我正在尝试使用gpipe包在Haskell的屏幕上随机位置绘制四边形。由于四边形将位于2D中，我还想设置正交投影。我怎么似乎没有得到适合光栅化的类型。这是我的代码（使用gpipe教程拼凑在一起）：
{-# LANGUAGE ScopedTypeVariables, PackageImports, FlexibleContexts, TypeFamilies #-}
module Main where
import Graphics.GPipe
import qualified "GPipe-GLFW" Graphics.GPipe.Context.GLFW as GLFW
import Control.Monad (unless)
import System.Random
-- Generate random points in 2d
getRandomPoints2D :: (Float, Float) -> StdGen -> [V4 Float]
getRandomPoints2D range sg = zipWith createPoint x y
where createPoint x y = V4 x y 0 1
(sg1, sg2) = split sg
x = randomRs range sg1
y = randomRs range sg2
getPoints :: Int -> StdGen -> [V4 Float]
getPoints n sg = take n $ getRandomPoints2D (-1,1) sg
examplePoints :: Int -> IO [V4 Float]
examplePoints n = newStdGen >>= return . getPoints n
-- Quads from point
quadFromPoint :: V4 Float -> [V4 Float]
quadFromPoint p = map (p +) [ v4 (-off) (-off), v4 (-off) off
, v4 off (-off), v4 off (-off)
, v4 off off, v4 (-off) off
]
where off = 0.1
v4 x y = V4 x y 0 0
main = do
let width = 720
height = 540
windowConf = GLFW.WindowConf width height "hs-visu"
context = GLFW.newContext' [] windowConf
let numPoints = 6
points <- examplePoints numPoints
let quadPoints = concatMap quadFromPoint points
pointsWithColor = zip quadPoints $ repeat (V3 1 0 0)
runContextT context (ContextFormatColor RGB8) $ do
-- Create buffers for the vertices
vertexBuffer :: Buffer os (B4 Float, B3 Float) <- newBuffer (length pointsWithColor)
writeBuffer vertexBuffer 0 pointsWithColor
-- Create a buffer for the uniform values
uniformBuffer :: Buffer os (Uniform (V4 (B4 Float))) <- newBuffer 1
shader <- compileShader $ do
-- Vertex shader
primitiveStream <- toPrimitiveStream primitives
modelViewProj <- getUniform (const (uniformBuffer, 0))
let projPrimitiveStream = proj modelViewProj <$> primitiveStream
-- Fragment shader
fragmentStream <- rasterize rasterOptions projPrimitiveStream
let colorOption = ContextColorOption NoBlending (pure True)
drawContextColor (const colorOption) fragmentStream
-- Run the loop
loop vertexBuffer shader uniformBuffer
loop vertexBuffer shader uniformBuffer = do
-- Construct the ModelViewProjection matrix
size@(V2 w h) <- getContextBuffersSize
let halfWidth = (fromIntegral w)/2.0
halfHeight = (fromIntegral h)/2.0
projMat = ortho (-halfWidth) halfWidth halfHeight (-halfHeight) -1.0 1.0
writeBuffer uniformBuffer 0 [projMat]
render $ do
clearContextColor 1 -- White background
clearContextDepth 1 -- Far plane
vertexArray <- newVertexArray vertexBuffer
let primitiveArray = toPrimitiveArray TriangleList vertexArray
shader $ ShaderEnvironment primitiveArray (FrontAndBack, ViewPort 0 size, DepthRange 0 1)
swapContextBuffers
closeRequested <- GLFW.windowShouldClose
unless closeRequested $ loop vertexBuffer shader uniformBuffer
data ShaderEnvironment = ShaderEnvironment
{ primitives :: PrimitiveArray Triangles (B4 Float, B3 Float)
, rasterOptions :: (Side, ViewPort, DepthRange)
}
-- Projects the position of the vertices with the ModelViewProjection matrix
proj :: (V4 (V4 VFloat)) -> (V4 VFloat, V3 VFloat) -> (V4 VFloat, V3 VFloat)
proj modelViewProj (p,c) = (modelViewProj !* p, c)
但是，如果我尝试编译我的代码，我将收到以下错误消息：
Main.hs:58:31:
Couldn't match type ‘Color c0 (S F (ColorElement c0))’
with ‘V3 FFloat’
The type variable ‘c0’ is ambiguous
Expected type: Shader
os
(ContextFormat c0 ds0)
ShaderEnvironment
(FragmentStream (FragColor c0))
Actual type: Shader
os
(ContextFormat c0 ds0)
ShaderEnvironment
(FragmentStream (FragmentFormat (V3 VFloat)))
In a stmt of a 'do' block:
fragmentStream <- rasterize rasterOptions projPrimitiveStream
In the second argument of ‘($)’, namely
‘do { primitiveStream <- toPrimitiveStream primitives;
modelViewProj <- getUniform (const (uniformBuffer, 0));
let projPrimitiveStream = proj modelViewProj <$> primitiveStream;
fragmentStream <- rasterize rasterOptions projPrimitiveStream;
.... }’
Main.hs:59:62:
Couldn't match type ‘Color c0 Bool’ with ‘f0 Bool’
The type variables ‘f0’, ‘c0’ are ambiguous
Expected type: ColorMask c0
Actual type: f0 Bool
Relevant bindings include
colorOption :: ContextColorOption c0 (bound at Main.hs:59:17)
fragmentStream :: FragmentStream (FragColor c0)
(bound at Main.hs:58:13)
In the second argument of ‘ContextColorOption’, namely
‘(pure True)’
In the expression: ContextColorOption NoBlending (pure True)
Main.hs:65:1:
Could not deduce (Fractional a0)
from the context (Fractional a,
Fractional a1,
Fractional (a1 -> a -> a -> M44 a),
Num (Color c Float),
Num (a -> a -> M44 a),
DepthRenderable ds,
ContextColorFormat c,
Control.Monad.IO.Class.MonadIO m,
Control.Monad.Exception.MonadException m,
HostFormat b ~ (a -> a -> M44 a))
bound by the inferred type for ‘loop’:
(Fractional a, Fractional a1, Fractional (a1 -> a -> a -> M44 a),
Num (Color c Float), Num (a -> a -> M44 a), DepthRenderable ds,
ContextColorFormat c, Control.Monad.IO.Class.MonadIO m,
Control.Monad.Exception.MonadException m,
HostFormat b ~ (a -> a -> M44 a)) =>
Buffer os (B4 Float, B3 Float)
-> (ShaderEnvironment -> Render os (ContextFormat c ds) ())
-> Buffer os b
-> ContextT GLFW.GLFWWindow os (ContextFormat c ds) m ()
at Main.hs:(65,1)-(83,66)
The type variable ‘a0’ is ambiguous
When checking that ‘loop’ has the inferred type
loop :: forall os (m :: * -> *) a a1 b c ds.
(Fractional a, Fractional a1, Fractional (a1 -> a -> a -> M44 a),
Num (Color c Float), Num (a -> a -> M44 a), DepthRenderable ds,
ContextColorFormat c, Control.Monad.IO.Class.MonadIO m,
Control.Monad.Exception.MonadException m,
HostFormat b ~ (a -> a -> M44 a)) =>
Buffer os (B4 Float, B3 Float)
-> (ShaderEnvironment -> Render os (ContextFormat c ds) ())
-> Buffer os b
-> ContextT GLFW.GLFWWindow os (ContextFormat c ds) m ()
Probable cause: the inferred type is ambiguous
我不明白为什么这些类型与发现的Hello World教程here不同。还有什么可以解决这个问题。
答案 0 :(得分：5)
当你遇到这样的错误时，一个好主意是首先通过添加类型注释来限制类型错误。像loop这样的顶级定义是很好的选择，所以在你的情况下添加：
loop :: Buffer os (B4 Float, B3 Float)
-> CompiledShader os (ContextFormat RGBFloat ()) ShaderEnvironment
-> Buffer os (Uniform (V4 (B4 Float)))
-> ContextT GLFW.GLFWWindow os (ContextFormat RGBFloat ()) IO ()
使用此类型注释可以显示真正的问题：
-1.0来电中遗忘了ortho周围的括号。clearContextDepth 1。修复这两个问题后，即使删除了loop的类型注释，模块也可以正常编译（但我建议你保留它）。