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 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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