[Haskell-cafe] "Universal" constraints?

Michael Walker mike at barrucadu.co.uk
Sun Jun 21 05:55:52 UTC 2015

Hi all,

I was wondering about constraints in class declarations. Suppose we have
something like this:

    class Monad m => Store m where
      type Var m :: * -> *
      new   :: a -> m (Var m a)
      read  :: Var m a -> m a
      write :: Var m a -> a -> m ()

Then some reasonable implementations would be IO and ST, using IORefs and

    instance Store IO where
      type Var IO = IORef
      new   = newIORef
      read  = readIORef
      write = writeIORef
    instance Store (ST s) where
      type Var (ST s) = STRef s
      new   = newSTRef
      read  = readSTRef
      write = writeSTRef

Now, both IORefs and STRefs have equality, which doesn't require equality of the
contained type,

    instance Eq (IORef a) where ...
    instance Eq (STRef s a) where ...

This corresponds to pointer equality and, because reading the values is a
monadic action, is as good an Eq instance as we can get. Given this, it would be
nice to be able to specify that all instances of our Store class have this
property. Something like,

    class (Monad m, Eq (Var m a)) => Store m where ...

But we can't write this! The `a` isn't in scope! What I really want to do is to
be able to write class constraints like this,

    class (forall a. C (T a)) => D x where ...

Or why not get really crazy:

    class (forall a. C a => D (T a)) => E x where ...

Is there some extension or combination of extensions that would make this work
without completely sacrificing type safety? FlexibleContexts allows it (the
simpler form, at least) for function constraints, but not for class constraints.

If not, has anyone thought about this sort of thing before?

Michael Walker (http://www.barrucadu.co.uk)

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