Panic when using syb with GHC API

[Ranjit Jhala]

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
>> 
>> _______________________________________________
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>> Glasgow-haskell-users at haskell.org
>> http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
>> 
> 
> 
> 
> -- 
> Push the envelope. Watch it bend.
> 
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