Add a -fprint-axiom-incomps option (#15546)

Supply branch incomps when building an IfaceClosedSynFamilyTyCon

`pprTyThing` now has access to incomps. This also causes them to be
written out to .hi files, but that doesn't pose an issue other than a
more faithful bijection between `tyThingToIfaceDecl` and `tcIfaceDecl`.

The machinery for displaying axiom incomps was already present but not
in use. Since this is now a thing that pops up in ghci's :info the
format was modified to look like a haskell comment.

Documentation and a test for the new feature included.

Test Plan: T15546

Reviewers: simonpj, bgamari, goldfire

Reviewed By: simonpj

Subscribers: rwbarton, carter

GHC Trac Issues: #15546

Differential Revision: https://phabricator.haskell.org/D5097

2 files changed, 50 insertions, 25 deletions

diff --git a/compiler/iface/IfaceSyn.hs b/compiler/iface/IfaceSyn.hs
index f284ae9373..7ed25b1927 100644
--- a/compiler/iface/IfaceSyn.hs
+++ b/compiler/iface/IfaceSyn.hs
@@ -47,6 +47,7 @@ import IfaceType
 import BinFingerprint
 import CoreSyn( IsOrphan, isOrphan )
 import PprCore()            -- Printing DFunArgs
+import DynFlags( gopt, GeneralFlag (Opt_PrintAxiomIncomps) )
 import Demand
 import Class
 import FieldLabel
@@ -66,7 +67,7 @@ import Binary
 import BooleanFormula ( BooleanFormula, pprBooleanFormula, isTrue )
 import Var( VarBndr(..), binderVar )
 import TyCon ( Role (..), Injectivity(..), tyConBndrVisArgFlag )
-import Util( dropList, filterByList )
+import Util( dropList, filterByList, notNull )
 import DataCon (SrcStrictness(..), SrcUnpackedness(..))
 import Lexeme (isLexSym)
 
@@ -545,6 +546,28 @@ to cross the separate compilation boundary.
 In general we retain all info that is left by CoreTidy.tidyLetBndr, since
 that is what is seen by importing module with --make
 
+Note [Displaying axiom incompatibilities]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+With -fprint-axiom-incomps we display which closed type family equations
+are incompatible with which. This information is sometimes necessary
+because GHC doesn't try equations in order: any equation can be used when
+all preceding equations that are incompatible with it do not apply.
+
+For example, the last "a && a = a" equation in Data.Type.Bool.&& is
+actually compatible with all previous equations, and can reduce at any
+time.
+
+This is displayed as:
+Prelude> :i Data.Type.Equality.==
+type family (==) (a :: k) (b :: k) :: Bool
+  where
+      (==) (f a) (g b) = (f == g) && (a == b)
+      (==) a a = 'True
+          -- incompatible indices: 0
+      (==) _1 _2 = 'False
+          -- incompatible indices: 1, 0
+The comment after an equation refers to all previous equations (0-indexed)
+that are incompatible with it.
 
 ************************************************************************
 *                                                                      *
@@ -571,13 +594,17 @@ pprAxBranch pp_tc (IfaceAxBranch { ifaxbTyVars = tvs
   = WARN( not (null _cvs), pp_tc $$ ppr _cvs )
     hang ppr_binders 2 (hang pp_lhs 2 (equals <+> ppr rhs))
     $+$
-    nest 2 maybe_incomps
+    nest 6 maybe_incomps
   where
     -- See Note [Printing foralls in type family instances] in IfaceType
     ppr_binders = pprUserIfaceForAll $ map (mkIfaceForAllTvBndr Specified) tvs
     pp_lhs = hang pp_tc 2 (pprParendIfaceAppArgs pat_tys)
-    maybe_incomps = ppUnless (null incomps) $ parens $
-                    text "incompatible indices:" <+> ppr incomps
+
+    -- See Note [Displaying axiom incompatibilities]
+    maybe_incomps
+      = sdocWithDynFlags $ \dflags ->
+        ppWhen (gopt Opt_PrintAxiomIncomps dflags && notNull incomps) $
+        text "--" <+> text "incompatible indices:" <+> interpp'SP incomps
 
 instance Outputable IfaceAnnotation where
   ppr (IfaceAnnotation target value) = ppr target <+> colon <+> ppr value
diff --git a/compiler/iface/MkIface.hs b/compiler/iface/MkIface.hs
index 261a8bfca2..e3be840006 100644
--- a/compiler/iface/MkIface.hs
+++ b/compiler/iface/MkIface.hs
@@ -1804,33 +1804,30 @@ coAxiomToIfaceDecl ax@(CoAxiom { co_ax_tc = tycon, co_ax_branches = branches
  where
    branch_list = fromBranches branches
 
--- 2nd parameter is the list of branch LHSs, for conversion from incompatible branches
--- to incompatible indices
+-- 2nd parameter is the list of branch LHSs, in case of a closed type family,
+-- for conversion from incompatible branches to incompatible indices.
+-- For an open type family the list should be empty.
 -- See Note [Storing compatibility] in CoAxiom
 coAxBranchToIfaceBranch :: TyCon -> [[Type]] -> CoAxBranch -> IfaceAxBranch
 coAxBranchToIfaceBranch tc lhs_s
-                        branch@(CoAxBranch { cab_incomps = incomps })
-  = (coAxBranchToIfaceBranch' tc branch) { ifaxbIncomps = iface_incomps }
+                        (CoAxBranch { cab_tvs = tvs, cab_cvs = cvs
+                                    , cab_eta_tvs = eta_tvs
+                                    , cab_lhs = lhs, cab_roles = roles
+                                    , cab_rhs = rhs, cab_incomps = incomps })
+
+  = IfaceAxBranch { ifaxbTyVars  = toIfaceTvBndrs tvs
+                  , ifaxbCoVars  = map toIfaceIdBndr cvs
+                  , ifaxbEtaTyVars = toIfaceTvBndrs eta_tvs
+                  , ifaxbLHS     = toIfaceTcArgs tc lhs
+                  , ifaxbRoles   = roles
+                  , ifaxbRHS     = toIfaceType rhs
+                  , ifaxbIncomps = iface_incomps }
   where
     iface_incomps = map (expectJust "iface_incomps"
-                        . (flip findIndex lhs_s
-                          . eqTypes)
+                        . flip findIndex lhs_s
+                        . eqTypes
                         . coAxBranchLHS) incomps
 
--- use this one for standalone branches without incompatibles
-coAxBranchToIfaceBranch' :: TyCon -> CoAxBranch -> IfaceAxBranch
-coAxBranchToIfaceBranch' tc (CoAxBranch { cab_tvs = tvs, cab_cvs = cvs
-                                        , cab_eta_tvs = eta_tvs
-                                        , cab_lhs = lhs
-                                        , cab_roles = roles, cab_rhs = rhs })
-  = IfaceAxBranch { ifaxbTyVars    = toIfaceTvBndrs tvs
-                  , ifaxbCoVars    = map toIfaceIdBndr cvs
-                  , ifaxbEtaTyVars = toIfaceTvBndrs eta_tvs
-                  , ifaxbLHS       = toIfaceTcArgs tc lhs
-                  , ifaxbRoles     = roles
-                  , ifaxbRHS       = toIfaceType rhs
-                  , ifaxbIncomps   = [] }
-
 -----------------
 tyConToIfaceDecl :: TidyEnv -> TyCon -> (TidyEnv, IfaceDecl)
 -- We *do* tidy TyCons, because they are not (and cannot
@@ -1911,7 +1908,8 @@ tyConToIfaceDecl env tycon
     to_if_fam_flav (ClosedSynFamilyTyCon (Just ax))
       = IfaceClosedSynFamilyTyCon (Just (axn, ibr))
       where defs = fromBranches $ coAxiomBranches ax
-            ibr  = map (coAxBranchToIfaceBranch' tycon) defs
+            lhss = map coAxBranchLHS defs
+            ibr  = map (coAxBranchToIfaceBranch tycon lhss) defs
             axn  = coAxiomName ax
 
     ifaceConDecls (NewTyCon { data_con = con })    = IfNewTyCon  (ifaceConDecl con)