Question about Haskell AST

Jane Ren j2ren at
Mon Jan 24 18:20:21 CET 2011

Hi Simon,

That is exactly what I needed.  However, although I was able to get the patterns from the parse tree for test modules that I wrote, I was not able to get the parsetrees for the Haskell base library modules.  
For example, I am trying to use Data/List.hs as a test.  Here's the code

    defaultErrorHandler defaultDynFlags $ do
      runGhc (Just libdir) $ do
        dflags <- getSessionDynFlags
	setSessionDynFlags dflags
        target <- guessTarget targetFile Nothing
        setTargets [target]
        load LoadAllTargets
        modSum <- getModSummary $ mkModuleName "Data.List"

When I try this, I get 
"AstWalker: panic! (the 'impossible' happened)
  (GHC version 7.0.1 for x86_64-apple-darwin):
	lexical error at character 'i'

It appears this error comes from "load LoadAllTargets"

Any ideas how I can get parse trees for the Haskell base modules?

Sure, I can augment that wiki page.

From: Simon Peyton-Jones [simonpj at]
Sent: Tuesday, January 11, 2011 12:06 AM
To: Jane Ren; glasgow-haskell-users at
Subject: RE: Question about Haskell AST

desugarModule returns a GHC.DesugaredModule
Inside a DesugaredModule is a field dm_core_module :: HscTypes.ModGuts
Inside a ModGuts is a field mg_binds :: [CoreSyn.CoreBind]

And there are your bindings!  Does that tell you what you wanted to know?


PS: When you have it clear, would you like to augment the Wiki to describe what you learned?  That way others can benefit.

| -----Original Message-----
| From: glasgow-haskell-users-bounces at [mailto:glasgow-haskell-
| users-bounces at] On Behalf Of Jane Ren
| Sent: 10 January 2011 17:21
| To: glasgow-haskell-users at
| Subject: Question about Haskell AST
| Hi,
| I need to be able to take a piece of Haskell source code and get an
| simplified, typed, intermediate representation of the AST, which means I need
| to use compiler/coreSyn/CoreSyn.lhs
| So I'm first trying to get the desguaredModule of the source code with
|         ...
|         modSum <- getModSummary $ mkModuleName "..."
|         p <- parseModule modSum
|         t <- typecheckModule p
|         d <- desugarModule t
| Now I'm really stuck on figuring out how to connect the variable d of type
| desugaredModule to compiler/coreSyn/CoreSyn.lhs to get Expr patterns like
| App, Let, Case, etc.
| Also, is it correct to get the deguaredModule first?  At least CoreSyn.lhs
| seems to suggest this.
| Any suggestions would be greatly apprecia
| _______________________________________________
| Glasgow-haskell-users mailing list
| Glasgow-haskell-users at

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