[Haskell-cafe] Binary constants in Haskell

[Don Stewart]

dons:
> claus.reinke:
> > >>>  From my point of view, the difference between 0b10111011 and
> > >>> (bin[1,0,1,1,1,0,1,1]) is 22-10 that is 12 characters.
> > 
> > how about using ghc's new overloaded strings for this?
> > 
> >    "10111011"::Binary
> > 
> > there used to be a way to link to ghc head's docs, but 
> > i can't find it right now. the test is 
> > 
> > http://darcs.haskell.org/testsuite/tests/ghc-regress/typecheck/should_compile/tc224.hs
> > 
> > and the xml docs would be
> > 
> > http://darcs.haskell.org/ghc/docs/users_guide/glasgow_exts.xml
> 
> Why not use a Num instance for Binary, with fromInteger :: Integer -> a,
> Yielding,
> 
>     10111011 :: Binary
> 
> Overloaded numeric literals seem better here than strings :)

Something like this:

    import Data.List
    import Data.Bits

    newtype Binary = Binary Integer deriving (Eq, Show)

    instance Num Binary where
        fromInteger n = Binary . roll . map (read.return) . show $ n
          where
            roll = foldl' unstep 0
        
            unstep a 1 = a `shiftL` 1 .|. fromIntegral 1
            unstep a 0 = a `shiftL` 1
            unstep a _ = error "Invalid character in binary literal"

Yielding,

    *A> 0 :: Binary
    Binary 0

    *A> 101 :: Binary
    Binary 5

    *A> 1111 :: Binary
    Binary 15

    *A> 1010101011010111 :: Binary
    Binary 43735

    *A> 42 :: Binary
    Binary *** Exception: Invalid character in binary literal