[GHC] #15039: Bizarre pretty-printing of inferred Coercible constraint in partial type signature

[GHC]

#15039: Bizarre pretty-printing of inferred Coercible constraint in partial type
signature
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.4.1
  checker)                           |             Keywords:
      Resolution:                    |  PartialTypeSignatures
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Poor/confusing    |  Unknown/Multiple
  error message                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 #15039

 I know what is going on here.  When we first introduced explicit
 equalities
 Richard arranged to make the pretty-printer conceal some of the menagerie,
 with some ad-hoc rules sketched in `IfaceType`:
 {{{
 Note [Equality predicates in IfaceType]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 GHC has several varieties of type equality (see Note [The equality types
 story]
 in TysPrim for details).  In an effort to avoid confusing users, we
 suppress
 the differences during "normal" pretty printing.  Specifically we display
 them
 like this:

  Predicate                         Pretty-printed as
                           Homogeneous case        Heterogeneous case
  ----------------        -----------------        -------------------
  (~)    eqTyCon                 ~                  N/A
  (~~)   heqTyCon                ~                  ~~
  (~#)   eqPrimTyCon             ~#                 ~~
  (~R#)  eqReprPrimTyCon         Coercible          Coercible

 By "homogeneeous case" we mean cases where a hetero-kinded equality
 (all but the first above) is actually applied to two identical kinds.
 Unfortunately, determining this from an IfaceType isn't possible since
 we can't see through type synonyms. Consequently, we need to record
 whether this particular application is homogeneous in IfaceTyConSort
 for the purposes of pretty-printing.

 All this suppresses information. To get the ground truth, use -dppr-debug
 (see 'print_eqs' in 'ppr_equality').

 See Note [The equality types story] in TysPrim.
 }}}
 There's a flag to control this: `-fprint-equality-relations`, and
 using that flag makes both oddities go away.

 In this particular case, although it displays `Coercible a b`, it is
 really pretty
 printing `a ~R# b`!  And that is why the kind looks wrong: it's the
 kind of `a ~R# b`.  So concealing the reality is jolly confusing here.

 Moreover, for reasons I don't understand, `-fprint-explicit-kinds`
 affects the behhaviour too, hence oddness (2).

 It's all in `IfaceType.ppr_equality`, which I reproduce below
 {{{
 ppr_equality :: TyPrec -> IfaceTyCon -> [IfaceType] -> Maybe SDoc
 ppr_equality ctxt_prec tc args
   | hetero_eq_tc
   , [k1, k2, t1, t2] <- args
   = Just $ print_equality (k1, k2, t1, t2)

   | hom_eq_tc
   , [k, t1, t2] <- args
   = Just $ print_equality (k, k, t1, t2)

   | otherwise
   = Nothing
   where
     homogeneous = case ifaceTyConSort $ ifaceTyConInfo tc of
                     IfaceEqualityTyCon -> True
                     _other             -> False
        -- True <=> a heterogeneous equality whose arguments
        --          are (in this case) of the same kind

     tc_name = ifaceTyConName tc
     pp = ppr_ty
     hom_eq_tc = tc_name `hasKey` eqTyConKey            -- (~)
     hetero_eq_tc = tc_name `hasKey` eqPrimTyConKey     -- (~#)
                 || tc_name `hasKey` eqReprPrimTyConKey -- (~R#)
                 || tc_name `hasKey` heqTyConKey        -- (~~)
     print_equality args =
         sdocWithDynFlags $ \dflags ->
         getPprStyle      $ \style  ->
         print_equality' args style dflags

     print_equality' (ki1, ki2, ty1, ty2) style dflags
       | print_eqs   -- No magic, just print the original TyCon
       = ppr_infix_eq (ppr tc)

       | hetero_eq_tc
       , print_kinds || not homogeneous
       = ppr_infix_eq (text "~~")

       | otherwise
       = if tc_name `hasKey` eqReprPrimTyConKey
         then pprIfacePrefixApp ctxt_prec (text "Coercible")
                                [pp TyConPrec ty1, pp TyConPrec ty2]
         else pprIfaceInfixApp ctxt_prec (char '~')
                  (pp TyOpPrec ty1) (pp TyOpPrec ty2)
       where
         ppr_infix_eq eq_op
            = pprIfaceInfixApp ctxt_prec eq_op
                  (parens (pp TopPrec ty1 <+> dcolon <+> pp TyOpPrec ki1))
                  (parens (pp TopPrec ty2 <+> dcolon <+> pp TyOpPrec ki2))

         print_kinds = gopt Opt_PrintExplicitKinds dflags
         print_eqs   = gopt Opt_PrintEqualityRelations dflags ||
                       dumpStyle style || debugStyle style

 }}}
 What to do?  I'm not sure.  But that's what is going on.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15039#comment:1>
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