The problem here is that unfortunately the Haskell type system cannot do coercion. There is however a toRational method "Real" typeclass that converts instances of Num and Ord to Rational data type and a fromRational method that converts back. So your code could be: myplus :: (Real a, Real b) => a -> b -> Float a `myplus` b = fromRational(toRational a + toRational b)
(1 :: Integer) `myplus` (2::Integer) 3.0 (1.5 :: Float) `myplus` (2::Integer) 3.5
-deech
[1] http://hackage.haskell.org/packages/archive/base/latest/doc/html/Prelude.htm...
[2] http://hackage.haskell.org/packages/archive/base/latest/doc/html/Prelude.htm...
On Tue, Dec 28, 2010 at 3:45 PM, william murphy
We were trying to make an addition function that, unlike the library one, could add numbers of different types from the typeclass 'Num.' We originally ran into a problem when trying to add (5 :: Integer) + 4.8, and were trying to define a new function that would be able to get that:
(+') :: (Num a, Num b) => a -> b -> Float x +' b = d + e where d :: Float d = x e :: Float e = b
This gave us the error:
Couldn't match expected type `Float' against inferred type `b' `b' is a rigid type variable bound by the type signature for `pl' at test.hs:9:18 In the expression: b In the definition of `e': e = b In the definition of `pl': pl x b = d + e where d :: Float d = x e :: Float e = b Failed, modules loaded: none.
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