{-# RULE "op1@C[a]" forall a, d:C a.
op1 [a] (df_i d) = op1_i a d #-}
-* We want to inline the dictionary function itself as vigorously as we
- possibly can, so that we expose that dictionary constructor to
- selectors as much as poss. We don't actually inline it; rather, we
- use a Builtin RULE for the ClassOps (see MkId.mkDictSelId) to short
- circuit such applications. But the RULE only applies if it can "see"
- the dfun's DFunUnfolding.
-
+Note [Instances and loop breakers]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
* Note that df_i may be mutually recursive with both op1_i and op2_i.
It's crucial that df_i is not chosen as the loop breaker, even
though op1_i has a (user-specified) INLINE pragma.
a RULE (the magic ClassOp rule above), and RULES work inside InlineRule
unfoldings. See Note [RULEs enabled in SimplGently] in SimplUtils
+Note [ClassOp/DFun selection]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+One thing we see a lot is stuff like
+ op2 (df d1 d2)
+where 'op2' is a ClassOp and 'df' is DFun. Now, we could inline *both*
+'op2' and 'df' to get
+ case (MkD ($cop1 d1 d2) ($cop2 d1 d2) ... of
+ MkD _ op2 _ _ _ -> op2
+And that will reduce to ($cop2 d1 d2) which is what we wanted.
+
+But it's tricky to make this work in practice, because it requires us to
+inline both 'op2' and 'df'. But neither is keen to inline without having
+seen the other's result; and it's very easy to get code bloat (from the
+big intermediate) if you inline a bit too much.
+
+Instead we use a cunning trick.
+ * We arrange that 'df' and 'op2' NEVER inline.
+
+ * We arrange that 'df' is ALWAYS defined in the sylised form
+ df d1 d2 = MkD ($cop1 d1 d2) ($cop2 d1 d2) ...
+
+ * We give 'df' a magical unfolding (DFunUnfolding [$cop1, $cop2, ..])
+ that lists its methods.
+
+ * We make CoreUnfold.exprIsConApp_maybe spot a DFunUnfolding and return
+ a suitable constructor application -- inlining df "on the fly" as it
+ were.
+
+ * We give the ClassOp 'op2' a BuiltinRule that extracts the right piece
+ iff its argument satisfies exprIsConApp_maybe. This is done in
+ MkId mkDictSelId
+
+ * We make 'df' CONLIKE, so that shared uses stil match; eg
+ let d = df d1 d2
+ in ...(op2 d)...(op1 d)...
+
+Note [Single-method classes]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+If the class has just one method (or, more accurately, just one elemen
+of {superclasses + methods}), then we want a different strategy.
+
+ class C a where op :: a -> a
+ instance C a => C [a] where op = <blah>
+
+We translate the class decl into a newtype, which just gives
+a top-level axiom:
+
+ axiom Co:C a :: C a ~ (a->a)
+
+ op :: forall a. C a -> (a -> a)
+ op a d = d |> (Co:C a)
+
+ df :: forall a. C a => C [a]
+ {-# INLINE df #-}
+ df = $cop_list |> (forall a. C a -> (sym (Co:C a))
+
+ $cop_list :: forall a. C a => a -> a
+ $cop_list = <blah>
+
+So the ClassOp is just a cast; and so is the dictionary function.
+(The latter doesn't even have any lambdas.) We can inline both freely.
+No need for fancy BuiltIn rules. Indeed the BuiltinRule stuff does
+not work well for newtypes because it uses exprIsConApp_maybe.
+
Note [Subtle interaction of recursion and overlap]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
mk_app fun arg_id = L loc (HsApp fun (L loc (wrapId arg_wrapper arg_id)))
arg_wrapper = mkWpApps dfun_lam_vars <.> mkWpTyApps (mkTyVarTys inst_tyvars')
- dfun_id_w_fun = dfun_id
- `setIdUnfolding` mkDFunUnfolding dict_constr (sc_ids ++ meth_ids)
+ dfun_id_w_fun | isNewTyCon (classTyCon clas)
+ = dfun_id -- Just let the dfun inline; see Note [Single-method classes]
+ | otherwise
+ = dfun_id -- Do not inline; instead give it a magic DFunFunfolding
+ -- See Note [ClassOp/DFun selection]
+ `setIdUnfolding` mkDFunUnfolding dict_constr (sc_ids ++ meth_ids)
`setInlinePragma` dfunInlinePragma
main_bind = noLoc $ AbsBinds
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