缺少 MonadReader 实例

Question

可从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 编译正常，但使用 ghc-7.10 编译失败并出现错误

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完全相同的依赖关系（mtl 2.2.1、transformers 0.5.2、lens 4.14、zippers 0.2）除了 base 只能是 4.8 ghc-7.10，4.9 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。我想让代码也适用于 ghc-7.10。

Answer 1

我还不知道为什么会出现这个问题。但是在 #haskell irc 频道上的用户 puregreen 的帮助下，我能够通过简单地将 view focus 替换为 (^. focus).[=14 来解决此设置中的这个问题=]

缺少 MonadReader 实例

Missing instance of MonadReader

haskell

typeclass

type-constraints

type-families