..在哪里定义? [英] Where is .. defined?

问题描述

我只是想知道 .. 是否实际上在任何地方的Haskell代码中定义，例如在

要对问题进行您显示的定义是默认定义"，在 Enum 类型类中.它在您链接的前奏曲中说得很对:

  class枚举a其中succ，pred :: a->一种toEnum :: Int->一种fromEnum :: a->诠释enumFrom :: a->[一个..]enumFromThen :: a->a->[a]-[n，n'..]enumFromTo :: a->a->[a]-[n..m]enumFromThenTo :: a->a->a->[a]-[n，n'.. m]-最小完整定义:-toEnum，来自Enum-_______-____-注意:这些默认方法仅对类型有意义-****注意-使用fromEnum射影映射到Int-和toEnum.succ = toEnum.(+1).来自枚举pred = toEnum.(减去1).来自枚举enumFrom x =映射到Enum [fromEnum x ..]enumFromTo x y =映射到Enum [fromEnum x .. fromEnum y]enumFromThen x y =映射到Enum [fromEnum x，fromEnum y ..]enumFromThenTo x y z =映射到Enum [fromEnum x，fromEnum y .. fromEnum z] 

因此，中 Enum 中的每个 a 类型都必须至少提供 toEnum :: Int->的定义.a 和 fromEnum :: a->Int .如果它没有提供自己的定义，例如 enumFromTo :: a->a->[a] ，则将使用其默认定义:

  enumFromTo :: a->a->[一种]enumFromTo x y =映射到Enum [fromEnum x .. fromEnum y]-└a┘└a┘└a┘└a┘-└──Int────┘└──Int────┘-└Int->a┘└─────────[Int]─────────-└──────────────[a]────────────────┘ 

对于 Int 类型本身，库中的某个地方定义了 specific 定义，因此未使用 enumFromTo 的默认定义.表示 Int ，因此没有恶性循环.

I'm just wondering whether .. is actually defined in Haskell code anywhere, like in the Prelude? Is enumFromTo the same thing? I don't get which is the definition?

enumFromTo x y  =  map toEnum [fromEnum x .. fromEnum y]

[ e1 .. e3 ]  =  enumFromTo e1 e3

解决方案

It's a part of the syntax, specified in the Report, section 3.10.

Yes it gets translated into code using enumFrom etc. functions:

To your edit of the question: the definitions you show are "default definitions" in the Enum typeclass. It says so right there in the Prelude you linked:

class  Enum a  where
    succ, pred       :: a -> a
    toEnum           :: Int -> a
    fromEnum         :: a -> Int
    enumFrom         :: a -> [a]             -- [n..]
    enumFromThen     :: a -> a -> [a]        -- [n,n'..]
    enumFromTo       :: a -> a -> [a]        -- [n..m]
    enumFromThenTo   :: a -> a -> a -> [a]   -- [n,n'..m]

        -- Minimal complete definition:
        --      toEnum, fromEnum
--             _______                                      -- ____
-- NOTE: these default methods only make sense for types    -- **** NB
--   that map injectively into Int using fromEnum
--  and toEnum.
    succ             =  toEnum . (+1) . fromEnum
    pred             =  toEnum . (subtract 1) . fromEnum
    enumFrom x       =  map toEnum [fromEnum x ..]
    enumFromTo x y   =  map toEnum [fromEnum x .. fromEnum y]
    enumFromThen x y =  map toEnum [fromEnum x, fromEnum y ..]
    enumFromThenTo x y z = 
                        map toEnum [fromEnum x, fromEnum y .. fromEnum z]

So each type a that is in Enum must provide the definitions for at least toEnum :: Int -> a and fromEnum :: a -> Int. If it doesn't provide its own definition for e.g. enumFromTo :: a -> a -> [a] then its default definition will be used:

enumFromTo :: a -> a -> [a]
enumFromTo x  y   =  map  toEnum  [fromEnum x .. fromEnum y ]
--        └a┘└a┘                           └a┘           └a┘
--                                 └──Int────┘   └──Int────┘
--                       └Int->a┘ └─────────[Int]───────────┘
--                   └───────────────[a]────────────────────┘

And for the Int type itself there is a specific definition defined somewhere in the library, so the default definition of enumFromTo is not used for Int, thus there is no vicious circle.

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