diff --git a/compiler/typecheck/TcTyClsDecls.hs b/compiler/typecheck/TcTyClsDecls.hs
index fd032f85301d9a2cf6de13e0c5c02cc750061dcf..5cbc0780159c05275afcb7e768250b3c21755c7d 100644
--- a/compiler/typecheck/TcTyClsDecls.hs
+++ b/compiler/typecheck/TcTyClsDecls.hs
@@ -913,8 +913,8 @@ Then:
* During TcHsType.tcTyVar we look in the *local* env, to get the
fully-known, not knot-tied TcTyCon for T.
- * Then, in TcHsSyn.zonkTcTypeToType (and zonkTcTyCon in particular) we look in
- the *global* env to get the TyCon.
+ * Then, in TcHsSyn.zonkTcTypeToType (and zonkTcTyCon in particular)
+ we look in the *global* env to get the TyCon.
This fancy footwork (with two bindings for T) is only necessary for the
TyCons or Classes of this recursive group. Earlier, finished groups,
@@ -929,19 +929,19 @@ is done by establishing an "initial kind", which is a rather uninformed
guess at a tycon's kind (by counting arguments, mainly) and then
using this initial kind for recursive occurrences.
-The initial kind is stored in exactly the same way during kind-checking
-as it is during type-checking (Note [Type checking recursive type and class
-declarations]): in the *local* environment, with ATcTyCon. But we still
-must store *something* in the *global* environment. Even though we
-discard the result of kind-checking, we sometimes need to produce error
-messages. These error messages will want to refer to the tycons being
-checked, except that they don't exist yet, and it would be Terribly
-Annoying to get the error messages to refer back to HsSyn. So we
-create a TcTyCon and put it in the global env. This tycon can
-print out its name and knows its kind,
-but any other action taken on it will panic. Note
-that TcTyCons are *not* knot-tied, unlike the rather valid but
-knot-tied ones that occur during type-checking.
+The initial kind is stored in exactly the same way during
+kind-checking as it is during type-checking (Note [Type checking
+recursive type and class declarations]): in the *local* environment,
+with ATcTyCon. But we still must store *something* in the *global*
+environment. Even though we discard the result of kind-checking, we
+sometimes need to produce error messages. These error messages will
+want to refer to the tycons being checked, except that they don't
+exist yet, and it would be Terribly Annoying to get the error messages
+to refer back to HsSyn. So we create a TcTyCon and put it in the
+global env. This tycon can print out its name and knows its kind, but
+any other action taken on it will panic. Note that TcTyCons are *not*
+knot-tied, unlike the rather valid but knot-tied ones that occur
+during type-checking.
Note [Declarations for wired-in things]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -1482,12 +1482,15 @@ tcTyFamInstEqn fam_tc mb_clsinfo
tcFamTyPats fam_tc mb_clsinfo tv_names pats
(kcTyFamEqnRhs mb_clsinfo hs_ty) $
\tvs pats res_kind ->
- do { rhs_ty <- solveEqualities $ tcCheckLHsType hs_ty res_kind
-
+ do { traceTc "tcTyFamInstEqn {" (ppr eqn_tc_name <+> ppr pats)
+ ; rhs_ty <- solveEqualities $ tcCheckLHsType hs_ty res_kind
+ ; traceTc "tcTyFamInstEqn 1" (ppr eqn_tc_name <+> ppr pats)
; (ze, tvs') <- zonkTyBndrsX emptyZonkEnv tvs
+ ; traceTc "tcTyFamInstEqn 2" (ppr eqn_tc_name <+> ppr pats)
; pats' <- zonkTcTypeToTypes ze pats
+ ; traceTc "tcTyFamInstEqn 3" (ppr eqn_tc_name <+> ppr pats $$ ppr rhs_ty)
; rhs_ty' <- zonkTcTypeToType ze rhs_ty
- ; traceTc "tcTyFamInstEqn" (ppr fam_tc <+> pprTyVars tvs')
+ ; traceTc "tcTyFamInstEqn 4" (ppr fam_tc <+> pprTyVars tvs')
; return (mkCoAxBranch tvs' [] pats' rhs_ty'
(map (const Nominal) tvs')
loc) }
diff --git a/compiler/types/CoAxiom.hs b/compiler/types/CoAxiom.hs
index 63c21627c8d82b0031023c0caba2f013aa571079..7f578ec696fcffcb2c4163ae16716ebcbea08c66 100644
--- a/compiler/types/CoAxiom.hs
+++ b/compiler/types/CoAxiom.hs
@@ -222,6 +222,8 @@ data CoAxBranch
-- See Note [CoAxiom locations]
, cab_tvs :: [TyVar] -- Bound type variables; not necessarily fresh
-- See Note [CoAxBranch type variables]
+ -- May be eta-reduded; see FamInstEnv
+ -- Note [Eta reduction for data families]
, cab_cvs :: [CoVar] -- Bound coercion variables
-- Always empty, for now.
-- See Note [Constraints in patterns]
diff --git a/compiler/types/FamInstEnv.hs b/compiler/types/FamInstEnv.hs
index 45d0842f8a6dbf2fef3fa259355b2166c8d2cf66..a59f9a684e958f6eaecbe5ced139a6b8149edbd6 100644
--- a/compiler/types/FamInstEnv.hs
+++ b/compiler/types/FamInstEnv.hs
@@ -191,22 +191,31 @@ Solution: eta-reduce both axioms, thus:
Now
d' = d |> Monad (sym (ax2 ; ax1))
-This eta reduction happens for data instances as well as newtype
-instances. Here we want to eta-reduce the data family axiom.
-All this is done in TcInstDcls.tcDataFamInstDecl.
+----- Bottom line ------
-See also Note [Newtype eta] in TyCon.
+For a FamInst with fi_flavour = DataFamilyInst rep_tc,
+
+ - fi_tvs (and cab_tvs of its CoAxiom) may be shorter
+ than tyConTyVars of rep_tc.
-Bottom line:
- For a FamInst with fi_flavour = DataFamilyInst rep_tc,
- - fi_tvs may be shorter than tyConTyVars of rep_tc.
- fi_tys may be shorter than tyConArity of the family tycon
i.e. LHS is unsaturated
+
- fi_rhs will be (rep_tc fi_tvs)
i.e. RHS is un-saturated
- But when fi_flavour = SynFamilyInst,
+ - This eta reduction happens for data instances as well
+ as newtype instances. Here we want to eta-reduce the data family axiom.
+
+ - This eta-reduction is done in TcInstDcls.tcDataFamInstDecl.
+
+But when fi_flavour = SynFamilyInst,
- fi_tys has the exact arity of the family tycon
+
+
+(See also Note [Newtype eta] in TyCon. This is notionally separate
+and deals with the axiom connecting a newtype with its representation
+type; but it too is eta-reduced.)
-}
-- Obtain the axiom of a family instance
diff --git a/compiler/types/TyCon.hs b/compiler/types/TyCon.hs
index 0a02adf0b65313f187bf1af9637465aa0a3f7901..d5347fc534e074f512d1e6360d56e2cfff5c159a 100644
--- a/compiler/types/TyCon.hs
+++ b/compiler/types/TyCon.hs
@@ -1172,6 +1172,9 @@ so the coercion tycon CoT must have
kind: T ~ []
and arity: 0
+This eta-reduction is implemented in BuildTyCl.mkNewTyConRhs.
+
+
************************************************************************
* *
TyConRepName