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我正在使用嵌入在 Haskell 中的语言。我的语言可以作为源代码打印出来，所以我创建了一个Compile类并为每个可以打印的程序元素创建了一个类实例。这样我就可以组合地转储我的代码。在考虑模式的概念之前，这很好用。

每种语言都可以在两种模式下使用（实现为 class 的实例Mode）。在简单模式下一切正常。在命名模式下，许多程序元素可以用字符串替换。（它像宏定义一样工作。）

我想保持所有表示类型安全。所以不同语言或不同模式的程序元素不能混用。

所以问题是：无论模式如何，如何转储语言？

{-# LANGUAGE TypeFamilies, MultiParamTypeClasses, FlexibleInstances #-}
class Compile a where
  comp :: a -> String

-- common elements in all languages
data ElemA l m = ElemA (ElemB l m)
data ElemB l m = ElemB

class Lang l where
  -- language-specific elements
  data Instructions l :: * -> *

-- common modes for all languages
class Mode l m where
  type MElemA l m :: *
  type MElemB l m :: *

-- mode with normal program elements
data SimpleMode
instance Mode l SimpleMode where
  type MElemA l SimpleMode = ElemA l SimpleMode
  type MElemB l SimpleMode = ElemB l SimpleMode

-- a mode where each program element can be replaced with a string
data NamedMode
instance Mode l NamedMode where
  type MElemA l NamedMode = Either String (ElemA l NamedMode)
  type MElemB l NamedMode = Either String (ElemB l NamedMode)

-- definition of Lang1 language
data Lang1
instance Lang Lang1 where
  data Instructions Lang1 m 
    = Add (MElemA Lang1 m) (MElemA Lang1 m) (MElemA Lang1 m)
    | Mul (MElemA Lang1 m) (MElemA Lang1 m) (MElemA Lang1 m)
    -- | ...

-- dumping the source code of Lang1 langauge

-- ILLEGAL TYPE SYNONYM FAMILY APPLICATION HERE
instance Compile (MElemA Lang1 m) where  
  comp _ = "A"
-- AND HERE
instance Compile (MElemB Lang1 m) where  
  comp _ = "B"

我知道类型同义词系列不适用于类，所以我正在寻找另一种解决方案。

我知道（但不想使用）的可能解决方案：

  • 使用多个函数而不是单个多态comp函数。
  • 仅NamedMode用于表示
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1 回答 1

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我的一个朋友 Zoltán Kelemen 给我发了一个解决方案。他使用包装类来封装指定语言的程序元素。通过这种方式，他从实例头中消除了类型族应用程序，而不会产生不必要的开销。

我也在寻找其他解决方案。

{-# LANGUAGE TypeFamilies, MultiParamTypeClasses, FlexibleInstances, UndecidableInstances #-}
class Compile a where
  comp :: a -> String

-- common elements in all languages
data ElemA l m = ElemA (ElemB l m)
data ElemB l m = ElemB

class Lang l where
  -- language-specific elements
  data Instructions l :: * -> *

-- wrapper classes for program elements of Lang1
data Lang1A m = WrapperA (ElemA Lang1 m)
data Lang1B m = WrapperB (ElemB Lang1 m)

-- common modes for all languages
class Mode l m where
  type MElemA l m :: *
  type MElemB l m :: *

-- mode with normal program elements
data SimpleMode
instance Mode l SimpleMode where
  type MElemA l SimpleMode = ElemA l SimpleMode
  type MElemB l SimpleMode = ElemB l SimpleMode

-- a mode where each program element can be replaced with a string
data NamedMode
instance Mode l NamedMode where
  type MElemA l NamedMode = Either String (ElemA l NamedMode)
  type MElemB l NamedMode = Either String (ElemB l NamedMode)

-- definition of Lang1 language
data Lang1
instance Lang Lang1 where
  data Instructions Lang1 m 
    = Add (MElemA Lang1 m) (MElemA Lang1 m) (MElemA Lang1 m)
    | Mul (MElemA Lang1 m) (MElemA Lang1 m) (MElemA Lang1 m)
    -- | ...

-- dumping the source code of Lang1 langauge

-- ILLEGAL TYPE SYNONYM FAMILY APPLICATION HERE
instance Compile (Lang1A m) where  
  comp (WrapperA e) = "A"
-- AND HERE
instance Compile (Lang1B m) where  
  comp (WrapperB e) = "B"
于 2013-08-09T05:39:21.790 回答