Re: TH: calling fail in Q monad gives uninformative error message, and error causes panic

On Wed, 2004-06-23 at 01:13, Duncan Coutts wrote:
> ghc makes it's own type checker monad TcM an instance of the Quasi class
> (which is rather cunning), however this means that if the user calls the
> Monad function 'fail' then the internal TcM fail method gets called.
> This monad (IOEnv) was never meant to fail (see
> ghc/compiler/utils/IOEnv.hs/) so the error message is:
> 
> Exception when trying to run compile-time code:
>         Code: genex_count
>         Exn: user error (IOEnv failure)

In the meantime (before a proper solution) here's a quick hack so you
can at least find the error message associated with the call to fail.

This patch is against recent ghc CVS:

===================================================================
RCS file: /cvs/fptools/ghc/compiler/utils/IOEnv.hs,v
retrieving revision 1.2
diff -C2 -r1.2 IOEnv.hs
*** IOEnv.hs    12 Jan 2004 15:47:53 -0000      1.2
--- IOEnv.hs    29 Jun 2004 15:30:10 -0000
***************
*** 45,49 ****
    (>>)   = thenM_
    return = returnM
!   fail s = failM      -- Ignore the string
  
  returnM :: a -> IOEnv env a
--- 45,49 ----
    (>>)   = thenM_
    return = returnM
!   fail s = failMWithMessage s
  
  returnM :: a -> IOEnv env a
***************
*** 60,63 ****
--- 60,65 ----
  failM = IOEnv (\ env -> ioError (userError "IOEnv failure"))
  
+ failMWithMessage :: String -> IOEnv env a
+ failMWithMessage msg = IOEnv (\ env -> ioError (userError $ "IOEnv failure: " 
++ msg))