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 <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?

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