Question

我有以下代码：

type Drawable = '["object" ::: Object, "transform" ::: M44 GL.GLfloat]
objXfrm :: "transform" ::: M44 GL.GLfloat
objXfrm = Field
objRec :: "object" ::: Object
objRec = Field

drawObject :: (Drawable `ISubset` a) => M44 GL.GLfloat -> PlainRec a -> IO ()
drawObject camera obj =
    withVAO vao $ do
        GL.currentProgram $= Just (program shdr)
        setUniforms shdr (modelView =: (rGet objXfrm obj !*! camera))
        GL.polygonMode $= (GL.Line, GL.Line)
        GL.drawElements GL.Triangles inds GL.UnsignedInt nullPtr
    where Object {objVAO = vao, objNumIndices = inds, objShader = shdr}
              = rGet objRec obj

当我摆脱drawObject上的类型时，它编译得很好，但是我得到的类型

  Could not deduce (IElem * ("transform" ::: V4 (V4 GL.GLfloat)) a)
      arising from a use of `rGet'
    from the context (ISubset * Drawable a)
...
  Could not deduce (IElem * ("object" ::: Object) a)
      arising from a use of `rGet'
    from the context (ISubset * Drawable a)

GHC为我推断的类型是

drawObject
  :: (IElem * ("object" ::: Object) rs,
      IElem * ("transform" ::: V4 (V4 GL.GLfloat)) rs) =>
     V4 (V4 GL.GLfloat)
     -> Rec rs Data.Functor.Identity.Identity -> IO ()

作为类型签名，它可以正常工作，但ISubset的签名则不行。如果我将参数交换为ISubset，则错误完全相同。这是怎么回事？

Answer 1

查看Vinyl的源代码，IElem x xs（Implicit (Elem x xs)的同义词）有两个实例：

instance Implicit (Elem x (x ': xs)) where
    implicitly = Here
instance Implicit (Elem x xs) => Implicit (Elem x (y ': xs)) where
    implicitly = There implicitly

请注意，此处未提及Subset。逻辑上，(x ∈ xs) ∧ (xs ⊆ ys) ⇒ (x ∈ ys)，但由于没有签名Implicit (Subset xs ys), Implicit (Elem x xs) => Implicit (Elem x ys)的实例，Haskell无法推断出适当的实例。此外，不能写这样的实例，因为这样做会导致一些讨厌的实例重叠。

作为一种可能的解决方法，我们可以直接操纵成员证人（Elem和Subset）以强制执行适当的实例（这完全未经测试，可能会失败）：

{-# LANGUAGE RankNTypes, ScopedTypeVariables, and possibly more... #-}

-- Lift an Elem value to a constraint.
withElem :: Elem x xs -> (forall r. IElem x xs => r) -> r
withElem Here x = x
withElem (There e) x = withElem e x

-- Witness of (x ∈ xs) ⇒ (xs ⊆ ys) ⇒ (x ∈ ys)
subsetElem :: Elem x xs -> Subset xs ys -> Elem x ys
subsetElem Here (SubsetCons e _) = e
subsetElem (There e) (SubsetCons _ s) = There (subsetElem e s)

-- Utility for retrieving Elem values from Fields (:::).
fieldElem :: IElem x xs => x -> Elem x xs
fieldElem _ = implicitly

inSubset :: IElem x xs => x -> Subset xs ys -> (forall r. IElem x ys => r) -> r
inSubset f s x = withElem (subsetElem (fieldElem f) s) x

drawObject :: forall a. (Drawable `ISubset` a) => M44 GL.GLfloat-> PlainRec a -> IO ()
drawObject camera obj =
    inSubset objRec subset $
    inSubset objXfrm subset $
        -- The desired instances should now be available here
        ...
  where
    subset = implicitly :: Subset Drawable a
    ...

Answer 2

通过简要地查看Vinyl代码，似乎整个“子集”的想法还没有完全实现（意味着，它需要更多的功能和实例才能真正有用）。你能不能只使用记录之间的“子类型”关系？然后就可以了

drawObject :: (PlainRec a <: PlainRec Drawable) => M44 GL.GLfloat -> PlainRec a -> IO ()
drawObject camera obj' =
    ... -- as before
  where
    obj :: PlainRec Drawable
    obj = cast obj'
    ... -- rest as before

乙烯基中ISubset的问题

2 个答案: