Haskell:没有自己类型的实例

本文关键字：类型 实例 自己 Haskell | 更新日期: 2023-09-27 17:54:07

我正在尝试使用以下代码的"call-haskell-to-anything"包导出自己的类型:

{-# 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

当我尝试编译这个文件时，我得到错误消息:

[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"。但是正确的语法是什么呢?我已经尝试添加:

instance Rectangle where

但是得到了这个错误信息:

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

Haskell:没有自己类型的实例

首先需要导入Data.MessagePack.Object。这将使MessagePack类进入作用域。

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

与MessagePack类的文档进行比较。

最后我在ErikR(如何派生数据实例)的帮助下解决了这个问题。Messagepack 1.0.0)。下面是最后的代码:

{-# 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)
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