[Martijn]

My Magic: the Gathering implementation uses a few GADTs. Interestingly they use GADTs in very different ways. Source code is here: https://github.com/MedeaMelana/Magic/blob/master/Magic/src/M...

Let me briefly highlight three of them:

1) Type Question models questions that the game might ask of players. Its constructors are tagged with the type of the answer to that question:

  data Question a where
    AskKeepHand :: Question Bool
    AskTarget   :: [EntityRef] -> Question EntityRef
    ...

This allows me to write cards that ask questions and later on plug in various interpreters of these questions (command line interface, JSON webserver, AI, test framework).

2) The ZoneRef datatype enumerates the various zones in a game of Magic (battlefield, hand, etc) but I've chosen to tag each ZoneRef constructor with a type that indicates what the type of a card is when you look up a reference to a card ("object" in Magic jargon):

  type ObjectRef ty = (ZoneRef ty, Id)
  lookupObject :: ObjectRef ty -> Magic (ObjectOfType ty)

Most functions that take object references as an argument like to constrain the type of the object referenced, and using a GADT allows them to do so. For example, dealing damage to an object (creature or planeswalker):

  data SimpleOneShotEffect = DamageObject Object (ObjectRef TyPermanent) Int ...

3) Type TargetList models zero or more targets a spell on the stack can target. Even though in essence it is just a list of targets, it is a GADT so that it can have a "return type" which is re-evaluated when one of more of the targets change (for example, by a card like Redirect or Reverberate).

Hope this helps!

[ufo]

What an interesting example. Not what I was expecting!

Can you think of an example of a bug that you avoided due to the extra type safety of gadts? Magic has some pretty complex rules so I imagine that writing exaustive test must be hard and that the extra static guarantees might be able to find something a dynamic checker wouldn't. (Its ok if you need some ingame jargon to explain that, I know the basic rules).

[Martijn]

It's a combination of avoiding potential bugs and just modelling things differently altogether:

If I had to leave out the answer type of questions, I'd have to come up with an entirely different way of modelling answers. For example, I might have had to write a datatype like:

  data Answer = AnswerBool Bool | AnswerInt Int | ...

and then processing the answer (through pattern matching) would have to discard any answer it didn't like, which is awkward. Or I would have had to use a type class to keep everything type-safe which would have resulted in more complicated type signatures throughout the code.

For the TargetList, the current setup allows me to write functions that consume a target list of exactly the right size, for example:

  \(target1, target2) -> ...

rather than a normal list:

  \[target1, target2] -> ...

which in this case results in a pattern match exception if you pass it a list of any length other than 2.