Failed OccName lookup in RnEnv with Template Haskell Hack

Simon Peyton-Jones simonpj at
Fri Nov 3 04:51:50 EST 2006

| The problem is that the RdrNames carried in the HsExpr are not found
| in the Renamer environment. To give you an example
| foobar =
|  $(return (InternalE (HsVar (mkUnqual varName
| 				      (mkFastString "myfoo")))))
| which should in the overall result "foobar = myfoo"
| Running this gives me a "Not in scope" error but the myfoo variable is
| obviously in the global environment as running this with
| -ddump-rn-trace shows:

Well that is indeed puzzling, but you are doing something quite tricky
here.  In TH, a TH splice uses only data types and functions declared in
the TH library.  But your splices use data types declared in GHC itself.
I'm not quite sure how you make the compilation and linking stuff work
out, but you certainly need to take great care that the TH splice code
is uses the same data types, compiled in the same way, as the compiler
it's linked with.  Yes, it's hard to think about; I don't have it fully
paged in myself.

So the ice is thin.  I can't account for the behaviour you are seeing --
but I can't see how to help you more.  Perhaps do the HsVar and VarE
thing in the *same* expression (in a pair, say), and confirm that they
do generate the same HsExpr. Then start tracing the global envt lookup
and its results.


|  test.hs:49:7:
|     Not in scope: `myfoo{v}'
|     Global envt is:
|        [cut]
|        myfoo{v}: main:MyAdd.myfoo{v rI9} imported from MyAdd at
|        [cut]
| In contrast, the TH variant works without any flaws:
|  $(return (VarE (mkName "myfoo")))
| I assert that the VarE variant and the InternalE variant are converted
| to exactly the same HsExpr by hsSyn/Convert.lhs. Using -ddump-slices
| -dppr-debug shows that both splices generate "myfoo{v}".
| Using a few more hacks, I even made sure that the printed FastStrings
| "myfoo" are equal in terms of "getKey# (getUnique str)", this pretty
| much rules out a bug in UniqFM.hs.
| Does anyone (especially the developers that are intimate with TH and
| HsSyn) know what could be the cause? I spent twelve fruitless hours
| searching for the cause.
| Many thanks in advance,
| ---
| Fruhwirth Clemens -
| for robots: sp4mtrap at
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