+ Quasi-quoting
+%* *
+%************************************************************************
+
+Note [Quasi-quote overview]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The GHC "quasi-quote" extension is described by Geoff Mainland's paper
+"Why it's nice to be quoted: quasiquoting for Haskell" (Haskell
+Workshop 2007).
+
+Briefly, one writes
+ [:p| stuff |]
+and the arbitrary string "stuff" gets parsed by the parser 'p', whose
+type should be Language.Haskell.TH.Quote.QuasiQuoter. 'p' must be
+defined in another module, because we are going to run it here. It's
+a bit like a TH splice:
+ $(p "stuff")
+
+However, you can do this in patterns as well as terms. Becuase of this,
+the splice is run by the *renamer* rather than the type checker.
+
+\begin{code}
+runQuasiQuote :: Outputable hs_syn
+ => HsQuasiQuote Name -- Contains term of type QuasiQuoter, and the String
+ -> Name -- Of type QuasiQuoter -> String -> Q th_syn
+ -> String -- Documentation string only
+ -> Name -- Name of th_syn type
+ -> (SrcSpan -> th_syn -> Either Message hs_syn)
+ -> TcM hs_syn
+runQuasiQuote (HsQuasiQuote _name quoter q_span quote) quote_selector desc meta_ty convert
+ = do { -- Check that the quoter is not locally defined, otherwise the TH
+ -- machinery will not be able to run the quasiquote.
+ ; this_mod <- getModule
+ ; let is_local = case nameModule_maybe quoter of
+ Just mod | mod == this_mod -> True
+ | otherwise -> False
+ Nothing -> True
+ ; traceTc (text "runQQ" <+> ppr quoter <+> ppr is_local)
+ ; checkTc (not is_local) (quoteStageError quoter)
+
+ -- Build the expression
+ ; let quoterExpr = L q_span $! HsVar $! quoter
+ ; let quoteExpr = L q_span $! HsLit $! HsString quote
+ ; let expr = L q_span $
+ HsApp (L q_span $
+ HsApp (L q_span (HsVar quote_selector)) quoterExpr) quoteExpr
+ ; recordThUse
+ ; meta_exp_ty <- tcMetaTy meta_ty
+
+ -- Typecheck the expression
+ ; zonked_q_expr <- tcTopSpliceExpr expr meta_exp_ty
+
+ -- Run the expression
+ ; traceTc (text "About to run" <+> ppr zonked_q_expr)
+ ; result <- runMeta convert zonked_q_expr
+ ; traceTc (text "Got result" <+> ppr result)
+ ; showSplice desc zonked_q_expr (ppr result)
+ ; return result
+ }
+
+runQuasiQuoteExpr quasiquote
+ = runQuasiQuote quasiquote quoteExpName "expression" expQTyConName convertToHsExpr
+
+runQuasiQuotePat quasiquote
+ = runQuasiQuote quasiquote quotePatName "pattern" patQTyConName convertToPat
+
+quoteStageError :: Name -> SDoc
+quoteStageError quoter
+ = sep [ptext (sLit "GHC stage restriction:") <+> ppr quoter,
+ nest 2 (ptext (sLit "is used in a quasiquote, and must be imported, not defined locally"))]
+\end{code}
+
+
+%************************************************************************
+%* *