[Haskell-beginners] Data declarations

[Christopher Howard]

On 06/02/2011 07:58 PM, aditya siram wrote:
>> data KeyValuePair a b = KeyValuePair String String
> 
> 'a' and 'b' are called phantom types and they are used to tag a
> datatype. Rather than explain what that means in some abstract way
> I'll just show you how I have used them:
> 
> I had the following simple program that retrieved values from and
> wrote to a dictionary data structure (like Oxford's or Webster's).
> Here are the types in Haskell:
> type Name = String
> type Definition = String
> type Dictionary = [(Name,Definition)]
> 
> The actual contents of the dictionary were stored in a SQLite database
> so I needed to read them into the "Dictionary" type whenever I started
> up the program.
> 
> And I had a number of functions that queried the "Dictionary" that
> required on a populated dictionary and I wanted to check *at compile
> time* that they would only be called on a populated dictionary. So I
> added the following two datatypes:
> data Populated = Populated
> data New = New
> 
> I then changed my "Dictionary" to take an extra argument that is never
> used just like your "KeyValuePair":
> type Dictionary a = [(Name,Definition)]
> 
> Now I could write a function that populated the dictionary:
> populateFromDatabase :: Dictionary New -> IO (Dictionary Populated)
> 
> and query functions:
> getDefinition :: Name -> Dictionary Populated -> Definition
> 
> Since "populateFromDatabase" is the only function that can create a
> "Dictionary Populated" the phantom type statically guarantees that
> "getDefinition" and any other function that expects a full dictionary
> can never be called before "populateFromDatabase".
> 
> The Haskell Wiki has a similar example [1] and Richard Jones has a
> nice article on they are used in Ocaml [2] .
> 
> hth,
> -deech
> 
> [1] http://www.haskell.org/haskellwiki/Phantom_type
> [2] http://camltastic.blogspot.com/2008/05/phantom-types.html
> 

Thank you. The first reference link was especially helpful in explaining
phantom types.

One more question, out of curiosity: In your above example, you could change

  data Populated = Populated
  data New = New

to simply

  data Populated
  data New

correct? You might as well, right, since your constructors take no
values, and you are just using them as phantom types?

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