为什么ghci给我一个"1 ++ 2"而不是ragequitting的类型？

Question

GHCI会给我一个类型1 ++ 2:

$ ghci
GHCi, version 7.4.2: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Prelude> :t 1 ++ 2
1 ++ 2 :: Num [a] => [a]

但这显然是错误的.如果我尝试评估它,而不是只是键入检查它,事情正确失败:

Prelude> 1 ++ 2

<interactive>:3:1:
    No instance for (Num [a0])
      arising from the literal `1'
    Possible fix: add an instance declaration for (Num [a0])
    In the first argument of `(++)', namely `1'
    In the expression: 1 ++ 2
    In an equation for `it': it = 1 ++ 2

是什么赋予了？

Answer 1

但这显然是错误的.

不,这是完全正确的.

类型(++)是

(++) :: [a] -> [a] -> [a]

和整数文字的类型是

1 :: Num n => n

因此[a],(++)必须具有的参数的类型Num n => n与文字所具有的类型一致

1 ++ 2 :: Num [a] => [a]

如果您有一个包含Num实例的列表类型,那么也可以计算该表达式.

但是,默认情况下,没有Num可用的列表类型的实例,因此当您尝试评估它时,ghci会抱怨它找不到Num实例[a].

例如:

Prelude> instance Num a => Num [a] where fromInteger n = Data.List.genericReplicate n 1 

<interactive>:2:10: Warning:
    No explicit method or default declaration for `+'
    In the instance declaration for `Num [a]'

<interactive>:2:10: Warning:
    No explicit method or default declaration for `*'
    In the instance declaration for `Num [a]'

<interactive>:2:10: Warning:
    No explicit method or default declaration for `abs'
    In the instance declaration for `Num [a]'

<interactive>:2:10: Warning:
    No explicit method or default declaration for `signum'
    In the instance declaration for `Num [a]'
Prelude> 1 ++ 2 :: [Int]
[1,1,1]

Answer 2

因为有人可以定义要被视为数字的列表:

instance Num a => Num [a] where
 (+) = zipWith (+)
 (*) = zipWith (*)
 (-) = zipWith (-)
 negate = map negate
 abs = map abs
 signum = map signum
 fromInteger x = [fromInteger x]

然后你输入的内容就可以了

  1++2 == fromInteger 1++fromInteger 2 == [1]++[2]

(并不是说这个Num实例会有意义..)