [mkShowableObject 1, mkShowableObject "abc", mkShowableObject ('a', True, [0..5])]
> show test
"[1,\"abc\",('a',True,[0,1,2,3,4,5])]"
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