No facility for thread-local storage exists yet in any Haskell compiler.
If we override 'fork', then we can implement a thread-local storage facility using 'ThreadId'. The following implementation uses one variable per type:
However, if many people are going to use thread-local storage then it would be best to have a standard implementation, for library compatibility.
1 Proposal 1
Robert Dockins has put forward a proposal which deals specially with initialization issues.
2 Proposal 2
Frederik Eaton posted an example API to the Haskell mailing list . It depends on two new functions 'withParams' and 'getParams'.
import qualified Data.Map as M import Data.Maybe import Data.Unique import Data.IORef import Data.Typeable -- only these 2 must be implemented: withParams :: ParamsMap -> IO () -> IO () getParams :: IO ParamsMap -- type ParamsMap = M.Map Unique Value data Value = forall a . (Typeable a) => V a type IOParam a = IORef (Unique, a) newIOParam :: Typeable a => a -> IO (IOParam a) newIOParam def = do k <- newUnique newIORef (k,def) withIOParam :: Typeable a => IOParam a -> a -> IO () -> IO () withIOParam p value act = do (k,def) <- readIORef p m <- getParams withParams (M.insert k (V value) m) act getIOParam :: Typeable a => IOParam a -> IO a getIOParam p = do (k,def) <- readIORef p m <- getParams return $ fromMaybe def (M.lookup k m >>= (\ (V x) -> cast x))
3 Proposal 3
Simon Peyton-Jones gave another proposal:
* The thoughts that Simon and were considering about thread-local state are quite close to Robert's proposal. For myself, I am somewhat persuaded that some form of implicitly-passed state in the IO monad (without explicit parameters) is useful. Examples I often think of are - Allocating unique identifiers - Making random numbers - Where stdin and stdout should go In all of these cases, a form of dynamic binding is just what we want: send stdout to the current thread's stdout, use the current thread's random number seed, etc. * There's no need to connect it to *state*. The key top-level thing you need is to allocate what Adrian Hey calls a "thing with identity". http://www.haskell.org/hawiki/GlobalMutableState. I'll call it a key. For example, rather than a 'threadlocal' declaration, one might just have: newkey foo :: Key Int where 'newkey' the keyword; this declares a new key with type (Key Int), distinct from all other keys. Now you can imagine that the IO monad could provide operations withBinding :: Key a -> a -> IO b -> IO b lookupBinding :: Key a -> IO a very much like the dynamic-binding primitives that have popped up on this thread. * If you want *state*, you can have a (Key (IORef Int)). Now you look up the binding to get an IORef (or MVar, whatever you like) and you can mutate that at will. So this separates a thread-local *environment* from thread-local *state*. * Keys may be useful for purposes other than withBinding and thread-local state. One would also want to dynamically create new keys: newKey :: IO (Key a) * I agree with Robert that a key issue is initialisation. Maybe it should be possible to associate an initialiser with a key. I have not thought this out. * A key issue is this: when forking a thread, does the new thread inherit the current thread's bindings, or does it get a freshly-initialised set. Sometimes you want one, sometimes the other, alas.
Frederik's response is:
The main difference between my and your proposals, as I see it, is that your proposal is based on "keys" which can be used for other things. I think that leads to an interface which is less natural. In my proposal, the IOParam type is quite similar to an IORef - it has a user-specified initial state, and the internal implementation is hidden from the user - yours differs in both of these aspects.
4 Cons of Thread local storage
Einar Karttunen expressed concern that extensive use of thread-local storage might cause problems with libraries that run actions in thread pools. He suggested that it would be better to define monads which contain all of the contextual state . Frederik Eaton pointed out that in many reasonable designs, an approach which carries state in custom monads requires code which is quadratic in the number of layers of context . Einar Karttunen also suggested a function which would solve the thread pool problem:
-- | Tie all TLS references in the IO action to the current -- environment rather than the environment it will actually -- be executed. tieToCurrentTLS :: IO a -> IO (IO a)