How to catch errors/unsolved constraints from GHC type checker?

[Andreas Abel]

Hi, I am stuck fixing issue #9582

   https://ghc.haskell.org/trac/ghc/ticket/9582

I need to test whether one type is a subtype of another and throw a 
custom error if this is not the case.  I tried

   (errMsgs, result) <- tryTcErrs $
     tcSubType ctOrigin userTypeCtxt local_meth_ty sig_ty

   case result of
     Just _coercion -> return ()
     Nothing -> badInstSigErr sel_name local_meth_ty

However, it does not seem to do what I expect.  It acts as if I had not 
tried to catch the error, it acts as just

   tcSubType ctOrigin userTypeCtxt local_meth_ty sig_ty

My hypothetical explanations:

   1. 'tryTcErrs' is not the correct function to catch errors in the 
type checker.  Or,

   2. 'tcSubType' does not generate errors, just postponed constraints, 
which are checked later in a clean-up phase.  Thus, I do not get an 
error now I could handle, but the error is generated later during the 
constraint solving phase, where I do not have a handler in place.  In 
this case, I would need something like Agda's noConstraints,

   tryTcErrs $ noConstraints $ tcSubType...

which forces 'tcSubType' to check without generating new unification 
constraints.

Any advice how to do is or where to continue searching?

Thanks,
Andreas


-- 
Andreas Abel  <><      Du bist der geliebte Mensch.

Department of Computer Science and Engineering
Chalmers and Gothenburg University, Sweden

andreas.abel at gu.se
http://www2.tcs.ifi.lmu.de/~abel/