缺少MonadReader实例

Question

从github获得的代码

{-# LANGUAGE TypeFamilies
           , TypeOperators #-}

module Test where

import Control.Monad.Reader

import Control.Lens
import Control.Zipper

class Monad f => Family f where
    type Set f

-- This does not work
f :: Family f => (Top :>> [Set f] :>> Set f) -> f (Set f)
f = return . view focus

-- This here works
g :: Top :>> [Int] :>> Int -> Int
g = view focus

使用ghc-8编译很好，但是不能用ghc-7.10编译错误

Test.hs:16:14:
Could not deduce (MonadReader
                    (Zipper (Top :>> [Set f]) Int (Set f))
                    ((->) ((Top :>> [Set f]) :>> Set f)))
  arising from a use of `view'
from the context (Family f)
  bound by the type signature for
             f :: Family f => (Top :>> [Set f]) :>> Set f -> f (Set f)
  at Test.hs:15:6-57
In the second argument of `(.)', namely `view focus'
In the expression: return . view focus
In an equation for `f': f = return . view focus

在这两种情况下，使用的依赖项完全相同(mtl 2.2.1、transformers 0.5.2、lens 4.14、zippers 0.2)，但base除外，后者只能是ghc-7.10的4.8，4.9只能用于ghc-8。我真的不明白为什么它应该用一个编译器而不是另一个编译器编译。我也想让代码为ghc-7.10工作。

Answer 1

我仍然不知道为何会出现这个问题。但是，在用户puregreen over的帮助下，在#haskell irc通道上，我能够通过简单地用(^. focus)替换view focus来克服这个问题。