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

[GHC]

#12665: Make Read instances 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 dfeuer):

 Replying to [comment:10 int-e]:
 > more prototypes for conversion from digits to an integer can be found
 here: http://int-e.eu/~bf3/haskell/FromDigits.hs ... `valInteger'` is the
 most interesting one of the bunch, and according to some preliminary
 testing, also the most efficient.

 There's an unfortunate problem with the memoized base list. Once GHC knows
 the base, it pulls the `bs` base list up to the top level. If someone
 parses a very large integer, I believe we could be stuck with its base
 list indefinitely. Now it turns out that `ReadP` is defined `newtype ReadP
 a = R (forall b . (a -> P b) -> P b)`. If `Text.ParserCombinators.ReadP`
 exposed the `R` constructor and the `P` type (which it does not) then we
 could almost certainly trick GHC into thinking that `bs` depends on the
 passed `a -> P b` function. Since it does not, we're in a bit of a pickle.
 I think the best approach is probably to add an "internal" module that
 exposes all these details. For experimental purposes, I guess I can just
 use `unsafeCoerce`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12665#comment:11>
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