Have addModFinalizer expose the local type environment.

Summary:
This annotates the splice point with 'HsSpliced ref e' where 'e' is the
result of the splice. 'ref' is a reference that the typechecker will fill with
the local type environment.

The finalizer then reads the ref and uses the local type environment, which
causes 'reify' to find local variables when run in the finalizer.

Test Plan: ./validate

Reviewers: simonpj, simonmar, bgamari, austin, goldfire

Reviewed By: goldfire

Subscribers: simonmar, thomie, mboes

Differential Revision: https://phabricator.haskell.org/D2286

GHC Trac Issues: #11832

1 files changed, 4 insertions, 0 deletions

diff --git a/libraries/template-haskell/Language/Haskell/TH/Syntax.hs b/libraries/template-haskell/Language/Haskell/TH/Syntax.hs
index dfcf471f1d..62bdd10aac 100644
--- a/libraries/template-haskell/Language/Haskell/TH/Syntax.hs
+++ b/libraries/template-haskell/Language/Haskell/TH/Syntax.hs
@@ -458,6 +458,10 @@ addTopDecls ds = Q (qAddTopDecls ds)
 
 -- | Add a finalizer that will run in the Q monad after the current module has
 -- been type checked. This only makes sense when run within a top-level splice.
+--
+-- The finalizer is given the local type environment at the splice point. Thus
+-- 'reify' is able to find the local definitions when executed inside the
+-- finalizer.
 addModFinalizer :: Q () -> Q ()
 addModFinalizer act = Q (qAddModFinalizer (unQ act))