Question

我正在尝试使用带有此代码的“call-haskell-to-anything”包将自己的类型导出到C＃：

{-# LANGUAGE ForeignFunctionInterface #-}
{-# LANGUAGE TemplateHaskell #-}

module RectangleMover where

import Foreign.C
import FFI.Anything.TypeUncurry.Msgpack

data Point = Point Int Int
movePoint :: Point -> Int -> Int -> Point
movePoint (Point x y) dx dy = Point (x + dx) (y + dy)

data Rectangle = Rectangle { corner :: Point, width :: Int, height :: Int }
move :: Rectangle -> Int -> Int -> Rectangle
move r@(Rectangle {corner=c}) dx dy = r { corner = movePoint c dx dy }

foreign export ccall move_export :: CString -> IO CString
move_export :: CString -> IO CString
move_export = export move

当我尝试编译此文件时，我收到错误消息：

[1 of 1] Compiling RectangleMover   ( test.hs, test.o )

test.hs:19:15:
    No instance for (Data.MessagePack.Object.MessagePack Rectangle)
      arising from a use of ‘export’
    In the expression: export move
    In an equation for ‘move_export’: move_export = export move

所以我想我必须为“矩形”类型写一个“实例”。但是这个的正确语法是什么？我已经尝试添加：

instance Rectangle where

但收到此错误消息：

test.hs:13:10:
    Expected a constraint, but ‘Rectangle’ has kind ‘*’
    In the instance declaration for ‘Rectangle’

Answer 1

首先，您需要导入Data.MessagePack.Object。这将使课程MessagePack进入范围。

然后你需要说：

instance MessagePack Rectangle where
   toObject rect = 
      -- Insert your implementation here, returning an Object
   fromObject obj = 
      -- Insert your implementation here, returning a Maybe Rectangle

将此与MessagePack class的文档进行比较。

Answer 2

最后，我在ErikR（How to derive instances of Data.Messagepack 1.0.0）的帮助下解决了这个问题。这是最终的代码：

{-# LANGUAGE ForeignFunctionInterface #-}
{-# LANGUAGE DeriveGeneric #-}

module RectangleMover where

import Foreign.C
import FFI.Anything.TypeUncurry.Msgpack
import qualified Data.MessagePack as MP
import Data.MessagePack.Object
import Data.MessagePack.Aeson
import Data.Aeson
import GHC.Generics

instance ToJSON Point
instance FromJSON Point
data Point = Point Int Int deriving (Generic, Show)
movePoint :: Point -> Int -> Int -> Point
movePoint (Point x y) dx dy = Point (x + dx) (y + dy)

instance ToJSON Rectangle
instance FromJSON Rectangle
data Rectangle = Rectangle { corner :: Point, width :: Int, height :: Int } deriving (Generic, Show)
move :: Rectangle -> Int -> Int -> Rectangle
move r@(Rectangle {corner=c}) dx dy = r { corner = movePoint c dx dy }

toMsgPack :: Rectangle  -> Maybe MP.Object
toMsgPack =   decode . encode

p = Point 1 2
rect = Rectangle p 10 20
test = toMsgPack (rect)

Haskell：没有自己类型的实例

2 个答案: