Friday, May 10, 2019

Handling GHC parser errors right

Handling GHC parser errors right

Did you know, a POk parse result from the GHC parser doesn't necessarily mean the parse was OK? This blog explains what's up with that. The source code below is from this example in the ghc-lib repo.

Here is code that tries to make a parse tree of a Haskell module.

parse :: String -> DynFlags -> String -> ParseResult (Located (HsModule GhcPs))
parse filename flags str =
  unP Parser.parseModule parseState
    location = mkRealSrcLoc (mkFastString filename) 1 1
    buffer = stringToStringBuffer str
    parseState = mkPState flags buffer location

The way to call the above code is like this.

case parse file flags s of
  PFailed s ->
    report flags $ snd (getMessages s flags)
  POk s m -> do
    report flags $ fst (getMessages s flags)
    analyzeModule flags m
In the PFailed s case (where s is the parse state), the expression snd (getMessages s flags) retrieves the errors and we report them. In the POk case, we report warnings and do whatever it is we wanted to do with the parse tree m right?

Not quite. The problem is that the parser produces two sorts of errors : "fatal" and "non-fatal". Thus far, we have only considered the "fatal" ones.

Fatal errors are such that production of a parse tree is impossible. Non-fatal parse errors are those that don't prevent construction of a parse tree. A parse that generates non-fatal errors is going to associate with a parse tree in some way non-conforming to the Haskell language specification.

The right way to write the POk case is like this.

POk s m -> do
  let (warns, errs) = getMessages s flags
  report flags warns
  report flags errs
  when (null errs) $ analyzeModule flags m
The key point is analyzeModule is called only if there are absolutely no parse errors at all.

A non-fatal error example is provided by the ImportQualifiedPost language extension (see this post for how to add a GHC language extension). Specifically, it is only legal to write import M qualified if the extension is in effect via pragma or the option -XImportQualifiedPost. In the event this syntax is used when the extension is not in effect, the user should see an error like

 test/MiniHlintTest_non_fatal_error.hs:6:18: error:
     Found `qualified' in postpositive position.
     To allow this, enable language extension 'ImportQualifiedPost'
and further analysis of the parse abandoned.