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我被这个 GHC（8.4.3 版）类型检查错误难住了。这是我正在研究的 Haskell Servant 代码库的摘录。如果有人可以解释此消息的原因，我将不胜感激。为代码的长度道歉，但我无法进一步减少它。

{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}

module Api2 where

import Control.Lens (preview)
import Control.Lens.Cons
import Control.Monad.Error.Class
import Control.Monad.Except (runExceptT)
import Control.Monad.IO.Class (liftIO, MonadIO)
import Control.Monad.Trans.Maybe (MaybeT, runMaybeT)
import Crypto.JOSE.Compact
import Crypto.JOSE.Error
import Crypto.JWT (JWT, JWK, JWTError, verifyClaims, decodeCompact,
               defaultJWTValidationSettings, stringOrUri, ClaimsSet, 
               AsJWTError)
import Data.Aeson (decode, encode)
import Data.Aeson.Types
import Data.Maybe (fromJust)
import Data.Text as T
import Data.Text.Encoding (decodeUtf8, encodeUtf8)
import Data.Word8(isSpace)
import Network.Wai (Request, requestHeaders)
import Servant.API.Experimental.Auth (AuthProtect)
import Servant.Auth.Server (FromJWT, ToJWT)
import Servant.Server.Experimental.Auth (AuthHandler, AuthServerData, 
         mkAuthHandler)
import Servant.Server.Internal.ServantErr
import qualified Data.ByteString as BS
import qualified Data.ByteString.Lazy as L

data User = User

authHandler :: AuthHandler Request User
authHandler = mkAuthHandler authF2

authF2 :: (MonadIO m,
       MonadError ServantErr m) =>
      Request -> m User
authF2 req = do
  case lookup "Authorization" (requestHeaders req) of
    Nothing -> throwError err401
    Just authH -> do
      let (b, rest) = BS.break isSpace authH

    if T.toLower (decodeUtf8 b) == "bearer"
      then do
        claims <- liftIO $ verifyJwt "" (L.fromStrict rest) 
                   ("audience"::String)
        return $ User
      else throwError err401

verifyJwt
  :: forall e s. (Crypto.JWT.AsJWTError JWTError, (AsError e), 
                 AsJWTError e,
              Control.Lens.Cons.Cons s s Char Char, Monoid s) =>
     FilePath
     -> L.ByteString
     -> s
     -> IO (Either e Crypto.JWT.ClaimsSet)
verifyJwt jwkFilename jwtData aud = do
  let
    aud' = fromJust $ preview stringOrUri aud
    conf = defaultJWTValidationSettings (== aud')

  Just k <-  decode <$> L.readFile jwkFilename
  result <- runExceptT
    (decodeCompact jwtData >>= verifyClaims conf (k :: JWK))
  return result

当我在 GHC 中加载此文件时，我收到以下错误。

• Could not deduce (AsError e0) arising from a use of ‘verifyJwt’
  from the context: (MonadIO m, MonadError ServantErr m)
    bound by the type signature for:
               authF2 :: forall (m :: * -> *).
                         (MonadIO m, MonadError ServantErr m) =>
                         Request -> m User
    at servant-api-server/src/Api2.hs:(37,1)-(39,27)
  The type variable ‘e0’ is ambiguous
  These potential instances exist:
    instance AsError Crypto.JOSE.Error.Error
      -- Defined in ‘Crypto.JOSE.Error’
    instance AsError JWTError -- Defined in ‘Crypto.JWT’
• In the second argument of ‘($)’, namely
    ‘verifyJwt "" (L.fromStrict rest) ("audience" :: String)’
  In a stmt of a 'do' block:
    claims <- liftIO
                $ verifyJwt "" (L.fromStrict rest) ("audience" :: String)
  In the expression:
    do claims <- liftIO
                   $ verifyJwt "" (L.fromStrict rest) ("audience" :: String)
       return $ User
   |
48 |           claims <- liftIO $ verifyJwt "" (L.fromStrict rest) ("audience"::String)

|                              
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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1 回答 1

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当您调用 时verifyJwt :: forall e s. ...，GHC 确定您要将s类型变量实例化为，String但它无法确定将e类型变量实例化为什么。

您可以通过为 提供显式类型签名来解决此问题claims，尽管您必须启用ScopedTypeVariables扩展。您可以选择您想要的实例。AsError

例如，挑选e ~ Error：

authF2 :: (MonadIO m,
       MonadError ServantErr m) =>
      Request -> m User
authF2 req = do
  case lookup "Authorization" (requestHeaders req) of
    Nothing -> throwError err401
    Just authH -> do
      let (b, rest) = BS.break isSpace authH

    if T.toLower (decodeUtf8 b) == "bearer"
      then do
        (claims :: Either Error Crypto.JWT.ClaimsSet) <- liftIO $ verifyJwt ""  (L.fromStrict rest) ("audience"::String)
        return $ User
      else throwError err401

与类型错误无关，但claims在Left返回User?

于 2018-08-23T06:53:54.247 回答