Linear thoughts

Posted on 2017-01-17 by Oleg Grenrus linear

I have been wondering about linear logic for a while, and after reading Edsko de Vries' blog post: Linearity, Uniqueness, and Haskell, I finally got to dump my thoughts on the topic.

Edsko starts with two example functions, where the second one is following:

frugal :: a -> (a, a)
frugal x = (x, x)

In Haskell functions can reuse their arguments, as frugal does. Later he shows that linear setting we can forbid such functions.

## Pairs

A taste of linear logic by Philip Wadler is a very nice introduction to the intuitionistic linear logic. One of the take aways, is that in linear logic, there are two kind of pairs, "ands". And there's still single "or".

So to recap, Take A to be the proposition "I have ten zlotys", B to be the proposition "I have a pizza", and C to be the proposition "I have a cake". We can say

  • A ⊢ B: for ten zlotys I may buy a pizza
  • A ⊢ C: for ten zlotys I may buy a cake

And there are three ways to "pair" the terms:

  • A, A ⊢ B ⊗ C: for twenty zlotys I can buy both a pizza and a cake,
  • A ⊢ B & C: for ten zlotys I can buy whichever I choose from a cake and a pizza,
  • A ⊢ B ⊕ C: for ten zlotys I can buy either a pizza or a cake, but I don't have a choice.

In natural language we'd might say: whichver I choose from a cake or a pizza. That's confusing.

In the Edsko's post, the pairs are assumed to be of the first kind: ⊗, tensors. We cannot write fst or snd variants for the tensor, but

uncurry :: (a -o b -o c) -o a ⊗ b -o c
uncurry f p = case p of (x, y) -> f x y

is ok.

However, if we work with the second kind of the pair, "with", we cannot write uncurry, but can have fst and snd:

fst :: a & b -o a
fst (x, _) = a

snd :: a & b -o b
snd (_, y) = y

And with this pair we can write something like:

haveAndEat :: Vector Double -o Vector Double & Vector Double
heveAndEat arr = (write 0 2.3 arr, arr)
-- one should really have different constructor for tensor and with

There is no problem, as a callee cannot use both sides of the with, they have to choose only (and exactly) one half.

I.e. we can write frugal as

frugal :: a -o a & a
frugal x = (x, x)

## With

IMHO with is a very interesting construction. It seems it cannot exist in a strict language: only when we apply fst or snd will force it. In addition, when we force one half, the thunk for the other can be released immediately.

I guess this can be useful in non-backtracking infinite solution-space traversals, by using with we could be sure we don't introduce space leaks by accidentally retaining other paths.

## Linear State

Recall a state monad

newtype State a = State { runState :: s -> (a, s) }

We can have linear version of it, and the Monad instance would type-check:

newtype LinearState a = LinearState { runLinearState :: s -o a ⊗ s }

instance Monad LinearState where
  return x = \s -> (x, s)
  m >>= k  = LinearState $ \s -> case runLinearState m s of
     (a, s') -> runLinearState (k a) s'

The interesting part, is that this type isn't an instance of MonadState. We cannot write get or put. And we cannot write MonadReader with ask = get either. But we can have a different class:

class Monad m => MonadLinearState s m | m -> s where
  linearModify :: (s -o s) -o m ()

instance MonadLinearState s (LinearState s) where
  linearModify f = LinearState $ \s -> ((), f s)

Also LinearWriter, i.e. the one without listen or pass.

AFAICS, we could have MonadLinearState RealWorld# IO, without problems!

## do-notation

It seems, that if we had linearity, then we can use linearity to handle resources. We can write bracket like functions, where we have to return the token at the end:

-- Not sure about the arrows, should they be lollipops?
withFile :: FilePath -> (Handle -o IO (Handle ⊗ a)) -> IO a
getFileContents :: Handle -o IO (Handle & String)

lineCount :: FilePath -> IO Int
lineCount fp = withFile fp $ \h -> do
  (h, contents) <- getFileContents h
  return (h, length $ lines contents)

That's something where LinearStateT will make code nicer. Or we can change getFileContents :: Handle OPENED -o IO (Handle CLOSED ⊗ String), or actually we'd need to have

  :: FilePath
  -> (forall s. Handle s OPENED -o IO (Handle s CLOSED ⊗ a))
  -> IO a

The type variable s will index our Handle, so don't pass a wrong one when we have nested withFiles.

But then we couldn't use do notation, as state variable will change. We'd need indexed monads, oh dear.

At this point, I have to mention, that linear types would give us great power, but explaining this kind of IO (or any "magic" involcing them) to a beginner won't be easy.

FWIW, withMutableArray from Edsko's post is simpler example of this idea of handling resources.

## Point free

phadej    @pl \xs -> map f (filter g xs) 
lambdabot map f . filter g              -- looks linear
phadej    @pl \x -> x * x
lambdabot join (*)                      -- but join isn't (Reader)
phadej    @pl \p -> (fst p, snd p)
lambdabot id                            -- sometimes pl is smart
phadej    @pl \p -> (fst p, snd p + 2)
lambdabot liftM2 (,) fst ((2 +) . snd)  -- sometimes it isn't
phadej    @pl \p -> uncurry (\x y -> (x, y + 2)) p
lambdabot second (2 +)                  -- so you have to help it

TL;DR concatative languages are linear by default, as there are special operators to contract and weaken!

## Traversals

It's probablty not obvious, but the type

traverse :: (Applicative f, Traversable t) => (a -> f b) -> t a -> f (t b)

is ok to linearise into:

  :: (LinearApplicative f, LinearTraversable t)
  => (a -o f b) -> t a -o f (t b)

Or can we reuse Applicative f? Hard to say at this point. What arrows should be there? Polymorphic in arrows, with some relations?

## Conclusion

In this post I used different syntax than in used by Edsko, the one I learned from watching lectures and reading papers on linear logic. And I have to agree, it's hard to evaluate how well type system will work in practice. Will we need with, how higher order functions or polymorphism will fit etc. I'm also looking forward how Haskell could support linearity, and what cool stuff we can do with it!

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