[GHC] #12665: Make Read instances for Integral types faster, and make them fail fast

[GHC]

#12665: Make Read instances for Integral types faster, and make them fail fast
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        Reporter:  dfeuer            |                Owner:  dfeuer
            Type:  feature request   |               Status:  new
        Priority:  high              |            Milestone:  8.2.1
       Component:  Core Libraries    |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Ben Gamari <ben@…>):

 In [changeset:"bef7e784d037f720697a215b9e21f13b385e6d3e/ghc" bef7e78/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="bef7e784d037f720697a215b9e21f13b385e6d3e"
 Read parentheses better

 Instead of pulling a token and looking for `'('` or `')'`,
 just look for the character itself. This prevents us from
 lexing every single item twice, once to see if it's a
 left parenthesis and once to actually parse it.

 Partially fixes #12665

 Make parens faster more aggressively

 * Strip spaces before parsing, so we never have to strip
 the same spaces twice.

 * String parsers together manually, to try to avoid unnecessary closure
 creation.

 Test Plan: Validate

 Reviewers: austin, hvr, bgamari

 Reviewed By: bgamari

 Subscribers: thomie

 Differential Revision: https://phabricator.haskell.org/D2630

 GHC Trac Issues: #12665
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12665#comment:19>
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