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Amazing type inference for field name in Haskell

Consider the following GHCi decoding, version 8.2.2:

GHCi, version 8.2.2: http://www.haskell.org/ghc/  :? for help
Prelude> :set -XRankNTypes
Prelude> data Functor f = Functor { fmap :: forall a b. (a -> b) -> f a -> f b }
Prelude> :t fmap
fmap :: Functor f1 -> (a -> b) -> f2 a -> f2 b
Prelude> :t Functor map
Functor map :: Functor []
Prelude> :t fmap (Functor map)
fmap (Functor map) :: (a -> b) -> [a] -> [b]

As you can see, the type fmapis inferred as Functor f1 -> (a -> b) -> f2 a -> f2 b. This is surprising because f1, and f2must be the same type variable, but there is no limit f1 ~ f2. However, if you apply fmapto Functor mapwhich has a type Functor [], the result still has the type (a -> b) -> [a] -> [b]as expected. What's going on here? I would expect fmapto have a type Functor f -> (a -> b) -> f a -> f b.

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, GHC 8.2.2, 8.0.2. : , 8.4.

, GHCi, , , :

{-# LANGUAGE RankNTypes #-}
module Inference where
data Functor f = Functor { fmap :: forall a b. (a -> b) -> f a -> f b }

ghc -ddump-tc, , GHC :

TYPE SIGNATURES
  ...
  Inference.fmap ::
    forall (f :: * -> *).
    Inference.Functor f -> forall a b. (a -> b) -> f a -> f b

GHC. :

main = print Inference.fmap

, ( " ( xxx)" ), .

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