Commit 39b9f94a authored by Simon Marlow's avatar Simon Marlow
Browse files

Add a big warning to the documentation for Weak (#7250)

parent 4f03d110
......@@ -55,7 +55,7 @@ It is not guaranteed that a finalizer will eventually run, and no
attempt is made to run outstanding finalizers when the program exits.
Therefore finalizers should not be relied on to clean up resources -
other methods (eg. exception handlers) should be employed, possibly in
addition to finalisers.
addition to finalizers.
References from the finalizer to the key are treated in the same way
as references from the value to the key: they do not keep the key
......@@ -76,6 +76,24 @@ such finalizer.
If there are no other threads to run, the runtime system will check
for runnable finalizers before declaring the system to be deadlocked.
WARNING: weak pointers to ordinary non-primitive Haskell types are
particularly fragile, because the compiler is free to optimise away or
duplicate the underlying data structure. Therefore attempting to
place a finalizer on an ordinary Haskell type may well result in the
finalizer running earlier than you expected. This is not a problem
for caches and memo tables where early finalization is benign.
Finalizers /can/ be used reliably for types that are created explicitly
and have identity, such as @IORef@ and @MVar@. However, to place a
finalizer on one of these types, you should use the specific operation
provided for that type, e.g. @mkWeakIORef@ and @addMVarFinalizer@
respectively (the non-uniformity is accidental). These operations
attach the finalizer to the primitive object inside the box
(e.g. @MutVar#@ in the case of @IORef@), because attaching the
finalizer to the box itself fails when the outer box is optimised away
by the compiler.
data Weak v = Weak (Weak# v)
......@@ -116,7 +134,7 @@ deRefWeak (Weak w) = IO $ \s ->
finalize :: Weak v -> IO ()
finalize (Weak w) = IO $ \s ->
case finalizeWeak# w s of
(# s1, 0#, _ #) -> (# s1, () #) -- already dead, or no finaliser
(# s1, 0#, _ #) -> (# s1, () #) -- already dead, or no finalizer
(# s1, _, f #) -> f s1
......@@ -94,15 +94,10 @@ mkWeakPtr key finalizer = mkWeak key key finalizer
when the key becomes unreachable).
Note: adding a finalizer to a 'Foreign.ForeignPtr.ForeignPtr' using
'addFinalizer' won't work as well as using the specialised version
'Foreign.ForeignPtr.addForeignPtrFinalizer' because the latter
version adds the finalizer to the primitive 'ForeignPtr#' object
inside, whereas the generic 'addFinalizer' will add the finalizer to
the box. Optimisations tend to remove the box, which may cause the
finalizer to run earlier than you intended. The same motivation
justifies the existence of
'Control.Concurrent.MVar.addMVarFinalizer' and
'Data.IORef.mkWeakIORef' (the non-uniformity is accidental).
'addFinalizer' won't work; use the specialised version
'Foreign.ForeignPtr.addForeignPtrFinalizer' instead. For discussion
see the 'Weak' type.
addFinalizer :: key -> IO () -> IO ()
addFinalizer key finalizer = do
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