Haskell - 无法从上下文错误中推断出...... OpenGL AsUniform 类类型

Question

我正在努力使我的数据类型通用，而不是采用 OpenGL 类型GLfloat。所以我开始让它输入a，然后用它替换所有东西。

现在，我已经到了设置统一变量的地步，但它们采用 GLfloat 的。我正在使用一个名为 GLUtil 的库，它提供了一个类 AsUniform 来检查该类型是否可以是统一变量，这使它变得更容易一些。我把它贴在我的类型签名中，但它仍然给我一个错误。代码如下：

-- | Sets the modelview and projection matrix uniform variables.
mvpUnif :: (GL.UniformComponent a, Num a, Epsilon a, Floating a, AsUniform a) => (GLState a) -> ShaderProgram  -> IO ()
mvpUnif state p = do
-- Check if view and projection matrices are there, else set them to the identity.
let vMat = case vMatrix state of
    Just v -> v
    Nothing -> getIdentity
let pMat = case pMatrix state of
    Just p -> p
    Nothing -> getIdentity
-- Multiply model and view matrix together.
let mvMatrix = vMat !*! mMatrix state
setUniform p uModelViewMatrixVar mvMatrix
setUniform p uProjectionMatrixVar pMat

和错误：

Could not deduce (AsUniform (V4 (V4 a)))
  arising from a use of `setUniform'
from the context (GL.UniformComponent a,
                  Num a,
                  Epsilon a,
                  Floating a,
                  AsUniform a)
  bound by the type signature for
             mvpUnif :: (GL.UniformComponent a, Num a, Epsilon a, Floating a
,
                         AsUniform a) =>
                        GLState a -> ShaderProgram -> IO ()
  at src\Graphics\FreeD\Shaders\DefaultShaders.hs:194:12-119
In a stmt of a 'do' block:
  setUniform p uModelViewMatrixVar mvMatrix
In the expression:
  do { let vMat = ...;
       let pMat = ...;
       let mvMatrix = vMat !*! mMatrix state;
       setUniform p uModelViewMatrixVar mvMatrix;
       .... }
In an equation for `mvpUnif':
    mvpUnif state p
      = do { let vMat = ...;
             let pMat = ...;
             let mvMatrix = ...;
             .... }

V4 是 AsUniform 的一个实例，而 M44 是 (V4 (V4 a)) 的一种类型，我认为这可能是问题所在，所以我不确定它为什么会这样。

这是课程的来源：

http://hackage.haskell.org/package/GLUtil-0.8.5/docs/Graphics-GLUtil-Linear.html

谢谢！

Answer 1

尝试在现有答案中添加-XFlexibleContexts 和约束：

{-# LANGUAGE FlexibleContexts #-}

mvpUnif :: ( GL.UniformComponent a
           , Num a
           , Epsilon a
           , Floating a
           , AsUniform a
           , AsUniform (V4 (V4 a))
           ) => (GLState a) -> ShaderProgram  -> IO ()

通常这是不可推断的约束的例程，或者约束需要被传递地包含在所有调用站点中。 MonadState 等人一直在我身上发生这种情况。在这种情况下，setUniform 是罪魁祸首。