Re: [Haskell-beginners] Restrict type in phantom data-type

I pereputal emails. --- David, hello! 1. Is it the same/different as: data family Day a data Sunny data Rainy data instance Day Sunny = SunnyDay deriving Show data instance Day Rainy = RainyDay deriving Show ..and here you can not create `Day Int` object because no `Day Int` constructor (but you can create such constructor) ? Or in case with type families there is possibility to extend it to `Day Int` and in case with DayaKinds it's totally impossible? 2. I read somewhere (on forums) that restrictions on data types... I don't remember exactly, but something like they are not real restrictions or are related to old extension which is/will be deprecated. I'm not sure. Also, I'm not sure is it - in your example - restriction (constraint) or something else. Am I wrong?

This is maybe edging toward haskell-cafe territory, but you can definitely do this in haskell.

{-# LANGUAGE DataKinds, KindSignatures #-}

data DayType = Sunny | Rainy

data Day (a :: DayType) = Day

sunnyDay :: Day Sunny sunnyDay = Day

rainyDay :: Day Rainy rainyDay = Day

-- impossibleDay :: Day () -- impossibleDay = Day

On Fri, Sep 1, 2017 at 10:18 AM, Baa wrote:

...

Hello, List!

For example, I have specialized (right nameis phantom?) type:

data Day a = Day { ... no `a` here } data Sunny data Rainy

joyToday :: Day Sunny -> IO () joyToday day = ...

melancholyToday :: Day Rainy -> IO () melancholyToday day = ...

And I can create (in spite of that it's phantom) some day:

let day1 = Day {...} :: Day Sunny joyToday day1

but no problem to create `Day Int`, `Day Char`, etc which is pointless actually (sure "creator"-function can be exported from the module only, but I'm talking about type-level solution).

I know that constraints (`... =>`) on data types are redundant/removed from the language. And I'm not sure how it's possible to restrict that parameter `a` (I know that it's possible to Java/C++/Perl6 (not sure), some other languages but how to add such restriction in Haskell? IMHO type families can help but I'm not sure how it will look (Sunny, Rainy are "nullary" type, so...).

Is it possible for Haskell too?

=== Best regards, Paul _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

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