[GHC] #12358: Read1/Read2 don't have methods defined in terms of ReadPrec

[GHC]

#12358: Read1/Read2 don't have methods defined in terms of ReadPrec
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:
            Type:  feature request   |               Status:  patch
        Priority:  normal            |            Milestone:  8.2.1
       Component:  libraries/base    |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D2379
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by RyanGlScott):

 * cc: core-libraries-committee@… (added)


Comment:

 Sorry, I should know by now to forward the CLC on things like this, seeing
 as how I'm on the CLC myself :) For now, I only have the above
 [https://mail.haskell.org/pipermail/libraries/2016-June/027102.html
 mailing list discussion] as evidence that this move is supported by the
 community.

 Anyways, adding `Read1` and `Read2` was decided in #11135 (associated
 mailing list discussion
 [https://mail.haskell.org/pipermail/libraries/2015-July/026014.html
 here]). `Read1/2` were originally defined in `transformers`, but migrated
 to `base` as part of an effort to bring in
 `Compose`/`Sum`/`Product`/`MonadIO` (and remove things from `transformers`
 that aren't actually, y'know, monad transformers).

 Ross's original definition of `Read1/2` uses `ReadS`, since that's what
 the report [https://www.haskell.org/onlinereport/basic.html#sect6.3.3
 dictates] the definition of `Read` should use, so that's what was migrated
 to `base`. It didn't occur to me at the time the problems that would pose
 when I began to work on [https://github.com/haskell-compat/deriving-
 compat/issues/3 automatic derivation] of `Read1/2` instances, since GHC's
 derived `Read` instances use `ReadPrec` instead of `ReadS`. This ticket
 just serves as a way to make the methods of `Read1/2` parallel to those of
 `Read`, and to eventually make deriving `Read1/2` easier.

 (P.S., if you're curious about whether kind polymorphism can obviate the
 need for multiple `Read(n)` classes, you might be interested in the
 discussion [https://github.com/haskell/deepseq/issues/8 here]. phadej has
 proposed one way to solve such a problem, although it involves some
 eyebrow-raising `-XTypeInType` tricks.)

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12358#comment:3>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler