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# Revision history of strict-checked-vars | ||
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## 0.1.0.0 | ||
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* Initial version, not released on Hackage. |
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Copyright 2019-2023 Input Output Global Inc (IOG). | ||
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# Strict `MVar`s and `TVar`s with invariant checking | ||
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The `strict-checked-vars` package provides a strict interface to mutable | ||
variables (`MVar`) and `TVar`s with invariant checking. It builds on top of | ||
`strict-mvar`, `strict-stm` and `io-classes`, and thus it provides the interface | ||
for `MVar`/`TVar` implementations for both | ||
[IO](https://hackage.haskell.org/package/base-4.18.0.0/docs/Prelude.html#t:IO) | ||
and [io-sim](https://hackage.haskell.org/package/io-sim). | ||
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## Checked and unchecked variants | ||
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There are currently two variant implementations of `StrictTVar`s. | ||
* From `strict-stm`: `Control.Concurrent.Class.MonadSTM.Strict.TVar` | ||
* From `strict-checked-vars`: `Control.Concurrent.Class.MonadSTM.Strict.TVar.Checked` | ||
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Similarly, there are currently two variant implementations of `StrictMVar`s. | ||
* From `strict-mvar`: `Control.Concurrent.Class.MonadMVar.Strict` | ||
* From `strict-checked-vars`: `Control.Concurrent.Class.MonadMVar.Strict.Checked` | ||
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The _unchecked_ modules provide the simplest implementation of strict variables: | ||
a light wrapper around lazy variables that forces values to WHNF before they are | ||
put inside the variable. The _checked_ module does the exact same thing, but it | ||
has the additional feature that the user can provide an invariant that is | ||
checked each time a new value is placed inside the variable. The checked modules | ||
are drop-in replacements for the unchecked modules, though invariants will be | ||
trivially true in that case. Non-trivial invariants can be set when creating a | ||
new variable. | ||
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```haskell | ||
newMVarWithInvariant :: MonadMVar m | ||
=> (a -> Maybe String) | ||
-> a | ||
-> m (StrictMVar m a) | ||
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newEmptyMVarWithInvariant :: MonadMVar m | ||
=> (a -> Maybe String) | ||
-> m (StrictMVar m a) | ||
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newTVarWithInvariant :: (MonadSTM m, HasCallStack) | ||
=> (a -> Maybe String) | ||
-> a | ||
-> STM m (StrictTVar m a) | ||
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newTVarWithInvariantIO :: (MonadSTM m, HasCallStack) | ||
=> (a -> Maybe String) | ||
-> a | ||
-> m (StrictTVar m a) | ||
``` | ||
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**Note:** though the checked modules are drop-in replacements for the unchecked | ||
modules, the `StrictMVar`/`StrictTVar` types are distinct. This means we can't | ||
make mixed use of the checked and unchecked modules. | ||
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## Guarantees for invariant checking on `StrictMVar`s | ||
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Although all functions that modify a checked `StrictMVar` will check the | ||
invariant, we do *not* guarantee that the value inside the `StrictMVar` always | ||
satisfies the invariant. Instead, we *do* guarantee that if the `StrictMVar` is | ||
updated with a value that does not satisfy the invariant, an exception is thrown | ||
*after* the new value is written to the `StrictMVar`. The reason for this weaker | ||
guarantee is that leaving an `MVar` empty can lead to very hard to debug | ||
"blocked indefinitely" problems. |
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