无法推断(显示 a10)由使用“def”引起

Could not deduce (Show a10) arising from a use of ‘def’

我已经被这个错误困扰了很长一段时间。我不确定我是不是很愚蠢,但我看不出它来自哪里...

数据类型Settings定义如下:

data Settings a = Settings { 
         _nodeDrawFunction :: a -> Diagram B
       , _dynamicHead :: Measure Double
       , _dynamicThick :: Measure Double
       , _directed :: Directed
       , _horizontalOrientation :: Maybe Bool
       , _layerSpacing :: Maybe Double
       , _nodeSpacing :: Maybe Double
       , _graphPadding :: Maybe Double
       , _colF :: Maybe (Int -> Colour Double)
       , _bgOp :: Maybe Double
       , _initPos :: Maybe Int
      }

makeLenses ''Settings用于为每条记录创建镜头。 _drawNodeFunction 的默认值为 drawDefaultNode.

当我创建一个使用镜头设置器覆盖其中一些值的函数时,它工作正常,例如:

test1 :: (Show a) => Graph a -> Settings a
test1 g = def & bgOp .~ Just 1

按预期工作。 Graph 的类型来自库 algebraic-graphs.

如果我尝试设置 nodeDrawFunction,例如:

test2 :: (Show a) => Graph a -> Settings a
test2 g = def & nodeDrawFunction .~ drawDefaultNode
              & bgOp .~ Just 1

产生以下错误:

Could not deduce (Show a10) arising from a use of ‘def’
  from the context: Show a
    bound by the type signature for:
               test2 :: forall a. Show a => Graph a -> Settings a
    at Visualise/Hierarchical.hs:78:1-42
  The type variable ‘a10’ is ambiguous
  These potential instances exist:
    instance Show n => Show (Duration n) -- Defined in ‘Data.Active’
    instance Show n => Show (Era n) -- Defined in ‘Data.Active’
    instance Show n => Show (Time n) -- Defined in ‘Data.Active’
    ...plus 74 others
    ...plus 264 instances involving out-of-scope types
    (use -fprint-potential-instances to see them all)
• In the first argument of ‘(&)’, namely ‘def’
  In the first argument of ‘(&)’, namely
    ‘def & nodeDrawFunction .~ drawDefaultNode’
  In the expression:
    def & nodeDrawFunction .~ drawDefaultNode & bgOp .~ Just 1

即使我尝试将 nodeDrawFunction 设置为默认值(以及任何其他有效函数)。

非常感谢任何帮助,谢谢!

您可能希望 defdef & nodeDrawFunction .~ drawDefaultNode 具有相同的类型。 nodeDrawFunction 足够灵活,允许它们有不同的类型,Settings aSettings a10。 GHC 从这个更一般的假设开始,然后无法为 a10 选择特定类型,因为它不受函数中任何内容的约束。

一种选择是用不太通用的类型定义 nodeDrawFunction

另一种选择是在 def 上添加类型签名以指导 GHC。我认为如果您添加 -XScopedTypeVariables:

这会起作用
(def :: Settings a) & nodeDrawFunction .~ drawDefaultNode

没有 ScopedTypeVariables 两个类型变量 a 被解释为不同的,并且在类型检查之前被赋予不同的唯一名称。如果 Settings 中有多个涉及 a 的字段,makeLenses 将导出无法更改 a 的镜头,因为没有单字段镜头可以这样做。