Can I specify the a in a phantom type to be limited to a sum type?

Cody Goodman

15 May 2015 15 May '15

6:47 a.m.

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 {-# LANGUAGE UndecidableInstances #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE GADTs #-} {-# LANGUAGE StandaloneDeriving #-} module Tutorial where data Gender = Male | Female deriving (Show) data Race = White | Black deriving (Show) type Age = Int data Answer a where Answer :: Gender -> Answer Gender deriving instance Show (Answer w) -- λ> -- This is pretty useful and accepts Answers with the Gender -- λ> Answer Male -- Answer Male -- λ> :t Answer Male -- Answer Male :: Answer Gender -- λ> -- how do I also accept Race and Age? -- below I try to sort out how to use type families (unsuccessfully) before bed -- type family Answer :: * -- type instance Answer Gender = Answer Gender

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Tom Ellis

15 May 15 May

6:51 a.m.

New subject: Can I specify the a in a phantom type to be limited to a sum type?

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

Andras Slemmer

9:12 a.m.

New subject: Can I specify the a in a phantom type to be limited to a sum type?

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@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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Cody Goodman

12:39 p.m.

New subject: Can I specify the a in a phantom type to be limited to a sum type?

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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Alexander Solla

8:24 p.m.

New subject: Can I specify the a in a phantom type to be limited to a sum type?

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 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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_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

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Alexander Solla
Andras Slemmer
Cody Goodman
Tom Ellis