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我跟随this walkthrough如何绘制到purescript中的HTML画布。不幸的是,我遇到了让代码工作的问题。我按照教程创建了下面的代码,但它不会编译:如何在没有有效计算的情况下返回一个Eff monad?
main = do
-- Canvas has type Maybe CanvasElement
canvas <- getCanvasElementById "canvas"
-- Type mismatch: CanvasElement expected, (Maybe CanvasElement) provided
context <- getContext2D canvas
runGraphics context $ do
setFillStyle "#00FFFF"
rect { x: 0, y: 0, w: 400, h: 600 }
fill
我试图通过执行以下操作来解决问题:
run Nothing = do return Unit -- Doesn't compile
run (Just canvas) = do
g2d <- GraphicsBuilder.getContext2D canvas
runGraphics g2d $ do
setFillStyle "#00FFFF"
rect { x: 0.0, y: 0.0, w: 400.0, h: 600.0 }
fill
main = do
mcanvas <- GraphicsBuilder.getCanvasElementById "canvas"
run mcanvas
我应该如何处理“跑没什么”案件?为了匹配类型签名,它需要返回一个Eff值,是否有一种简单的方法将计算包装在Eff monad中?
太棒了,谢谢! –