Panic when using syb with GHC API

[Simon Peyton-Jones]

Feel free to propose better solutions.

The underlying issue is that before type checking GHC (obviously) doesn't know the types of things, while afterwards it does.  The whole HsSyn tree is parameterised over the types of identifiers:

  Parsed:       HsExpr RdrNames
  Renamed:      HsExpr Name
  Typechecked:  HsExpr Id

One alternative would be to parameterise the tree over the type of type-decorations, so instead of 'PostTcType' you'd have 'ty' (a variable) instead.  So we'd have

  Renamed:     HsExpr Name ()
  Typechecked: HsExpr Id   Type

To me this seems like a bit of a sledgehammer to crack a nut; and I think there are a couple of other similar things (like SyntaxExpr).  But it might be possible.

Another possibility would be for those PostTcTypes to be (Maybe Type), which would be less convenient when you know they are there.

Simon

| -----Original Message-----
| From: glasgow-haskell-users-bounces at haskell.org [mailto:glasgow-haskell-users-
| bounces at haskell.org] On Behalf Of Ranjit Jhala
| Sent: 25 August 2011 22:47
| To: Thomas Schilling
| Cc: glasgow-haskell-users at haskell.org
| Subject: Re: Panic when using syb with GHC API
| 
| Hi,
| 
| I ran into a similar issue earlier -- you might also look at this
| 
|    	http://mistuke.wordpress.com/category/vsx/
| 
| (also linked from http://haskell.org/haskellwiki/GHC/As_a_library#Links)
| 
| Hope to elaborate the text there one of these days...
| 
| Ranjit.
| 
| 
| On Aug 25, 2011, at 2:22 AM, Thomas Schilling wrote:
| 
| > GHC's parse tree contains lots of placeholders.  You are not supposed
| > to look at them until a specific phase has been run.  For example,
| > anything of type "SyntaxExpr" is an error thunk until the renamer has
| > been run.  Unfortunately, SyntaxExpr is just a type synonym, so
| > there's no way to distinguish them via SYB.
| >
| > The simplest workaround is to adapt the default traversal code for the
| > nodes which may contain such error thunks.  A better solution would be
| > to change the GHC AST to wrap such possibly undefined nodes with
| > newtypes, but that would only take effect once the next version of GHC
| > is released.
| >
| > On 24 August 2011 23:11, Simon Hengel <simon.hengel at wiktory.org> wrote:
| >> Hello,
| >> I'm trying to query a type-checked module with syb, this works for a
| >> plain binding.  But as soon as I add a type signature for that binding,
| >> I get an "panic!"
| >>
| >> I experienced similar problems with a renamed module.
| >>
| >> Are those data structures meant to be used with syb?  And if yes, what
| >> did I miss?
| >>
| >> Bellow is some code to reproduce my issue.  Any help is very much
| >> appreciated.
| >>
| >>    -- A.hs
| >>    module Main where
| >>
| >>    import GHC
| >>    import Outputable
| >>    import Data.Generics
| >>    import GHC.Paths (libdir)
| >>
| >>    import Bag
| >>
| >>    main :: IO ()
| >>    main = do
| >>      m <- parse
| >>      putStrLn $ showSDoc $ ppr $ m
| >>      putStrLn "\n---\n"
| >>      putStrLn $ showSDoc $ ppr $ selectAbsBinds m
| >>
| >>    parse = runGhc (Just libdir) $ do
| >>      _ <- getSessionDynFlags >>= setSessionDynFlags
| >>      target <- guessTarget "B.hs" Nothing
| >>      setTargets [target]
| >>      Succeeded <- load LoadAllTargets
| >>      modSum <- getModSummary $ mkModuleName "B"
| >>      m <- parseModule modSum >>= typecheckModule
| >>      return $ typecheckedSource m
| >>
| >>    selectAbsBinds :: GenericQ [HsBindLR Id Id]
| >>    selectAbsBinds = everything (++) ([] `mkQ` f)
| >>      where
| >>        f x@(AbsBinds _ _ _ _ _) = [x]
| >>        f _ = []
| >>
| >>
| >>    -- B.hs
| >>    module B where
| >>
| >>    foo :: Char
| >>    foo = 'f'
| >>
| >> Cheers,
| >> Simon
| >>
| >> _______________________________________________
| >> Glasgow-haskell-users mailing list
| >> Glasgow-haskell-users at haskell.org
| >> http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
| >>
| >
| >
| >
| > --
| > Push the envelope. Watch it bend.
| >
| > _______________________________________________
| > Glasgow-haskell-users mailing list
| > Glasgow-haskell-users at haskell.org
| > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
| 
| 
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