Re: [GHC] #12874: Read/Show Incompatibility in base

22 Aug 2017

      #12874: Read/Show Incompatibility in base
-------------------------------------+-------------------------------------
        Reporter:  andrewthad        |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       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:D3871
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by Ryan Scott <ryan.gl.scott@…>):

 In [changeset:"8fd959998e900dffdb7f752fcd42df7aaedeae6e/ghc"
 8fd95999/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="8fd959998e900dffdb7f752fcd42df7aaedeae6e"
 Make the Read instance for Proxy (and friends) ignore precedence

 Summary:
 The `Read` instance for `Proxy`, as well as a handful of other data
 types in `base` which only have a single constructor, are doing something
 skeevy: they're requiring that they be surrounded by parentheses if the
 parsing
 precedence is sufficiently high. This means that `"Thing (Proxy)"` would
 parse,
 but not `"Thing Proxy"`. But the latter really ought to parse, since
 there's no
 need to surround a single constructor with parentheses. Indeed, that's the
 output of `show (Thing Proxy)`, so the current `Read` instance for `Proxy`
 violates `read . show = id`.

 The simple solution is to change `readParen (d > 10)` to `readParen False`
 in
 the `Read` instance for `Proxy`. But given that a derived `Read` instance
 would
 essentially accomplish the same thing, but with even fewer characters,
 I've
 opted to just replace the hand-rolled `Read` instance with a derived one.

 Test Plan: make test TEST=T12874

 Reviewers: ekmett, austin, hvr, goldfire, bgamari

 Reviewed By: bgamari

 Subscribers: rwbarton, thomie

 GHC Trac Issues: #12874

 Differential Revision: https://phabricator.haskell.org/D3871
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12874#comment:3>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler