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
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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