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 <paul.rein...@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? > > 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 > > > > _______________________________________________ > 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