可鍵入類型的“模式匹配”

Question

例如，假設我們有以下數據結構：

data Foo = Bool Bool | Int Int | Double Double

現在，有沒有更簡單的方法來做到這一點：

foo :: Typeable a => a -> Foo
foo x = maybe (error "i dunno") id $
  liftM Bool   (cast x) `mplus`
  liftM Int    (cast x) `mplus`
  liftM Double (cast x)

有人想過為Typeable類型的模式匹配制定語法嗎？

Answer 1

使用typeOf和守衛：

foo x
    | tx == typeOf "str" = "string"
    | tx == typeOf True  = "bool"
    | otherwise          = "i dunno"
  where tx = typeOf x

Answer 2

您可以在此處使用視圖模式，它們是非常方便的擴展。

{-# LANGUAGE ViewPatterns #-}

import Data.Data

data Foo = Bool Bool | Int Int | Double Double
         deriving (Show)

foo :: Typeable a => a -> Foo
foo (cast -> Just x) = Int x
foo (cast -> Just x) = Bool x
foo (cast -> Just x) = Double x
foo _ = error "i dunno"

結果：

*Main> :l foo_typeable.hs 
[1 of 1] Compiling Main             ( foo_typeable.hs, interpreted )
Ok, modules loaded: Main.
*Main> foo "123"
*** Exception: i dunno
*Main> foo 1
*** Exception: i dunno
*Main> foo (1 :: Int)
Int 1
*Main> foo (1 :: Integer)
*** Exception: i dunno
*Main> foo (1 :: Double)
Double 1.0

Answer 3

此版本不限於Bool 、 Int或Double ，但String以[Char]的形式出現。

foo :: Typeable a => a -> String
foo = show . typeOf