如何从Javascript与GHCJS调用Haskell [英] How to call Haskell from Javascript with GHCJS

问题描述

我一直在玩GHCJS。 FFI可以用来从Haskell调用JavaScript，但我无法弄清楚如何做到相反。假设我有一个超级有用的实用函数，我在Haskell中写道：

  sayHello :: String  - > IO（）
 sayHello name = print $hello，++ name 
  

可能做些什么，所以我可以从Javascript调用它？最近我注意到 h $ main（h $ main2CMainzimain）会触发我的Haskell主函数。

解决方案

这是一种使其工作的方法。假设我们有一些有用的函数，比如

  revString :: String  - >字符串
 revString =反向
 
 somethingUseful :: JSString  - > IO JSString 
 somethingUseful = return。 toJSString。 revString。 fromJSString 
  

为了导出，我们需要通过 *回调函数在 GHCJS.Foreign 中。但是这些会丢弃返回值，所以我们需要一个将结果放入第二个参数的包装：

  returnViaArgument ::（ JSRef a  - > IO（JSRef b）） - > JSRef a  - > JSRef c  - > IO（）
 returnViaArgument f arg retObj = do 
r < -  f arg 
 setPropretr retObj 
  

我的 main 函数创建回调函数，并将其保存为JavaScript的全局函数：

  foreign import javascript unsafesomethingUseful_ = $ 1
 js_set_somethingUseful :: JSFun a  - > IO（）
 
 main = do 
回调<  -  syncCallback2 NeverRetain False（returnViaArgument somethingUseful）
 js_set_somethingUseful回调
  

最后，我们需要在JS方面稍微解包一下：

  function somethingUseful（arg）{x = {}; somethingUseful_（arg，x）; return x.ret}; 
  

现在我们可以使用我们很好的Haskell实现的函数：

  somethingUseful（Hello World！）
！dlroW olleH
  

我在真实世界的应用程序中使用这个技巧。在 JsInterface.hs 中，它定义为<$ 可执行文件的c $ c> main-in /8fa98e59a7b4414d0a78d2e327394cee9a174591/logic/incredible-logic.cabalrel =noreferrer> Cabal file ， main 函数设置全局Java脚本变量 incredibleLogic _ ，而 JavaScript胶合代码负责打包和解包参数。

I've been playing about with GHCJS. The FFI can be used to call javascript from Haskell but I can't figure out how do go the other way round. Say I had a super useful utility function I wrote in Haskell:

sayHello :: String -> IO ()
sayHello name = print $ "hello, " ++ name

Is it possible do something so I could call it from Javascript? The closest I've got is noticing that h$main(h$main2CMainzimain) will trigger my Haskell main function.

解决方案

Here is a way to make it work. Assume we have some useful function, like

revString :: String -> String
revString = reverse

somethingUseful :: JSString -> IO JSString
somethingUseful = return . toJSString . revString  . fromJSString

In order to export that, we need to make it a callback via one of the *Callback functions in GHCJS.Foreign. But these would discard the return value, so we need a wrapper that puts the result into a second argument:

returnViaArgument :: (JSRef a -> IO (JSRef b)) -> JSRef a -> JSRef c -> IO ()
returnViaArgument f arg retObj = do
    r <- f arg
    setProp "ret" r retObj

My main function creates the callback, and saves it as something that’s global to JavaScript:

foreign import javascript unsafe "somethingUseful_ = $1"
    js_set_somethingUseful :: JSFun a -> IO ()

main = do
    callback <- syncCallback2 NeverRetain False (returnViaArgument somethingUseful)
    js_set_somethingUseful callback

Finally, we need a little un-wrapper on the JS side:

function somethingUseful (arg) {x = {}; somethingUseful_(arg, x); return x.ret};

and now we can use our nice Haskell-implemented function:

somethingUseful("Hello World!")
"!dlroW olleH"

I am using this trick in a real-world application. In JsInterface.hs, which is defined as main-in of the executable in the Cabal file, the main function sets the global java script variable incredibleLogic_, while the JavaScript glue code takes care of packing and unpacking the parameters.

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