[GHC] #9908: Improve enumFromX support for OverloadedLists

[GHC]

#9908: Improve enumFromX support for OverloadedLists
-------------------------------------+-------------------------------------
       Reporter:  dfeuer             |                   Owner:  ekmett
           Type:  feature request    |                  Status:  new
       Priority:  normal             |               Milestone:  7.12.1
      Component:  Core Libraries     |                 Version:  7.9
       Keywords:                     |        Operating System:
   Architecture:  Unknown/Multiple   |  Unknown/Multiple
     Difficulty:  Unknown            |         Type of failure:
     Blocked By:                     |  None/Unknown
Related Tickets:                     |               Test Case:
                                     |                Blocking:
                                     |  Differential Revisions:
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 At present, `OverloadedLists` desugars `[m..n]` as `fromList (enumFromTo m
 n)`. This is sometimes okay, but terrible for `Data.Sequence`. In
 particular, I want, for example,

 {{{#!hs
 [m..n] :: Seq Int
 }}}

 to end up as `fromFunction (n-m+1) (+m)`, which performs much better.
 There are a few approaches that look reasonable:

 1. Try to catch `fromList (enumFromTo m n)` before `enumFromTo` becomes
 `eftInt`. This does not, unfortunately, seem to work.

 2. Grab `eftInt` after it gets written back from `eftIntFB`. This would
 probably work, but it's currently impossible because `eftInt` is not
 exported from `GHC.Exts` or even `GHC.Enum`.

 3. Add `enumFromTo`, etc., to the `IsList` class. Default to the current
 behavior, but if possible use `fromListN` for well-known `Enum` types.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9908>
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