Add Unlifted List type to base (#15996) · Issues · Glasgow Haskell Compiler / GHC

Add Unlifted List type to base

data UList (a :: TYPE 'UnliftedRep) where
  UNil :: UList a
  UCons :: a -> UList a -> UList a

This 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
BlockedBy
Related
Blocking
CC
Operating system
Architecture

```hs
data UList (a :: TYPE 'UnliftedRep) where
  UNil :: UList a
  UCons :: a -> UList a -> UList a
```

This 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:

```hs
data QSem = QSem !(MVar (Int, [MVar ()], [MVar ()]))
```

this could instead be:

```hs
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`.

<details><summary>Trac metadata</summary>

| Trac field             | Value          |
| ---------------------- | -------------- |
| Version                | 8.6.2          |
| Type                   | FeatureRequest |
| TypeOfFailure          | OtherFailure   |
| Priority               | normal         |
| Resolution             | Unresolved     |
| Component              | libraries/base |
| Test case              |                |
| Differential revisions |                |
| BlockedBy              |                |
| Related                |                |
| Blocking               |                |
| CC                     |                |
| Operating system       |                |
| Architecture           |                |

</details>