这个GHC.Generic函数签名错误消息是什么意思？

Question

I'm learning Haskell. Why can't I change Person to type variable a in the method signature of getName

{-# LANGUAGE DeriveGeneric #-}
module Main where

import GHC.Generics


data Person = Person { id   :: Int
                     , name :: String
                     , age  :: Int }
            deriving (Generic, Show)

p = from $ Person 12 "J" 12

getName :: (Rep Person) x -> String
-- getName :: Generic a => (Rep a) x -> String
----^ Err: Couldn't match type ‘Rep a0’ with ‘Rep a
getName f = concat [ moduleName f
                   , "_"
                   , datatypeName f
                   ]

n = getName p

Answer 1

Rep is an associated type-synonym ie the actual type can be anything depending on the implementation of the specific type-class instance. There is no guarantee that Rep a for any arbitrary a is a type that is compatible with the type required by the function moduleName .

Rep Person works because the compiler is able to check that the associated type for the type-class instance of Generic Person fits the type expected by moduleName (which is Datatype d => td (f :: * -> *) a ).

Instead of using Rep a you can instead use

getName :: Datatype d => t d (f :: * -> *) a -> String
getName f = concat [ moduleName f
                   , "_"
                   , datatypeName f
                   ]

Although you also need to enable the KindSignatures extension.

Answer 2

Just omit the type signature for getName .

Running your code in a ghci session yields this:

ghci> :t getName
getName :: Datatype d => t d f a -> [Char]

ghci> getName p
"Lib9_Person"

ghci> :t getName p
getName p :: [Char]

Note: My code resides in the module Lib9 which explains the output for getName p .

If you want to give it a type signature, enable KindSignatures and use:

getName :: Datatype d => (t :: * -> (* -> *) -> * -> *) d f a -> [Char]