Re: [Haskell-cafe] multi type addition

[Henning Thielemann]

On Tue, 28 Dec 2010, aditya siram wrote:


The problem here is that unfortunately the Haskell type system cannot
do coercion.


I'd omit the unfortunately. For me it is a good thing that Haskell does 
not silently apply lossy number conversions, as e.g. MatLab does.



'realToFrac' allows conversion to Float. It requires 'Real' constraint. 
Your 'Num' constraint is too weak, since 'Complex' is also in 'Num' type 
class.


http://www.haskell.org/haskellwiki/Converting_numbers
http://www.haskell.org/haskellwiki/Generic_number_type


___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe

[Haskell-cafe] multi type addition

[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.
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe

Re: [Haskell-cafe] multi type addition

[aditya siram]

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.html#t:Real
[2] 
http://hackage.haskell.org/packages/archive/base/latest/doc/html/Prelude.html#t:Rational

On Tue, Dec 28, 2010 at 3:45 PM, william murphy will.t.mur...@gmail.com wrote:
 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.


 ___
 Haskell-Cafe mailing list
 Haskell-Cafe@haskell.org
 http://www.haskell.org/mailman/listinfo/haskell-cafe



___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe