[x,y] = mkTemplateLocals [alphaTy, openBetaTy]
rhs = mkLams [alphaTyVar,openBetaTyVar,x,y] (Case (Var x) x openBetaTy [(DEFAULT, [], Var y)])
+ -- See Note [Built-in RULES for seq]
seq_cast_rule = BuiltinRule { ru_name = fsLit "seq of cast"
, ru_fn = seqName
, ru_nargs = 4
, ru_try = match_seq_of_cast
}
-match_seq_of_cast :: [CoreExpr] -> Maybe CoreExpr -- Note [RULES for seq]
+match_seq_of_cast :: [CoreExpr] -> Maybe CoreExpr
+ -- See Note [Built-in RULES for seq]
match_seq_of_cast [Type _, Type res_ty, Cast scrut co, expr]
= Just (Var seqId `mkApps` [Type (fst (coercionKind co)), Type res_ty,
scrut, expr])
c) It has quite a bit of desugaring magic.
See DsUtils.lhs Note [Desugaring seq (1)] and (2) and (3)
-d) There is some special rule handing: Note [RULES for seq]
+d) There is some special rule handing: Note [User-defined RULES for seq]
-Note [RULES for seq]
-~~~~~~~~~~~~~~~~~~~~
+Note [User-defined RULES for seq]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Roman found situations where he had
case (f n) of _ -> e
where he knew that f (which was strict in n) would terminate if n did.
done in Note [seqId magic] item (c) is *not* done on the LHS of a rule.
Or rather, we arrange to un-do it, in DsBinds.decomposeRuleLhs.
-We also have the following builtin rule:
+Note [Built-in RULES for seq]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+We also have the following built-in rule for seq
seq (x `cast` co) y = seq x y
This eliminates unnecessary casts and also allows other seq rules to
-match more often.
+match more often. Notably,
+
+ seq (f x `cast` co) y --> seq (f x) y
+
+and now a user-defined rule for seq (see Note [User-defined RULES for seq])
+may fire.
+
Note [lazyId magic]
~~~~~~~~~~~~~~~~~~~
rebuildCase env scrut case_bndr alts@[(_, bndrs, rhs)] cont
| all isDeadBinder (case_bndr : bndrs) -- So this is just 'seq'
- = -- For this case, see Note [RULES for seq] in MkId
+ = -- For this case, see Note [User-defined RULES for seq] in MkId
do { let rhs' = substExpr env rhs
out_args = [Type (substTy env (idType case_bndr)),
Type (exprType rhs'), scrut, rhs']
I# x# -> let x = x' `cast` sym co
in rhs
-so that 'rhs' can take advantage of the form of x'. Notice that Note
-[Case of cast] may then apply to the result. We only do this if x is actually
-used in the rhs. There is no point in adding the cast if this is really just a
-seq and doing so would interfere with seq rules (Note [RULES for seq]), in
-particular with the one that removes casts.
-
-This showed up in Roman's experiments. Example:
+so that 'rhs' can take advantage of the form of x'.
+
+Notice that Note [Case of cast] may then apply to the result.
+
+Nota Bene: We only do the [Improving seq] transformation if the
+case binder 'x' is actually used in the rhs; that is, if the case
+is *not* a *pure* seq.
+ a) There is no point in adding the cast to a pure seq.
+ b) There is a good reason not to: doing so would interfere
+ with seq rules (Note [Built-in RULES for seq] in MkId).
+ In particular, this [Improving seq] thing *adds* a cast
+ while [Built-in RULES for seq] *removes* one, so they
+ just flip-flop.
+
+You might worry about
+ case v of x { __DEFAULT ->
+ ... case (v `cast` co) of y { I# -> ... }}
+This is a pure seq (since x is unused), so [Improving seq] won't happen.
+But it's ok: the simplifier will replace 'v' by 'x' in the rhs to get
+ case v of x { __DEFAULT ->
+ ... case (x `cast` co) of y { I# -> ... }}
+Now the outer case is not a pure seq, so [Improving seq] will happen,
+and then the inner case will disappear.
+
+The need for [Improving seq] showed up in Roman's experiments. Example:
foo :: F Int -> Int -> Int
foo t n = t `seq` bar n
where
Here we'd like to avoid repeated evaluating t inside the loop, by
taking advantage of the `seq`.
-At one point I did transformation in LiberateCase, but it's more robust here.
-(Otherwise, there's a danger that we'll simply drop the 'seq' altogether, before
-LiberateCase gets to see it.)
-
-
+At one point I did transformation in LiberateCase, but it's more
+robust here. (Otherwise, there's a danger that we'll simply drop the
+'seq' altogether, before LiberateCase gets to see it.)
\begin{code}
-> SimplM (SimplEnv, OutExpr, OutId)
-- Note [Improving seq]
improveSeq fam_envs env scrut case_bndr case_bndr1 [(DEFAULT,_,_)]
- | not (isDeadBinder case_bndr)
+ | not (isDeadBinder case_bndr) -- Not a pure seq! See the Note!
, Just (co, ty2) <- topNormaliseType fam_envs (idType case_bndr1)
= do { case_bndr2 <- newId (fsLit "nt") ty2
; let rhs = DoneEx (Var case_bndr2 `Cast` mkSymCoercion co)
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