GHCi 如何为类型变量选择名称？
使用交互式 GHC 解释器时，可以请求表达式的推断类型：
Prelude> :t map
map :: (a -> b) -> [a] -> [b]
它似乎从签名中获取类型变量的名称，因为 map 是 定义如
map :: (a -> b) -> [a] -> [b]
map _ [] = []
map f (x:xs) = f x : map f xs
前奏中所示。这很有道理！我的问题是：当没有给出签名时，如何选择类型变量名称？
例如，
Prelude> :t map fst
map fst :: [(b, b1)] -> [b]
它选择了名称 b 和 b1。很明显，必须进行重命名，但只需以 a、b、... 开头即可
map fst :: [(a, b)] -> [a]
，我发现这更具可读性。
When using the interactive GHC interpreter, it's possible to ask for the inferred type of an expression:
Prelude> :t map
map :: (a -> b) -> [a] -> [b]
It seems that it takes the names of the type variables from the signature since map is defined as
map :: (a -> b) -> [a] -> [b]
map _ [] = []
map f (x:xs) = f x : map f xs
in the Prelude. That makes a lot of sense! My question is: how are type variable names picked when there is no signature given?
An example would be
Prelude> :t map fst
map fst :: [(b, b1)] -> [b]
where it picked names b and b1. It's clear that renaming must take place, but simply starting with a, b, ... would have given
map fst :: [(a, b)] -> [a]
instead, which I find slightly more readable.
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据我了解，ghci 按照推断类型的顺序选择名称。它使用您提到的命名方案来决定结果的类型名称，即
[b]，因为这是map定义中指定的类型名称。然后它决定作为map的第一个参数的函数也应该返回b类型的内容。因此，要命名的剩余类型变量是 fst 参数元组中第二个元素的类型变量，并且它再次查看 fst 的定义来决定哪个要使用的名称。
fst::(a, b) -> 的定义a，因此b将是此处的首选名称，但由于b已被采用，因此它会附加一个1，以便它变为b1。我认为这个系统在您不处理任意类型的情况下具有优势，就像这里的情况一样。如果结果类型看起来像这样，例如：
... 它可以说比： ... 更具可读性
，因为您主要可以依赖
n#表示数字类型，因为类定义Num是class Num n where ...。编辑：是的，我知道
castAdd是不可能实现的，但这只是一个类型示例。As I understand it,
ghcichooses names in the same order that it infers the types. It uses the naming scheme as you mentioned to decide the type name of the result, which is[b]because that is the type name specified in the definition ofmap. It then decides that the function that is the first parameter tomapshould return something of typebalso.The remaining type variable to be named is thus the type variable for the second element in the argument tuple to
fst, and again, it looks at the definition offstto decide which name to use. The definition offst :: (a, b) -> a, sobwould be the preferred name here, but sincebis already taken, it appends a1so that it becomesb1.I think that this system has advantages in situations where you don't deal with arbitrary types as is the case here. If the resulting type looks something like this, for example:
... it is arguably more readable than:
... because you can mostly rely on that
n#signifies a numeric type, since the class definition forNumisclass Num n where ....EDIT: Yes, I know that
castAddis impossible to implement, but it's just a type example.