On Wed, Oct 3, 2012 at 6:12 PM, Nathan Hüsken
On 10/03/2012 02:04 PM, Ertugrul Söylemez wrote:
When you write 'pure 1.0', then you're using, say, Double's Fractional instance directly. If you write just '1.0', then you're using Wire's Fractional instance, which is just defined as:
fromRational = pure . fromRational
So it's the same thing.
Let me simplify the example:
testW1 :: WireP () Double testW1 = pure 1.0
testW2 :: WireP () Double testW2 = 1.0
main = do let (res, _) = stepWire 1.0 () testW putStrLn $ show res
(set testW to testW1 or testW2). When I test this with testW=testW2 I see that "fromRational" is called which converts a Rational (=Ration Integer) to a WireP () Double (I tested this by adding a "trace" to the fromRational Control/Wire/Classes.hs). This means the "1.0" is converted to a Rational and then back to a WireP () Double, correct?
1.0 _is_ a Rational in the first place, it's a number literal that is not integral, so it is translated to a fraction (Rational) by the parser then converted at runtime (only once so no worry about efficiency) to the proper type in the Fractional type class by fromRational, like Double, Float, Ratio, ... Here it's a WireP () Double. This is how this code works :
let x :: (Fractional a) => a x = 1.23 y = (3.5 :: Double) + x z = (2.7 :: Float) + x
Note : Does GHC optimize the case were the final type is monomorphic and standard (Double, Float) if -O2 is given ? Sure one fromRational by literal shouldn't hurt much but... -- Jedaï