pprRuntimeTrace

In the process of trying to de-lazify TypeReps, I found it somewhat helpful to write a simple mechanism for injecting runtime calls to Debug.Trace.trace in the desugarer. Why? When there is an infinite loop in one of the functions in Data.Typeable.Internal, trying to print out information about its arguments is likely to fall into an infinite loop. Oy! The function I came up with is

-- | Inject a trace message into the compiled program.
pprRuntimeTrace :: String   -- ^ header
                -> SDoc     -- ^ information to output
                -> CoreExpr -- ^ expression
                -> DsM CoreExpr
pprRuntimeTrace str doc expr = do
  traceId <- dsLookupGlobalId traceName
  unpackCStringId <- dsLookupGlobalId unpackCStringName
  dflags <- getDynFlags
  let message :: CoreExpr
      message = App (Var unpackCStringId) $
                Lit $ mkMachString $ showSDoc dflags (hang (text str) 4 doc)
  return $ mkApps (Var traceId) [Type (exprType expr), message, expr]

pprRuntimeTrace header doc expr will produce an expression that looks like trace (header+doc) expr.

I've put up Phab:D4162 to add this utility to DsMonad.