Read the paper "Data types a la carte"[1], and check out the compdata[2]
library for an implementation.
The idea is that instead of making a big union type like:
data Answers = SexAnswer | AgeAnswer | etc...
you make
data Sex = M | F | etc
newtype Age = Age Int
and then you construct functions that accept sum types:
ageOrSex :: Age :+: Sex -> m Bool
...
A single type class, called :<: in the paper, handles dispatching from sums
the underlying values to sums.
[1]: http://www.cs.ru.nl/~W.Swierstra/Publications/DataTypesALaCarte.pdf
[2: https://hackage.haskell.org/package/compdata
On Fri, May 15, 2015 at 5:39 AM, Cody Goodman wrote: 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
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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