Question

代码的出现意味着一段时间以来我第一次使用megaparsec。自上次以来，情况已有所改变。假设

import qualified Text.Megaparsec as MP
import qualified Text.Megaparsec.Char as C

我尝试了

MP.parseTest (C.char '+') "+"

GHCI中的

，它给出以下相当无用的错误消息：

interactive>:121:1: error:
* Ambiguous type variable `e0' arising from a use of `MP.parseTest'
  prevents the constraint `(MP.ShowErrorComponent
                              e0)' from being solved.
  Probable fix: use a type annotation to specify what `e0' should be.
  These potential instance exist:
    one instance involving out-of-scope types
    (use -fprint-potential-instances to see them all)
* In the expression: MP.parseTest (C.char '+') "+"
  In an equation for `it': it = MP.parseTest (C.char '+') "+"

<interactive>:121:15: error:
* Ambiguous type variable `e0' arising from a use of `C.char'
  prevents the constraint `(Ord e0)' from being solved.
  Probable fix: use a type annotation to specify what `e0' should be.
  These potential instances exist:
    instance (Ord a, Ord b) => Ord (Either a b)
      -- Defined in `Data.Either'
    instance Ord Ordering -- Defined in `ghc-prim-0.5.3:GHC.Classes'
    instance Ord Integer
      -- Defined in `integer-gmp-1.0.2.0:GHC.Integer.Type'
    ...plus 25 others
    ...plus 131 instances involving out-of-scope types
    (use -fprint-potential-instances to see them all)
* In the first argument of `MP.parseTest', namely `(C.char '+')'
  In the expression: MP.parseTest (C.char '+') "+"
  In an equation for `it': it = MP.parseTest (C.char '+') "+"

我正确使用了吗？我该怎么做才能解决此问题？

Answer 1

用户定义的最新版本的“ megaparsec”支持custom error types。 ParsecT类型由自定义错误的类型参数化。为了支持精美打印（例如parseTest），此类自定义错误必须是Ord和ShowErrorComponent类型类的实例。

如果我们从不抛出任何自定义错误，则错误类型将保持多态。但是最后，在打印结果时，我们必须提供一个具体的类型。我们可以使用“ void”包中的无人居住类型Void，并通过使用显式类型签名来通知类型检查器。

文档recommends定义了type Parser = Parsec Void Text之类的类型同义词，并在您的签名中使用它。

另一种避免歧义的方法是使用visible type application：

Prelude Text.Megaparsec Data.Void> :t parseTest
parseTest
  :: (ShowErrorComponent e, Show a, Stream s) =>
     Parsec e s a -> s -> IO ()
Prelude Text.Megaparsec Data.Void> :set -XTypeApplications
Prelude Text.Megaparsec Data.Void> :t parseTest @Void
parseTest @Void
  :: (Show a, Stream s) => Parsec Void s a -> s -> IO ()

在megaparsec 7.0.4中使用parseTest

1 个答案: