[GHC] #12724: Be lazier about reducing type-function applications

17 Oct 2016

      #12724: Be lazier about reducing type-function applications
-------------------------------------+-------------------------------------
           Reporter:  simonpj        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Consider this:
 {{{
 f :: F 20 -> F 20
 f x = x
 }}}
 That ought to typecheck in a trice, right?  But if `F` is a type function,
 GHC currently (8.0) eagerly reduces `F 20` to normal form.  Let's say `F
 20` unltimately reduces to `Int`, after a long time.  Then we get
 {{{
    f = \x . (x |> g1) |> g2
 where
    g1 :: F 20 ~ Int
    g2 :: Int ~ F 20
 }}}
 Here both `g1` and `g2` are big coercions.  So we waste time reducing `F
 20` ''and'' we generate giant coercions by doing so.  Maybe the optimiser
 gets rid of them again; more probably not.  But it's clearly stupid.

 This is one reason that the program in #5030 typechecks so slowly.  We
 have
 {{{
 cnst :: Integer -> Either (Immediate DummyCPU) (RegVar DummyCPU)
 cnst x = Left (Const x)
 }}}
 and there is absolutely no need to reduce either argument of the `Either`
 to normal form.

 Richard and I have ideas about how to fix this, but I'm recording it in a
 ticket.
 and

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12724
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