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Thread-local storage

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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.

Robert Dockins has put forward a proposal[1].

Frederik Eaton posted an example API to the Haskell mailing list [2]. 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))

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 [3]. 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 [4]. 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)

Simon Marlow and Simon Peyton-Jones have both expressed that they support some form of thread-local storage.[5][6]

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".
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,