[Haskell-cafe] Can I specify the a in a phantom type to be limited to a sum type?

[Cody Goodman]

Tom, I'm trying to make a well-typed API to Question/Answers on
medical forms. The questions will be lined with a specific code, so I
want to enforce that certain codes can only contain certain types of
answers.

Andras, thanks. I'll give that a try later today and let you know how it works.

On Fri, May 15, 2015 at 4:12 AM, Andras Slemmer <0slemi0 at gmail.com> wrote:
> You can do this, although you still need a datastructure that allows you to
> use the contained type:
>
> {-# LANGUAGE GADTs #-}
> {-# LANGUAGE StandaloneDeriving #-}
> {-# LANGUAGE TypeSynonymInstances #-}
> module Tutorial where
>
> data Gender = Male | Female deriving (Show)
>
> data Race = White | Black deriving (Show)
>
> type Age = Int
>
> data Answer a where
>   Answer :: RacistAgistSexist a => a -> Answer a
>
> deriving instance Show w => Show (Answer w)
>
> data GenderRaceAge
>   = Gender Gender
>   | Race Race
>   | Age Age
>
> class RacistAgistSexist a where
>   genderRaceAge :: a -> GenderRaceAge
> instance RacistAgistSexist Gender where
>   genderRaceAge = Gender
> instance RacistAgistSexist Race where
>   genderRaceAge = Race
> instance RacistAgistSexist Age where
>   genderRaceAge = Age
>
> -- You can use genderRaceAge to get a GenderRaceAge out of the contained
> type if you don't know 'a'
>
>
> On 15 May 2015 at 08:51, Tom Ellis
> <tom-lists-haskell-cafe-2013 at jaguarpaw.co.uk> wrote:
>>
>> On Fri, May 15, 2015 at 01:47:49AM -0500, Cody Goodman wrote:
>> > How can I create Answers of type Gender, Race, or Age?
>> >
>> > These should be possible:
>> >
>> > λ> Answer Male
>> > λ> Answer White
>> > λ> Answer Black
>> > λ> Answer 28
>> >
>> > Others such as using a string should not be possible:
>> >
>> > λ> Answer "a string" -- should throw type error
>>
>> It would probably help if you tell us why precisely you want this, and in
>> particular why
>>
>> data Answer = AnswerGender Gender
>>             | AnswerRace   Race
>>             | AnswerAge    Int
>>
>> is not satisfactory.
>>
>> Tom
>>
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>
>
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