-- arising from specialisation pragmas
\end{code}
-
-We must do some gentle simplification on the template (but not the RHS)
-of each rule. The case that forced me to add this was the fold/build rule,
+Note [Simplifying the left-hand side of a RULE]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+We must do some gentle simplification on the lhs (template) of each
+rule. The case that forced me to add this was the fold/build rule,
which without simplification looked like:
fold k z (build (/\a. g a)) ==> ...
This doesn't match unless you do eta reduction on the build argument.
+Similarly for a LHS like
+ augment g (build h)
+we do not want to get
+ augment (\a. g a) (build h)
+otherwise we don't match when given an argument like
+ augment (\a. h a a) (build h)
\begin{code}
simplRule env rule@(BuiltinRule {})
-- alone leaves tons of crud.
-- Used (a) for user expressions typed in at the interactive prompt
-- (b) the LHS and RHS of a RULE
+-- (c) Template Haskell splices
--
-- The name 'Gently' suggests that the SimplifierMode is SimplGently,
-- and in fact that is so.... but the 'Gently' in simplExprGently doesn't
-- enforce that; it just simplifies the expression twice
+-- It's important that simplExprGently does eta reduction; see
+-- Note [Simplifying the left-hand side of a RULE] above. The
+-- simplifier does indeed do eta reduction (it's in Simplify.completeLam)
+-- but only if -O is on.
+
simplExprGently env expr = do
expr1 <- simplExpr env (occurAnalyseExpr expr)
simplExpr env (occurAnalyseExpr expr1)