[ghc-steering-committee] Proposal UnliftedArray#: recommend acceptance

[Simon Peyton Jones]

Yes, looks good to me.

Simon

From: ghc-steering-committee <ghc-steering-committee-bounces at haskell.org> On Behalf Of Simon Marlow
Sent: 08 May 2018 17:17
To: ghc-steering-committee at haskell.org
Subject: [ghc-steering-committee] Proposal UnliftedArray#: recommend acceptance

UnliftedArray# has been proposed by Andrew Martin:
https://github.com/ghc-proposals/ghc-proposals/pull/112
In a nutshell, the proposal is to add new primitive types UnliftedArray# and MutableUnliftedArray#

data UnliftedArray# :: TYPE 'UnliftedRep -> TYPE 'UnliftedRep
data MutableUnliftedArray# :: TYPE 'LiftedRep -> TYPE 'UnliftedRep -> TYPE 'UnliftedRep
with the appropriate set of primops, e.g.

indexUnliftedArray# :: forall (a :: TYPE 'UnliftedRep). UnliftedArray# a -> Int# -> a
This makes complete sense and it is what we would have done from the start, except that we only recently gained the ability to talk about type variables that range over unlifted types.
UnliftedArray# subsumes some existing rather ugly sets of primops that were added because we lacked the ability to define the more general version, like ArrayArray# And ByteArrayArray#, also including the hilarious

indexArrayArrayArray# :: ArrayArray# -> Int# -> ArrayArray#
All of this can be defined in terms of the general UnliftedArray#. (the migration will be easier if we can use UnliftedNewtypes to define ArrayArray# in terms of UnliftedArray#, otherwise we have to keep the original versions in the compiler, but I don't think the exact mechanism matters all that much for the purposes of accepting or rejecting the proposal).
Incidentally we might also want UnliftedMutVar#, UnliftedMVar#, UnliftedTVar#, and maybe UnliftedStablePtr# but that's for another proposal.

I propose that we accept this. Any objections?
Cheers
Simon
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