let with no bindings in do notation causes parse error

[bucklereed]

$ cat Test.hs 
foo = do
  let
  Just 3

bar :: ()
bar = ()
$ hlint --version
HLint v2.0.9, (C) Neil Mitchell 2006-2017
$ hlint Test.hs
Test.hs:5:1: Error: Parse error
Found:
      Just 3
    
  > bar :: ()
    bar = ()
  

1 hint
$ ghci Test.hs 
GHCi, version 8.0.2: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Main             ( Test.hs, interpreted )
Ok, modules loaded: Main.

It's weird, but apparently valid according to GHC.

[ndmitchell]

Thanks for the report. I've raised it as an issue in the underlying haskell-src-exts parser at haskell-suite/haskell-src-exts#373.

[EdmundsEcho]

I hope this is a useful place to add my experience with hlint generating a parse error with a let statement. The code compiles with ghc.

tests = testSpec "The Measurements Node" $ do
  describe "Measurement" $ do
    it "Pull a measurement with the type `Rx`" $ do
      let root = Q.rootHandler h
      let query = [r| query {
          measurement(type:"Rx") { 
            measurementType
          }
      }
      |]
      response <- interpretAnonymousQuery @Q.RootQuery root query
      let expected =                                        <<<<<<<< Error: Parse error found
            object
            [ "data" .= object
              [ "measurement" .= object
                [ "measurementType" .= ("Rx" :: Text)
                ]
              ]
            ]
      toJSON (toValue response) `shouldBe` expected

The error is present with hlint versions as recent as 2.1.8

[ndmitchell]

Thanks @EdmundsEcho - probably a bug in the underlying parser HLint uses. I'll raise upstream.

[ndmitchell]

@EdmundsEcho actually an entirely different problem. You don't have QuasiQuotes in your .hs file at the top, so HLint parses with it disabled, and then it gives the error you see. If you rerun with -XQuasiQuotes it works and gives you 2 hints.

[ndmitchell]

@EdmundsEcho you also have to enable TypeApplications as well.

[EdmundsEcho]

@ndmitchell thank you for the follow-up. I tested out your idea. When I run:

stack exec -- hlint tests/ETL/Queries/SubjectTests.hs

I get the following:

I have the QuasiQuotes language pragma included in the the code being parsed, and in my package.yaml file. Does hlint parse the stack configuration file?

default-extensions:
  - NoImplicitPrelude
  - OverloadedStrings
  - ScopedTypeVariables
  - RecordWildCards
  - TypeApplications
  - DataKinds
  - TypeOperators
  - DeriveGeneric
  - QuasiQuotes     <<<
  - KindSignatures
  - StandaloneDeriving

[ndmitchell]

No, it does not parse things from the package.yaml file or any .cabal files. It probably could do, and probably should do, but doesn't yet. See #479 to track adding it.

[EdmundsEcho]

Understood re parsing the stack config file. If you defined a specific task to implement the feature, I would be willing to help implement the code if you like. - E

[ndmitchell]

Task #479 is to implement the feature. I've added some notes to that ticket about how I envisaged it being implemented (but I'm not overly wedded to it).

[ndmitchell]

Confirmed works with HEAD that doesn't use HSE for parsing, so fixed in the next release.