Re: [Haskell-beginners] Check constructor's field numeric value at compile time

Also, if you want your mind blown, check out Idris. On Tuesday, September 2, 2014, Alexandre Lucchesi < alexandrelucchesi@gmail.com> wrote:

You can't do that within the data type declaration.

In order to add such constraints you should define "constructor functions" and apply the validation there, i.e.:

newSecondsInterval :: Float -> Maybe Interval newSecondsInterval n | n >= 1 = Just $ Seconds n | otherwise = Nothing

Note that the "Maybe" type is only a way to handle the case where the supplied value is invalid. Also, in order to provide encapsulation and ensure no one is going to create a new "Interval" by using "Seconds" and "MicroSeconds", you should hide these value constructors in the export list of your module.

On Tue, Sep 2, 2014 at 4:32 PM, Dmitriy Matrosov javascript:_e(%7B%7D,'cvml','sgf.dma@gmail.com');> wrote:

...

Hi.

I have a type

data Interval = Seconds Float | MicroSeconds Int

The Float field of data constructor Seconds should be >= 1, and Int field of constructor MicroSeconds should be in the range from 0 to 1000000.

How can i write this constraints so they're checked at compile time, not at runtime?

-- Dmitriy Matrosov _______________________________________________ Beginners mailing list Beginners@haskell.org javascript:_e(%7B%7D,'cvml','Beginners@haskell.org'); http://www.haskell.org/mailman/listinfo/beginners

-- alexandre lucchesi

*Perfection is achieved, not when there is nothing more to add, but when there is nothing left to take away!*

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