[GHC] #8697: Type rationals

[GHC]

#8697: Type rationals
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       Reporter:  MikeIzbicki       |             Owner:
           Type:  feature request   |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Compiler          |           Version:  7.6.3
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 I've used GHC's Type Nats to implement my own type level rationals (code
 below).  This works fine, but the syntax is a little funky because you
 have to write out the number as a fraction.  For example, you must write
 13/10 instead of 1.3; or even worse, 13/1 instead of 13.  It would be nice
 to just write the decimal notation.

 I only see one potential problem with this feature: the dot in the decimal
 notation could interfere with the dot in forall statements.  I guess this
 can be parsed unambiguously, but maybe not.

 {{{
 data Frac = Frac Nat Nat

 data instance Sing (n::Frac) = SFrac Integer Integer

 instance (SingI a, SingI b) => SingI ('Frac a b) where
     sing = SFrac (fromSing (sing :: Sing a)) (fromSing (sing :: Sing b))

 instance Fractional r => SingE (Kind :: Frac) r where
     fromSing (SFrac a b) = fromIntegral a/fromIntegral b

 type (/) a b = 'Frac a b
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8697>
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