在私有数据构造函数上进行模式匹配 [英] Pattern matching on a private data constructor

问题描述

我正在为网格轴编写一个简单的ADT。在我的应用程序中，网格可能是正常的（在坐标之间有一个固定的步长），或者不规则（否则）。当然，规则网格只是一个不规则网格的特殊情况，但在某些情况下可能需要区分它们（例如，执行一些优化）。所以，我宣布我的ADT如下：

$ p $ data $ GridAxis = RegularAxis（Float，Float）Float - （min，max ）delta
| IrregularAxis [Float] - [xs]

但我不希望用户创建格式不正确的轴 max <分钟或无序 xs 列表。因此，我添加了更智能的构造函数，它们执行一些基本检查： p>

  regularAxis ::（Float，Float） - >浮动 - > GridAxis 
 regularAxis（a，b）dx = RegularAxis（min a b，max a b）（abs dx）
 
 irregularAxis :: [Float]  - > GridAxis 
 irregularAxis xs = IrregularAxis（sort xs）
  

我不希望用户创建所以我不会在模块导出列表中添加 GridAxis 数据构造函数：

  module GridAxis（
 GridAxis，
 regularAxis，
 irregularAxis，
）其中
  

但事实证明，完成这一步之后，我无法再对 GridAxis 使用模式匹配。尝试使用它

 将合格的GridAxis导入为GA 
 
 test :: GA.GridAxis  - > Bool 
测试轴= 
的情况轴GA.RegularAxis  - >真
 GA.IrregularAxis  - >假
  

给出以下编译器错误：

  src / Physics / ImplicitEMC.hs：7：15：
不在范围内：数据构造函数`GA.RegularAxis'
 
 src / Physics / ImplicitEMC.hs：8：15：
不在范围内：数据构造函数GA.IrregularAxis'
  

有什么可以解决这个问题吗？

解决方案

您可以定义构造函数模式的同义词。这可以让你使用相同的名字进行智能建设和哑模式匹配。

  { - ＃LANGUAGE PatternSynonyms＃ - } 
 
模块GridAxis（GridAxis，模式RegularAxis，模式IrregularAxis）其中
导入Data.List 
 
数据GridAxis = RegularAxis_（Float，Float）Float  - （min，最大）增量
 | IrregularAxis_ [Float]  -  [xs] 
 
  - 带有& - 的行定义匹配行为
  - 带=的行定义构造函数行为
模式RegularAxis minmax delta < -  RegularAxis_minmax delta其中
 RegularAxis（a，b）dx = RegularAxis_（minab，maxab）（absdx）
 
 pattern IrregularAxis xs < - IrregularAxis_ xs其中
 IrregularAxis xs = IrregularAxis_（sort xs）
  

现在你可以这样做：

  module Foo 
导入GridAxis 
 
 foo :: GridAxis  - > a 
 foo（RegularAxis（a，b）d）= ... 
 foo（IrregularAxis xs）= ... 
  

并且使用 RegularAxis 和 IrregularAxis 作为智能构造函数。

I'm writing a simple ADT for grid axis. In my application grid may be either regular (with constant step between coordinates), or irregular (otherwise). Of course, the regular grid is just a special case of irregular one, but it may worth to differentiate between them in some situations (for example, to perform some optimizations). So, I declare my ADT as the following:

data GridAxis = RegularAxis (Float, Float) Float -- (min, max) delta
              | IrregularAxis [Float]            -- [xs]

But I don't want user to create malformed axes with max < min or with unordered xs list. So, I add "smarter" construction functions which perform some basic checks:

regularAxis :: (Float, Float) -> Float -> GridAxis
regularAxis (a, b) dx = RegularAxis (min a b, max a b) (abs dx)

irregularAxis :: [Float] -> GridAxis
irregularAxis xs = IrregularAxis (sort xs)

I don't want user to create grids directly, so I don't add GridAxis data constructors into module export list:

module GridAxis (
    GridAxis,
    regularAxis,
    irregularAxis,
    ) where

But it turned out that after having this done I cannot use pattern matching on GridAxis anymore. Trying to use it

import qualified GridAxis as GA

test :: GA.GridAxis -> Bool
test axis = case axis of
              GA.RegularAxis -> True
              GA.IrregularAxis -> False  

gives the following compiler error:

src/Physics/ImplicitEMC.hs:7:15:
    Not in scope: data constructor `GA.RegularAxis'

src/Physics/ImplicitEMC.hs:8:15:
    Not in scope: data constructor `GA.IrregularAxis'

Is there something to work this around?

解决方案

You can define constructor pattern synonyms. This lets you use the same name for smart construction and "dumb" pattern matching.

{-# LANGUAGE PatternSynonyms #-}

module GridAxis (GridAxis, pattern RegularAxis, pattern IrregularAxis) where
import Data.List

data GridAxis = RegularAxis_ (Float, Float) Float -- (min, max) delta
              | IrregularAxis_ [Float]            -- [xs]

-- The line with "<-" defines the matching behavior
-- The line with "=" defines the constructor behavior
pattern RegularAxis minmax delta <- RegularAxis_ minmax delta where
  RegularAxis (a, b) dx = RegularAxis_ (min a b, max a b) (abs dx)

pattern IrregularAxis xs <- IrregularAxis_ xs where
  IrregularAxis xs = IrregularAxis_ (sort xs)

Now you can do:

module Foo
import GridAxis

foo :: GridAxis -> a
foo (RegularAxis (a, b) d) = ...
foo (IrregularAxis xs) = ...

And also use RegularAxis and IrregularAxis as smart constructors.

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