On Sun, Jan 2, 2011 at 01:37, Mathijs Kwik
The original package http://hackage.haskell.org/packages/archive/reaction-logic/2010.11.17/doc/ht... has a constructor function (mkReactor) with exactly the constraints I'm after. However, the constraint is just on this constructor function, not on the data itself. All functions I write are inferred as "Reactor (State s) c", while the use of that constructor ensures that s == c.
The haskell98 way to do this, is to restate the constraints in every function where you use your data type. So every function that uses a "Reactor m c" and needs the fact that it has a "MonadState c m", needs this in its context: "MonadState c m => Reactor m c -> ..." Using the GADTs language extension, you can create a constructor that contains the proof that the type contained is a member of a certain type class. For example: data HasShow a where HasShow :: (Show a) => a -> HasShow a newShow :: HasShow a -> String newShow (HasShow x) = show x Note that newShow doesn't need a "Show a" constraint. Erik