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

11 Dec 2017

      #12001: RFC: Add pattern synonyms to base
-------------------------------------+-------------------------------------
        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:

Old 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
}}}
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 <- (isInfinite -> True)
   where Infinity = 1 / 0
 }}}

 Used
 [https://github.com/cchalmers/optical/blob/a85484ad23a7f4d6b5da1dcb78781ea9c4...
 here]

--

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12001#comment:14
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler