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

1 files changed, 1 insertions, 0 deletions

diff --git a/libraries/base/tests/all.T b/libraries/base/tests/all.T
index d97d79afe3..970fb7e454 100644
--- a/libraries/base/tests/all.T
+++ b/libraries/base/tests/all.T
@@ -205,6 +205,7 @@ test('T12494', normal, compile_and_run, [''])
 test('T12852', when(opsys('mingw32'), skip), compile_and_run, [''])
 test('lazySTexamples', normal, compile_and_run, [''])
 test('T11760', normal, compile_and_run, ['-threaded -with-rtsopts=-N2'])
+test('T12874', normal, compile_and_run, [''])
 test('T13191',
         [ stats_num_field('bytes allocated',
                           [ (wordsize(64), 185943272, 5) ])