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 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?