[Haskell-cafe] Question about data

[MigMit]

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.reiners at 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
> 
> 
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