Haskell中的约束值类型

Question

是否有可能在Haskell中定义约束类型，即我希望能够表达，

Prelude> let legalCharacters = ' ':['A'..'Z']
Prelude> legalCharacters
" ABCDEFGHIJKLMNOPQRSTUVWXYZ"

作为一种类型，如果可能的话。

Answer 1

可以在现代GHC中完成（> = 7.10，也许已经是7.8）。

{-# LANGUAGE KindSignatures, DataKinds, MonoLocalBinds #-}
import GHC.TypeLits

newtype LegalChar (legalSet :: Symbol)
   = LegalChar {getLegalChar :: Char}
  deriving (Show)

fromChar :: KnownSymbol legal => Char -> Maybe (LegalChar legal)
fromChar c
   | c`elem`symbolVal r = Just r
   | otherwise          = Nothing
 where r = LegalChar c

然后

*Main> fromChar 'a' :: Maybe (LegalChar "abc")
Just (LegalChar {getLegalChar = 'a'})
*Main> fromChar 'x' :: Maybe (LegalChar "abc")
Nothing

我认为在GHC-8中你甚至可以给legalSet一个String并取消KnownSymbol约束，不知道它是如何工作的。