This is not a valid data declaration. You can't have a "Float" field
without any constructor name and have it still of type
"MathExpression". I suggest you do something like:
data MathExpr = MathFloat Float
....
So you may declare pi:
let mathPi = MathFloat pi -- note "pi" is defined in the prelude alread
Cheers,
Thomas
On Fri, Aug 19, 2011 at 1:40 PM, Paul Reiners
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?
This doesn't work:
*Main> let pi = 3.14 :: MathExpression
<interactive>:1:10: No instance for (Fractional MathExpression) arising from the literal `3.14' Possible fix: add an instance declaration for (Fractional MathExpression) In the expression: 3.14 :: MathExpression In an equation for `pi': pi = 3.14 :: MathExpression
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