[GHC] #10068: Make the runtime reflection API for names, modules, locations more systematic

[GHC]

#10068: Make the runtime reflection API for names, modules, locations more
systematic
-------------------------------------+-------------------------------------
        Reporter:  simonpj           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.0.1
       Component:  Compiler          |              Version:  7.8.4
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
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Comment (by Ben Gamari <ben@…>):

 In [changeset:"bef2f03e4d56d88a7e9752a7afd6a0a35616da6c/ghc"
 bef2f03e/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="bef2f03e4d56d88a7e9752a7afd6a0a35616da6c"
 Generate Typeable info at definition sites

 This patch implements the idea floated in Trac #9858, namely that we
 should generate type-representation information at the data type
 declaration site, rather than when solving a Typeable constraint.

 However, this turned out quite a bit harder than I expected. I still
 think it's the right thing to do, and it's done now, but it was quite
 a struggle.

 See particularly

  * Note [Grand plan for Typeable] in TcTypeable (which is a new module)
  * Note [The overall promotion story] in DataCon (clarifies existing
 stuff)

 The most painful bit was that to generate Typeable instances (ie
 TyConRepName bindings) for every TyCon is tricky for types in ghc-prim
 etc:

  * We need to have enough data types around to *define* a TyCon
  * Many of these types are wired-in

 Also, to minimise the code generated for each data type, I wanted to
 generate pure data, not CAFs with unpackCString# stuff floating about.

 Performance
 ~~~~~~~~~~~
 Three perf/compiler tests start to allocate quite a bit more. This isn't
 surprising, because they all allocate zillions of data types, with
 practically no other code, esp. T1969

  * T3294:   GHC allocates 110% more (filed #11030 to track this)
  * T1969:   GHC allocates 30% more
  * T4801:   GHC allocates 14% more
  * T5321FD: GHC allocates 13% more
  * T783:    GHC allocates 12% more
  * T9675:   GHC allocates 12% more
  * T5642:   GHC allocates 10% more
  * T9961:   GHC allocates 6% more

  * T9203:   Program allocates 54% less

 I'm treating this as acceptable. The payoff comes in Typeable-heavy
 code.

 Remaining to do
 ~~~~~~~~~~~~~~~

  * I think that "TyCon" and "Module" are over-generic names to use for
    the runtime type representations used in GHC.Typeable. Better might be
    "TrTyCon" and "TrModule". But I have not yet done this

  * Add more info the the "TyCon" e.g. source location where it was
    defined

  * Use the new "Module" type to help with Trac Trac #10068

  * It would be possible to generate TyConRepName (ie Typeable
    instances) selectively rather than all the time. We'd need to persist
    the information in interface files. Lacking a motivating reason I have
    not done this, but it would not be difficult.

 Refactoring
 ~~~~~~~~~~~
 As is so often the case, I ended up refactoring more than I intended.
 In particular

  * In TyCon, a type *family* (whether type or data) is repesented by a
    FamilyTyCon
      * a algebraic data type (including data/newtype instances) is
        represented by AlgTyCon This wasn't true before; a data family
        was represented as an AlgTyCon. There are some corresponding
        changes in IfaceSyn.

      * Also get rid of the (unhelpfully named) tyConParent.

  * In TyCon define 'Promoted', isomorphic to Maybe, used when things are
    optionally promoted; and use it elsewhere in GHC.

  * Cleanup handling of knownKeyNames

  * Each TyCon, including promoted TyCons, contains its TyConRepName, if
    it has one. This is, in effect, the name of its Typeable instance.

 Requires update of the haddock submodule.

 Differential Revision: https://phabricator.haskell.org/D757
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10068#comment:16>
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