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

18 Apr 2017


      #12001: RFC: Add pattern synonyms to base
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        Reporter:  Iceland_jack      |                Owner:  (none)
            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:                    |
-------------------------------------+-------------------------------------
Description changed by Iceland_jack:

@@ -46,0 +46,3 @@
+ import Data.Complex (Complex, conjugate, polar, mkPolar)
+ import qualified Data.Complex as C
+
@@ -56,2 +59,2 @@
- pattern Real r <- r :+ _
-   where Real r =  r :+ 0
+ pattern Real r <- r C.:+ _
+   where Real r =  r C.:+ 0
@@ -60,2 +63,2 @@
- pattern Imaginary i <- _ :+ i
-   where Imaginary i =  0 :+ i
+ pattern Imaginary i <- _ C.:+ i
+   where Imaginary i =  0 C.:+ i
@@ -63,0 +66,2 @@
+ pattern (:+) :: a -> a -> Complex a
+ pattern (:+) { realPart, imagPart } = realPart C.:+ imagPart

New description:

 Since we have pattern synonyms it's worth considering if some belong in
 base:

 [https://hackage.haskell.org/package/lens-4.14/docs/Data-Complex-Lens.html
 Data.Complex.Lens] contains patterns that could be defined in base, here
 are some more suggestions:

 === Data.Array ===
 {{{#!hs
 pattern ArrayIx :: Ix i => (i, i) -> [(i, e)] -> Array i e
 pattern ArrayIx low'high xs <- ((\arr -> (bounds arr, assocs arr)) ->
 (low'high, xs))
   where ArrayIx low'high xs = array low'high xs
 }}}

 === Data.Bits ===
 {{{#!hs
 pattern ZeroBits :: (Eq a, Bits a) => a
 pattern ZeroBits <- ((== zeroBits) -> True)
   where ZeroBits = zeroBits

 pattern BitSize :: Bits a => Int -> a
 pattern BitSize n <- (bitSizeMaybe -> Just n)

 pattern Signed :: Bits a => a
 pattern Signed <- (isSigned -> True)

 pattern Unsigned :: Bits a => a
 pattern Unsigned <- (isSigned -> False)

 pattern PopCount :: Bits a => Int -> a
 pattern PopCount n <- (popCount -> n)
 }}}

 === Data.Char ===
 {{{#!hs
 pattern ControlChar :: Char
 pattern ControlChar <- (isControl -> True)

 pattern SpaceChar :: Char
 pattern SpaceChar <- (isSpace -> True)
 }}}

 === Data.Complex ===
 {{{#!hs
 import Data.Complex (Complex, conjugate, polar, mkPolar)
 import qualified Data.Complex as C

 pattern Conjugate :: Num a => Complex a -> Complex a
 pattern Conjugate a <- (conjugate -> a)
   where Conjugate a = conjugate a

 pattern Polar :: RealFloat a => a -> a -> Complex a
 pattern Polar m theta <- (polar -> (m, theta))
   where Polar m theta = mkPolar m theta

 -- See https://github.com/ekmett/lens/issues/653
 pattern Real :: Num a => a -> Complex a
 pattern Real r <- r C.:+ _
   where Real r =  r C.:+ 0

 pattern Imaginary :: Num a => a -> Complex a
 pattern Imaginary i <- _ C.:+ i
   where Imaginary i =  0 C.:+ i

 pattern (:+) :: a -> a -> Complex a
 pattern (:+) { realPart, imagPart } = realPart C.:+ imagPart
 }}}

 === GHC.Float ===
 {{{#!hs
 pattern NegativeZero :: RealFloat a => a
 pattern NegativeZero <- (isNegativeZero -> True)
   where NegativeZero = -0

 pattern Denormalized :: RealFloat a => a
 pattern Denormalized <- (isDenormalized -> True)

 pattern NaN :: RealFloat a => a
 pattern NaN <- (isNaN -> True)
   where NaN = 0 / 0

 -- How ever negative infinity is handled
 pattern Infinity :: RealFloat a => a
 pattern Infinity <- ((== 1 / 0) -> True)
   where Infinity = 1 / 0
 }}}

--

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12001#comment:12
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler

    