Tom Pledger wrote:
>
> Where do units of measure fit into a type system?
In Haskell this should be quite easy. Off my head I would suggest
something like
class Unit a where
unit :: Float -> a
value :: a -> Float
newtype Metres = Metres Float
newtype Seconds = Seconds Float
instance Unit Metres where
unit = Metres
value(Metres x) = x
instance Unit Seconds where
unit = Seconds
value(Seconds x) = x
newtype Prod a b = Prod Float
newtype Quot a b = Quot Float
instance (Unit a, Unit b) => Unit(Prod a b) where
unit = Prod
value(Prod x) = x
instance (Unit a, Unit b) => Unit(Quot a b) where
unit = Quot
value(Quot x) = x
infix 7 *$, /$
infix 6 +$, -$
(+$) :: (Unit a) => a -> a -> a
(-$) :: (Unit a) => a -> a -> a
(*$) :: (Unit a, Unit b) => a -> b -> Prod a b
(/$) :: (Unit a, Unit b) => a -> b -> Quot a b
x +$ y = unit(value x + value y)
x -$ y = unit(value x - value y)
x *$ y = Prod(value x * value y)
x /$ y = Quot(value x / value y)
m = Metres 5
s = Seconds 2
m' = m +$ m -- OK: Metres
m2 = m *$ m -- OK: Prod Metres Metres
v = m /$ s -- OK: Quot Metres Seconds
a = m /$ (s *$ s) -- OK: Quot Metres (Prod Seconds Seconds)
x = m -$ s -- error
It would be nicer if Haskell had infix type constructors:
newtype a :* b = Prod Float
newtype a :/ b = Quot Float
Cheers,
- Andreas
--
Andreas Rossberg, [EMAIL PROTECTED]
:: be declarative. be functional. just be. ::
