Implement UnliftedNewtypes proposal

The unlifted newtypes proposal is described here: https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0013-unlifted-newtypes.rst

I am going to be working on it.

Trac metadata

Trac field	Value
Version	8.4.3
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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Architecture

mentioned in issue #1311 (closed)

changed milestone to %8.6.1

changed weight to 5

added Ttask Trac import labels

(Does case of keywords even matter?)

changed milestone to %8.8.1

These won't be addressed in GHC 8.6.

Question: Would it be possible to introduce UnliftedNewtypes in one of the following releases (something like 8.6.3?)?

I have a use-case where levity-polymorphism would help, but it crucially relies on having levity polymorphic newtypes.

No, new releases are for bug fixes. 8.8 will not be a very long time away. I don't even think the feature is implemented yet.

We will definitely need to wait until 8.8. I've made some attempts at it on https://phabricator.haskell.org/D4777, but it still doesn't work right.

Trac metadata

Trac field	Value
Related	- → #1311 (closed)

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Differential revisions	- → D477

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Trac field	Value
Differential revisions	D477 → D4777

changed milestone to %8.10.1

Bumping milestones of low-priority tickets.

just for motivation: I am still very much looking forward to this feature :) But I understand that there might be other priorities.

This feature is nearly complete, but it has to wait for 8.10 since the 8.8 branch has already been cut. At this point, I'm just fixing little problems and documenting the implementation better. I'm looking forward to having this as well.

added ghc proposal label

What's the status on this? I would like for 8.10 to have:

#if WORD_SIZE_IN_BITS == 32
newtype Int# = Int# Int32#
newtype Word# = Word# Word32#
#elif WORD_SIZE_IN_BITS == 64
newtype Int# = Int# Int64#
newtype Word# = Word# Word64#
#else
#  error unsupported native word size
#endif

I have a few preliminary things to tackle, but after that I would be blocked on this.

This will certainly make it into 8.10. The MR is at !364 (closed). There is a single outstanding regression at the point.

Ryan Scott figured out what caused the regression. This will likely get merged later today. Also, might I add, I'm very happy to see that someone has come up with a use for this extension that I didn't consider when writing the proposal. I guess this means that the kind of Word# would vary by platform and that you'd want WordRep as a type synonym:

#if WORD_SIZE_IN_BITS == 32
type WordRep = 'Word32Rep
#elif WORD_SIZE_IN_BITS == 64
type WordRep = 'Word64Rep
#else
error unsupported native word size
#endif

I look forward to seeing an MR for this stuff. cc me when you put something up.

mentioned in commit 414e9735

mentioned in commit effdd948

Fixed in effdd948.

Testsuite-tests... well, there are too many to list here in a compact fashion. Just look at the commit diff :)

closed

added UnliftedNewtypes label

added Pnormal label