haskell - 如何消除选择器功能的歧义？

Question

在 GHC 8 中:

{-# LANGUAGE DuplicateRecordFields #-}

data Dog = Dog { name::String }
data Human = Human { name::String }

dog = Dog "Spike"


main = putStrLn $ name dog

此代码无法编译:

Ambiguous occurrence `name'
It could refer to either the field `name', defined at A.hs:4:22
                      or the field `name', defined at A.hs:3:18

如何正确检索我的狗的名字？

Answer 1

这应该工作:

main = putStrLn $ name (dog :: Dog)

见 DuplicateRecordFields详情:

Bare uses of the field refer only to the selector function, and work only if this is unambiguous.

和

However, we do not infer the type of the argument to determine the datatype, or have any way of deferring the choice to the constraint solver.

那里的例子和你的很像:

bad (p :: Person) = personId p

当有另一条记录为 personId 时，这将不起作用范围内的字段 - 即使看起来很明显:(