[ghc-steering-committee] Proposal #111: Linear Types

[Simon Peyton Jones]

Is this essentially proposing that we don't make any change to datatypes? That would mean that a library that wishes to have linear datatypes would have to explicitly declare them as such. I think this is a stable middle ground.

Are you proposing that pairs, lists, Maybe etc all have non-linear types?   Thus Just :: a -> Maybe a.  That would negate one of the principal merits of the proposal, namely that it allows all the existing data types and library functions to work for linear types too.    We don't want to force people to implement carbon copies of existing libraries!

I think we want both (Just :: a -o Maybe a) and (fromJust :: Maybe a -o a); but suppressing the lollipops when printing for the user unless -XLinear is on.

That is, without -XLinear we behave as if the Prelude data types were ordinary pre-linear types; but when you switch -XLinear on, you see that the Prelude types are in fact linear, so that they are useful in a linear setting.

Or am I misunderstanding the "stable middle ground"?

Simon

From: ghc-steering-committee <ghc-steering-committee-bounces at haskell.org> On Behalf Of Richard Eisenberg
Sent: 27 August 2018 22:33
To: Iavor Diatchki <iavor.diatchki at gmail.com>
Cc: ghc-steering-committee <ghc-steering-committee at haskell.org>
Subject: Re: [ghc-steering-committee] Proposal #111: Linear Types

Let's keep the conversation going here. I know this is a *big* proposal, but we owe it to the authors to form a quorum-based consensus (I'd love more voices in these threads!) and offer a response.


On Aug 7, 2018, at 1:50 AM, Iavor Diatchki <iavor.diatchki at gmail.com<mailto:iavor.diatchki at gmail.com>> wrote:

The main high level concern I have is the overloading of the function space arrow.  We already have it overloaded in all kinds of ways, and adding one more dimension without any kind of underlying system seems too much.  I am not at all convinced that we will be able to "hide" multiplicities when the extension is not enabled.

I agree. I've mused about making (->) be indexed by a type-level record. Right now, that record would look like

> data ArrowIndex = ArrowIndex { argRep :: RuntimeRep, resRep :: RuntimeRep }
> (->) :: forall (ind :: ArrowIndex). TYPE (argRep ind) -> TYPE (resRep ind) -> Type

With this proposal, we move up to

> data ArrowIndex = ArrowIndex { argRep :: RuntimeRep, resRep :: RuntimeRep, multiplicity :: Multiplicity }
> (->) :: forall (ind :: ArrowIndex). TYPE (argRep ind) -> TYPE (resRep ind) -> Type

But I don't have great ideas of how to make this work syntactically -- never mind the fact we don't have type-level records yet.



Overall, while I like the core ideas, I would prefer a different way of integrating them into Haskell, one that is more modular, even at the cost of having to duplicate some code.  My reasoning is that while linearity might be useful in some cases, most of the time it is not something that we'd care about much, so we should not add it (at least straight away) to the core part of the language (functions and data types).

Is this essentially proposing that we don't make any change to datatypes? That would mean that a library that wishes to have linear datatypes would have to explicitly declare them as such. I think this is a stable middle ground. But, if we can guarantee good error-message behavior, etc., I think the "default datatypes to linear" behavior is the right one.

Richard
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