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

15 May 2015

      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@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
 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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