有什么正确的方法让模板haskell包装一个带有源信息的函数（例如行号） [英] what's the correct way to have template haskell wrap a function with source information (e.g. line number)

问题描述

假设我从一个函数开始

  fromJust Nothing = errorfrom just nothing nothing！ 
 fromJust（Just x）= x 
  

然后，我想通过Template添加源信息Haskell更好的错误消息。让我们想象一下，我可以为函数添加一个额外的参数。

  fromJust'loc Nothing = error $from Just'Nothing at+ +（loc_filename loc）
 fromJust'loc（Just x）= x 
  

一些 fromJust 我可以在源代码中使用的宏，如$ / $>

  x = $ fromJust $ Map.lookup km 
  

hack

通过使用quasiquotes并提取源文件名的字符串，我设法破解了它。看来 Loc 没有Lift实例。有没有更好的方法？

  fromJustErr'l（Nothing）= 
 error $ printf[internal] fromJust error \ 
 \\\\
（在文件％s中）l 
 fromJustErr'l（Just x）= x 
 fromJustErr = do 
l < -  location 
让fn = loc_filename l 
 fnl :: Q Exp = TH.lift fn 
 [| fromJustErr'$ fnl |] 
  

谢谢！

< （我知道通过 Maybe 函数可以比 fmap 更好，而不是使用 fromJust ，但我有时需要破解。）

解决方案

这里试图让这个模式更加可重用。
$ b 关键的想法是将一个自定义的错误传递给我们的函数，该函数将包含错误消息中的位置。您可以像这样使用它：

  fromJust'::（String  - > a） - >也许是 - >一个
 fromJust'的错误Nothing =错误from just nothing nothing！ 
 fromJust'error（Just x）= x 
 
 fromJust :: Q Exp 
 fromJust = withLocatedError [| from'''] 
  

使用此函数与您的原始方法类似：

  main = print（1 + $ fromJust Nothing）
  

$
$ b

  withLocatedError :: Q Exp  - >现在，对于使这项工作成功的模板Haskell：
 
 
  withLocatedError :: Q Exp  - > Q Exp 
 withLocatedError f = do 
 let error = locatedError =<<位置
 appE f错误
 
位置错误:: Loc  - > Q Exp 
 locatedError loc = do 
 let postfix =at++ formatLoc loc 
 [| \msg  - >错误（msg ++ $（litE $ stringL postfix））|] 
 
 formatLoc :: Loc  - > String 
 formatLoc loc = let file = loc_filename loc 
（line，col）= loc_start loc 
 in concat [file，：，show line，：，show col] 
  
  locatedError 产生定制的错误函数，给定一个位置。  withLocatedError 将此内容反馈给 fromJust'以将所有内容挂钩在一起。  formatLoc 只是很好地将位置格式化为一个字符串。
 
 
 
运行这个程序给了我们想要的结果：
  FromJustTest：fromJust nothing nothing！在FromJustTest.hs：5：19 
  
 
Suppose I start with a function
fromJust Nothing = error "fromJust got Nothing!"
fromJust (Just x) = x
Then, I want to add source information via Template Haskell for better error messages. Let's imagine that I could add an extra parameter to the function
fromJust' loc Nothing = error $ "fromJust got Nothing at " ++ (loc_filename loc)
fromJust' loc (Just x) = x
and then have some fromJust macro that I could use in source code like,
x = $fromJust $ Map.lookup k m


hack

I did manage to hack it, by using quasiquotes and lifting the string of the source filename. It seems that Loc doesn't have a Lift instance. Is there a better way?
fromJustErr' l (Nothing) =
    error $ printf "[internal] fromJust error\
        \\n    (in file %s)" l
fromJustErr' l (Just x) = x
fromJustErr = do
    l <- location
    let fn = loc_filename l
        fnl :: Q Exp = TH.lift fn
    [| fromJustErr' $fnl |]
Thanks!

(I know it's nicer to fmap stuff via the Maybe functor than use fromJust, but I need to hack sometimes.)
解决方案
Here's an attempt at making this pattern somewhat more reusable.

The key idea is to pass a customized error to our function which will include the location in the error message. You'd use it like this:
fromJust' :: (String -> a) -> Maybe a -> a
fromJust' error Nothing = error "fromJust got Nothing!"
fromJust' error (Just x) = x

fromJust :: Q Exp
fromJust = withLocatedError [| fromJust' |]
Using this function is similar to your original approach:
main = print (1 + $fromJust Nothing)
Now, for the Template Haskell that makes this work:
withLocatedError :: Q Exp -> Q Exp
withLocatedError f = do
    let error = locatedError =<< location
    appE f error

locatedError :: Loc -> Q Exp
locatedError loc = do
    let postfix = " at " ++ formatLoc loc
    [| \msg -> error (msg ++ $(litE $ stringL postfix)) |]

formatLoc :: Loc -> String
formatLoc loc = let file = loc_filename loc
                    (line, col) = loc_start loc
                in concat [file, ":", show line, ":", show col]
locatedError produces the customized error function, given a location. withLocatedError feeds this to fromJust' to hook everything together. formatLoc just formats the location nicely into a string.

Running this gives us the result we wanted:
FromJustTest: fromJust got Nothing! at FromJustTest.hs:5:19


                        
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