In addition to fixing 'Float', you should define the 'Fractional' instance for MathExpression. This would let you use:

let pi = 3.14 :: MathExpression

So your instance of Fractional would look like:

instance Num MathExpression where

(+) a b = Add a b

(*) a b = Multiply a b

(-) a b = Subtract a b

negate b = Subtract 0 b

fromInteger = MathFloat . fromInteger

abs = undefined

signum = undefined

instance Fractional MathExpression where
fromRational = MathFloat . fromRational
(/) a b = Divide a b
recip b = Divide 1 b

That would give 'fromRational' and 'fromInteger' which are the main numeric conversion functions.

Regards,

Dave

On Fri, Aug 19, 2011 at 1:40 PM, Paul Reiners <paul.reiners@gmail.com> wrote:

I've created a simple type declaration:

data MathExpression = Float
    | Add MathExpression MathExpression
    | Subtract MathExpression MathExpression
    | Multiply MathExpression MathExpression
    | Divide MathExpression MathExpression
    deriving (Show)

Now how do I create an instance of MathExpression which is just a Float?