David Barbour dmbarbour at gmail.com
Mon Aug 22 21:53:32 CEST 2011

```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 at gmail.com>wrote:

> I've created a simple type declaration:
>
> data MathExpression = Float
>     | 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?
>
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