[Haskell-cafe] RFC: Top level mutable state in terms of pure-function initialization?

[Roman Cheplyaka]

1. Well, that creates a loophole in the type system:

  let
    x :: IORef Int
    x = enumToIORef 0

    y :: IORef Bool
    y = enumToIORef 0
  in writeIORef x (5 :: Int) >> (readIORef y :: IO Bool)

2. There's also a less obvious loophole with your Defaultable version:

  let
    x :: Defaultable a => IORef a
    x = enumToIORef 0
  in writeIORef x (5 :: Int) >> (readIORef x :: IO Bool)

3. In your default-less version there's a problem of doing this:

  x = enumToIORef 0 True
  y = enumToIORef 0 False

Now, x and y presumably refer to the same IORef, but the initial
contents of that IORef will depend on whether you access it through x or
y.

4. Finally, the implementation of enumToIORef can only inspect it by
converting to Int, and cannot distinguish different enums that
correspond to the same number.

This last problem can be solved by adding a Typeable constraint,
because TypeReps carry the information about where (module, package) the
type was defined.

Roman

* EatsKittens <temporalabstraction at gmail.com> [2013-12-15 12:39:20+0100]
> A pure function (enumToIORef :: (Enum a, Defaultable b) => a -> IORef b).
> This function returns referentially transparently an IORef as a function of
> its "seed" with the guarantee that the IORef returned is identical if and
> only if the seed is. This function can be used to implement top level
> mutable state in a module. The module can specifically create an enumerated
> type for this and not export it, thereby removing any possibility of
> another module passing the seed type and conflicting.
> 
> The Defaultable class is added in this case to implement only a single
> method (defaultValue :: Defaultable a => a -> a). This is conceptionally
> the 'simplest' value of the type, such as the empty list, the number 0,
> False, the null character &c. The IORef returned by enumToIORef would be
> initialized before being written to to this specific default value of its
> type. This approach is chosen because it is impossible to initialize it to
> user specified value because enumToIORef can be called twice with the same
> seed but a different initial value.
> 
> In the alternative it is also possible to do without the default value and
> say the IORef returned is the same if and only if the seed and the initial
> value given are the same. Allowing the function to remain referentially
> transparent as well. This would probably require for good semantics the
> underlying type of the IORef to be a member of EQ...?
> 
> All this would of course require that newIORef and enumToIORef never
> produce the same IORef.
> 
> Aside its limitations of the type IORef's initialized with this method can
> carry, I do believe they cover the vast majority of use cases of top level
> mutable state?
> 
> Caveats?

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