[GHC] #12001: RFC: Add pattern synonyms to base

[GHC]

#12001: RFC: Add pattern synonyms to base
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        Reporter:  Iceland_jack      |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  libraries/base    |              Version:  7.10.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Iceland_jack):

 === GHC.Generics ===
 {{{#!hs
 pattern From :: Generic a => (Rep a) x -> a
 pattern From rep <- (from → rep)
   where From rep = to rep

 pattern From1 :: Generic1 f => (Rep1 f) a -> f a
 pattern From1 rep <- (from1 → rep)
   where From1 rep = to1 rep
 }}}

 or corresponding `To`, `To1` patterns.

 === Text.Read ===
 {{{#!hs
 pattern Read :: Read a => a -> String
 pattern Read a <- (readMaybe -> Just a)
 }}}

 with the caveat that they parse different types,

 {{{#!hs
 foo :: String -> String
 foo (Read 42) = "answer"
 foo (Read n)  = "some other number " ++ show n
 foo _         = "can't parse"
 }}}

 {{{
 ghci> foo "()"
 "some other number ()"
 }}}

 If the types are made explicit with #11350

 {{{#!hs
 foo :: String -> String
 foo (Read @Integer 42) = "answer"
 foo (Read @()      n)  = "some other number " ++ show n
 foo _                  = "can't parse"
 }}}

 it is a common action and pattern (search for `pattern` and `readMaybe ->
 Just`), and is often use to pattern match on numbers. Artificially
 restricting the type avoids it matching `()`

 {{{#!hs
 pattern ReadNumber :: (Num a, Read a) => a -> String
 }}}

 This is cool

 {{{
 ghci> [ n | ReadNumber n <- words "from 2010 ... 2016 they were 4" ]
 [2010,2016,4]
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12001#comment:7>
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