This page is meant to contain discussion about global state and "things with identity". It's a companion to Adrian's http://www.haskell.org/hawiki/GlobalMutableState, but that's in the old Wiki, so I thought it better to start a new page.
See also Ian Stark's ACIO message http://www.haskell.org/pipermail/haskell-cafe/2004-November/007664.html.
As Brian Hulley put it, at the moment, there is a strange unnatural discrepancy between the fixed set of built-in privileged operations such as Data.Unique.newUnique which are "allowed" to make use of global state, and user defined operations which have to rely on a shaky hack in order to preserve natural abstraction barriers between components such as a user-defined Unique, Atom, and anything involving memoisation or device management etc.
The kind of applications we have in mind (please add more) are:
- A source of random numbers, or of unique numbers. This should be on a per-thread basis.
- The value of 'stdin' or 'stdout'. We don't want to mutate this (although note that the handle itself contains mutable state), but we might want to set the value for sub-computations, including any spawned threads.
A straw man
The basic idea is this:
- Each thread has access (via the IO or STM monad) to a "dictionary" that maps a (typed) key to a value of that type.
- The dictionary is not mutable, but the values might themselves be mutable cells (think of 'stdin').
(I don't know how to put code inside a bullet, and carry on the bullet afterwards, so the formatting below is a mess.)
- A new built-in data type,
Key a, an instance of Eq, but not Ord. It's a "thing with identity" (TWI).
- A new built-in data type of dictionaries,
Dict, which maps a typed key to a typed value:
lookup :: Dict -> Key a -> Maybe a insert :: Dict -> Key a -> a -> Dict union :: Dict -> Dict -> Dict etc
Yes, it'll need a massive interface, like Data.Map.
- You can allocate a fresh (globally unique) key in the IO monad
newKey :: IO (Key a)
- However, we add a new top-level declaration to allocate new top-level key:
key rng :: Key StdGen
This is useful independent of mutability. For example, GHC has lots of code going
thenIdKey = mkPreludeIdKey 3 returnIdName = mkPreludeIdKey 4
etc, where we hand out unique identifiers by hand. Easy to screw up.
- Each thread has an implicit, thread-specific dictionary:
getDict :: IO Dict setDict :: Dict -> IO a -> IO a
(Think of myThreadId.) Notice that setDict does not mutate the dictionary; it just sets the implicit dictionary for a nested sub-computation.
- A forked thread inherits its parent's dictionary.
Initialisation is the big question here. A library may want to allocate a key, and an initialiser to be run the first time the key is accessed, without the client of the library needing to know about it at all. A second issue that we may sometimes want to implicitly (?) re-initialise (or clear) all or part of the dictionary when forking a thread.