[GHC] #12973: No levity-polymorphic arguments

[GHC]

#12973: No levity-polymorphic arguments
-------------------------------------+-------------------------------------
        Reporter:  goldfire          |                Owner:  goldfire
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
                                     |  LevityPolymorphism
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by Richard Eisenberg <rae@…>):

 In [changeset:"e7985ed23ddc68b6a2e4af753578dc1d9e8ab4c9/ghc"
 e7985ed2/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="e7985ed23ddc68b6a2e4af753578dc1d9e8ab4c9"
 Update levity polymorphism

 This commit implements the proposal in
 https://github.com/ghc-proposals/ghc-proposals/pull/29 and
 https://github.com/ghc-proposals/ghc-proposals/pull/35.

 Here are some of the pieces of that proposal:

 * Some of RuntimeRep's constructors have been shortened.

 * TupleRep and SumRep are now parameterized over a list of RuntimeReps.
 * This
 means that two types with the same kind surely have the same
 representation.
 Previously, all unboxed tuples had the same kind, and thus the fact
 above was
 false.

 * RepType.typePrimRep and friends now return a *list* of PrimReps. These
 functions can now work successfully on unboxed tuples. This change is
 necessary because we allow abstraction over unboxed tuple types and so
 cannot
 always handle unboxed tuples specially as we did before.

 * We sometimes have to create an Id from a PrimRep. I thus split PtrRep
 * into
 LiftedRep and UnliftedRep, so that the created Ids have the right
 strictness.

 * The RepType.RepType type was removed, as it didn't seem to help with
 * much.

 * The RepType.repType function is also removed, in favor of typePrimRep.

 * I have waffled a good deal on whether or not to keep VoidRep in
 TyCon.PrimRep. In the end, I decided to keep it there. PrimRep is *not*
 represented in RuntimeRep, and typePrimRep will never return a list
 including
 VoidRep. But it's handy to have in, e.g., ByteCodeGen and friends. I can
 imagine another design choice where we have a PrimRepV type that is
 PrimRep
 with an extra constructor. That seemed to be a heavier design, though,
 and I'm
 not sure what the benefit would be.

 * The last, unused vestiges of # (unliftedTypeKind) have been removed.

 * There were several pretty-printing bugs that this change exposed;
 * these are fixed.

 * We previously checked for levity polymorphism in the types of binders.
 * But we
 also must exclude levity polymorphism in function arguments. This is
 hard to check
 for, requiring a good deal of care in the desugarer. See Note [Levity
 polymorphism
 checking] in DsMonad.

 * In order to efficiently check for levity polymorphism in functions, it
 * was necessary
 to add a new bit of IdInfo. See Note [Levity info] in IdInfo.

 * It is now safe for unlifted types to be unsaturated in Core. Core Lint
 * is updated
 accordingly.

 * We can only know strictness after zonking, so several checks around
 * strictness
 in the type-checker (checkStrictBinds, the check for unlifted variables
 under a ~
 pattern) have been moved to the desugarer.

 * Along the way, I improved the treatment of unlifted vs. banged
 * bindings. See
 Note [Strict binds checks] in DsBinds and #13075.

 * Now that we print type-checked source, we must be careful to print
 * ConLikes correctly.
 This is facilitated by a new HsConLikeOut constructor to HsExpr.
 Particularly troublesome
 are unlifted pattern synonyms that get an extra void# argument.

 * Includes a submodule update for haddock, getting rid of #.

 * New testcases:
   typecheck/should_fail/StrictBinds
   typecheck/should_fail/T12973
   typecheck/should_run/StrictPats
   typecheck/should_run/T12809
   typecheck/should_fail/T13105
   patsyn/should_fail/UnliftedPSBind
   typecheck/should_fail/LevPolyBounded
   typecheck/should_compile/T12987
   typecheck/should_compile/T11736

 * Fixed tickets:
   #12809
   #12973
   #11736
   #13075
   #12987

 * This also adds a test case for #13105. This test case is
 * "compile_fail" and
 succeeds, because I want the testsuite to monitor the error message.
 When #13105 is fixed, the test case will compile cleanly.
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12973#comment:4>
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