[Git][ghc/ghc][master] Modify ThreadId documentation and comments

[Marge Bot (@marge-bot)]

Marge Bot pushed to branch master at Glasgow Haskell Compiler / GHC


Commits:
7131b705 by David Feuer at 2023-03-25T00:04:04-04:00
Modify ThreadId documentation and comments

For a long time, `GHC.Conc.Sync` has said

```haskell
-- ToDo: data ThreadId = ThreadId (Weak ThreadId#)
-- But since ThreadId# is unlifted, the Weak type must use open
-- type variables.
```

We are now actually capable of using `Weak# ThreadId#`, but the
world has moved on. To support the `Show` and `Ord` instances, we'd
need to store the thread ID number in the `ThreadId`. And it seems
very difficult to continue to support `threadStatus` in that regime,
since it needs to be able to explain how threads died. In addition,
garbage collection of weak references can be quite expensive, and it
would be hard to evaluate the cost over he whole ecosystem. As discussed
in
[this CLC issue](https://github.com/haskell/core-libraries-committee/issues/125),
it doesn't seem very likely that we'll actually switch to weak
references here.

- - - - -


1 changed file:

- libraries/base/GHC/Conc/Sync.hs


Changes:

=====================================
libraries/base/GHC/Conc/Sync.hs
=====================================
@@ -133,9 +133,6 @@ infixr 0 `par`, `pseq`
 -----------------------------------------------------------------------------
 
 data ThreadId = ThreadId ThreadId#
--- ToDo: data ThreadId = ThreadId (Weak ThreadId#)
--- But since ThreadId# is unlifted, the Weak type must use open
--- type variables.
 {- ^
 A 'ThreadId' is an abstract type representing a handle to a thread.
 'ThreadId' is an instance of 'Eq', 'Ord' and 'Show', where
@@ -146,10 +143,9 @@ useful when debugging or diagnosing the behaviour of a concurrent
 program.
 
 /Note/: in GHC, if you have a 'ThreadId', you essentially have
-a pointer to the thread itself.  This means the thread itself can\'t be
-garbage collected until you drop the 'ThreadId'.
-This misfeature will hopefully be corrected at a later date.
-
+a pointer to the thread itself. This means the thread itself can\'t be
+garbage collected until you drop the 'ThreadId'. This misfeature would
+be difficult to correct while continuing to support 'threadStatus'.
 -}
 
 -- | @since 4.2.0.0



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/7131b705d54ed7ed20e9946d9ace45228ec5febd

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View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/7131b705d54ed7ed20e9946d9ace45228ec5febd
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