Your MathExpression data type has nothing to do with numbers of any kind. Your "Float" data constructor doesn't mean that float numbers are a part of your type; instead it means that you have a SINGLE value of type MathExpression, and this value is named "Float".
You should modify your data declaration as "data MathExpression = Float Float | …", and after that you can write something like "a = Float 4.2", which would automatically make "a" a value of type MathExpession". Отправлено с iPad 20.08.2011, в 0:40, Paul Reiners <paul.rein...@gmail.com> написал(а): > 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
