[GHC] #8776: Displaying pattern synonym for a GADT

[GHC]

#8776: Displaying pattern synonym for a GADT
-------------------------------------+------------------------------------
        Reporter:  monoidal          |            Owner:
            Type:  bug               |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.7
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
-------------------------------------+------------------------------------

Comment (by simonpj):

 Yes, this does pop up, and there is only half of an idiomatic solution.

  * When pretty-printing (a type, say), the idiomatic solution is not to
 "rename type variables on the fly", but rather to "tidy" the type (which
 gives each variable a distinct print-name), and then pretty-print it
 (without renaming).  Separate the two concerns. Functions like `tidyType`
 do this.

  * Alas, for type constructors, `TyCon`, tidying does not work well,
 because a `TyCon` includes `DataCon`s which include `Type`s, which mention
 `TyCon`s.  And tidying can't tidy a mutually recursive data structure
 graph, only trees.

  * One alternative would be to ensure that `TyCons` get type variables
 with distinct print-names.  That's ok for type variables but less easy for
 kind variables.  Processing data type declarations is already so
 complicated that I don't think it's sensible to add the extra requirement
 that it generates only "pretty" types and kinds.

  * One place the non-pretty names can show up is in GHCi.  But another is
 in interface files.  Look at `MkIface.tyThingToIfaceDecl` which converts a
 `TyThing` (i.e. `TyCon`, `Class` etc) to an `IfaceDecl`. '''And it does
 tidying as part of that conversion.'''  Why?  Because interface files
 contains fast-strings, not uniques, so the names must at least be
 distinct.  (You can see this happening for pattern synonyms in
 `patSynToIfaceDecl`.

  * SO MY PLAN is that the `:info` stuff in GHCi should work in two stages:
    * Use `tyThingToIfaceDecl` to convert the `TyThing` to an `IfaceDecl`
    * Pretty print that.

 Nothing very hard. It requires quite a bit of re-working in `PprTyThing`,
 but I think fairly routine.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8776#comment:6>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler