Loading package GHC in GHCi

[Sean Seefried]

Hi Simon (and others),

I have managed to build package GHC *and* load it into GHCi.  Initially 
this did not work. When I loaded up ghci with the -package ghc flag I 
was assaulted with the following error message.

  GHCi runtime linker: fatal error: I found a duplicate definition for 
symbol
  _OutOfHeapHook
  whilst processing object file
  /Users/sseefried/cvs-ghc/PLUGGABLE/working/ghc/compiler/HSghc.o
  This could be caused by:
  * Loading two different object files which export the same symbol
  * Specifying the same object file twice on the GHCi command line
  * An incorrect `package.conf' entry, causing some object to be
  loaded twice.
  GHCi cannot safely continue in this situation. Exiting now. Sorry.

I tracked down this symbol in the GHC source and found it in 
ghc/compiler/parser/hschooks.c.  The purpose of redefining the 
functions within this file is apparently to improve the quality of the 
error messages. That is, the symbols generated are meant to override 
those in the RTS. Unfortunately GHCi doesn't like this at all. At the 
moment it prohibits loading a symbol that is already in the RTS, which 
seems very reasonable from a certain perspective - I can see that a 
duplicate symbol would usually be an error. Except that in the case we 
really *do* want to load it so that it overrides the old one.

The only solution I can come up with is a modification to the 
package.conf syntax so that one can specify symbols which are part of 
the RTS package and the package in question.  We could then modify the 
dynamic linker of GHC so that these symbols were removed from the RTS's 
symbol table.  The symbols would then be loaded back in again in with 
the package thus overriding the old symbols.

What do you think of this proposal? I haven't implemented it yet 
because the alternative - that of simply removing the conflicting 
symbols from package ghc might be what you want.  My temporary fix for 
getting package ghc to load into GHCi is just this - I strip the 
symbols that overlap between the RTS and package ghc.  However, this 
means that the code that was supposed to be overridden in the RTS is no 
longer being overridden.  Although this works this doesn't seem to be 
what we want.

Cheers,

Sean