Add Unlifted List type to base
data UList (a :: TYPE 'UnliftedRep) where
UNil :: UList a
UCons :: a -> UList a -> UList aThis would guarantee that values stored inside the list would not be thunks. It would likely live in something like GHC.List.Unlifted, since it uses GHC-specific things.
An example of something it might improve is the implementation of Control.Concurrent.QSem.QSem:
data QSem = QSem !(MVar (Int, [MVar ()], [MVar ()]))this could instead be:
type MVarIO = MVar# RealWorld -- type synonym for brevity
data QSem = QSem (MVarIO (Int, UList (MVarIO ()), UList (MVarIO ())))Note that in this example the tuple inside the outermost MVarIO boxes the Int - this tuple could be represented as a datatype which ensures the unboxing on the Int.
The main idea here though is that now you can use MVar# inside of the UList.
Trac metadata
| Trac field | Value |
|---|---|
| Version | 8.6.2 |
| Type | FeatureRequest |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | libraries/base |
| Test case | |
| Differential revisions | |
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| Blocking | |
| CC | |
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| Architecture |