Introducing the mutable library

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mutable: documentation / reference / github

(Note: This post has been heavily revised to reflect mutable-, as of July 2020. For reference, the original post is available on github.)

I’m excited to announce the release of the mutable library!

The library offers what I call beautiful mutable values1 — automatic, composable piecewise-mutable references for your data types. Sort of like an automatically generated MVector, but for all your ADTs.

My high-level goal was a composable and overhead-free solution for dealing with mutable values in Haskell in a type-safe and clean way. After all, why do imperative languages have to have all the fun? In Haskell, we can have the best of both worlds: efficient and clean mutable algorithms and type safety.

The official documentation and homepage is here, so it’s a good read if you want to be introduced to how to use the library and where it is most effective. But I’m going to use this blog post to talk about why I wrote the library, some of the neat things you can do with it, and the techniques that went into writing it.


The original motivation for this comes from my development of backprop and backprop-learn, as I was trying to adapt my Functional Models framework to efficient Haskell code.

To properly train Artificial Neural Networks with Haskell, you need to do a lot of independent piecewise mutations to matrices and vectors. This becomes inefficient, quickly, because you have to do a lot of copying in the process for pure vectors and neural network weights. This problem also comes up for efficient simulations that require mutating many different components independently under a tight loop.


First of all, what do I mean by “piecewise-mutable”? Well, a simple example is the mutable vector type, where piecewise-mutable edits are able to save a lot of time and memory allocation.

If we want to edit the first item in a vector multiple times, this is extremely inefficient with a pure vector:

addFirst :: Vector Double -> Vector Double
addFirst xs = iterate incr xs !! 1000000
    incr v = v V.// [(0, (v V.! 0) + 1)]

That’s because addFirst will copy over the entire vector for every step — every single item, even if not modified, will be copied one million times. It is \(O(n*l)\) in memory updates — it is very bad for long vectors or large matrices.

However, this is extremely efficient with a mutable vector:

addFirst :: Vector Double -> Vector Double
addFirst xs = runST $ do
    v <- V.thaw xs
    replicateM_ 1000000 $ do
        MV.modify v 0 (+ 1)
    V.freeze v

(this action is run in ST, the monad for mutable actions that is provided by GHC)

This is because all of the other items in the vector are kept the same and not copied-over over the course of one million updates. It is \(O(n+l)\) in memory updates. It is very good even for long vectors or large matrices.

This situation is somewhat contrived, but it isolates a problem that many programs face. A more common situation might be that you have two functions that each modify different items in a vector in sequence, and you want to run them many times interleaved, or one after the other.

Composite Datatype

That was an example of using piecewise mutability for vectors, but it’s not exactly scalable. That’s because it always requires having a separate type for the pure type and the value type. We’re lucky enough to have one for Vector…but what about for our own custom types? That’s a lot of headache.

data TwoVec = TV { tv1 :: Vector Double
                 , tv2 :: Vector Double
  deriving Generic

To use this in a “piecewise-mutable” way, we would need a separate “mutable” version:

data TwoVecRef s = TVR { tvr1 :: MVector s Double
                       , tvr2 :: MVector s Double

Then we can do things like “mutate only the first item in the first vector” a million times, and be efficient with it.

We’d have to write functions to “thaw” and “freeze”

thawTwoVec :: TwoVec -> ST s (TwoVecRef s)
thawTwoVec (TV x y) = TVR <$> V.thaw x <*> V.thaw y

freezeTwoVec :: TwoVecRef s -> ST s TwoVec
freezeTwoVec (TVR u v) = TV <$> V.freeze u <*> V.freze v

It just doesn’t scale in a composable way. You’d have to create a second version of every data type.


The library provides the Mutable typeclass and the GRef type, where GRef s X is the automatically derived piecewise-mutable version of X.

instance Mutable s TwoVec where
    type Ref s TwoVec = GRef s TwoVec

The type GRef s TwoVec is exactly the TwoVecRef that we defined earlier: it is a tuple of two MVectors. It can do this because Vector itself has a Mutable instance, where its mutable version is MVector. GRef s TwoVec is essentially the “MVector” of TwoVec.

This now gives us thawRef :: TwoVec -> ST s (GRef s TwoVec) and freezeRef :: GRef s TwoVec -> ST s TwoVec, for free, so we can write:

addFirst :: TwoVec -> TwoVec
addFirst xs = runST $ do
    v <- thawRef xs
    replicateM_ 1000000 $ do
      withField #tv1 v $ \u ->
        MV.modify u 0 (+ 1)
    freezeRef v

This will in-place edit only the first item in the tv1 field one million times, without ever needing to copy over the contents tv2. Basically, it gives you a version of TwoVec that you can modify in-place piecewise. You can compose two functions that each work piecewise on TwoVec:

mut1 :: Ref s TwoVec -> ST s ()
mut1 v = do
    withField #tv1 v $ \u ->
      MV.modify u 0 (+ 1)
      MV.modify u 1 (+ 2)
    withField #tv2 v $ \u ->
      MV.modify u 2 (+ 3)
      MV.modify u 3 (+ 4)

mut2 :: Ref s TwoVec -> ST s ()
mut2 v = do
    withField #tv1 v $ \u ->
      MV.modify u 4 (+ 1)
      MV.modify u 5 (+ 2)
    withField #tv2 v $ \u ->
      MV.modify u 6 (+ 3)
      MV.modify u 7 (+ 4)

doAMillion :: TwoVec -> TwoVec
doAMillion xs = runST $ do
    v <- thawRef xs
    replicateM_ 1000000 $ do
      mut1 v
      mut2 v
    freezeRef v

The end result? You can now modify only a single component of your large composite data type (and even single items in vectors in them) without making nested copies every time.

Neat Consequences

Mutable Sum Types

While developing the library, I accidentally also stumbled into a way of automatically deriving useful mutable sum types and data structures in Haskell. This was more or less a complete accident — I was writing the code to automatically generate GRef, and needed to account for sum types somehow. The result was actually useful!

For example, it is a publicly kept secret that Haskell’s list type — “linked lists”, are actually very different from the mutable linked lists encountered as a standard data structure in languages like Java and C++. As it turns out, using GRef m [a] gives us exactly the mutable linked list type … for free!

data List a = Nil | Cons a (List a)
  deriving (Show, Generic)
infixr 5 `Cons`

instance Mutable s a => Mutable m (List a) where
    type Ref s (List a) = GRef s (List a)

Here we are re-implementing the List data structure from scratch just to show that there is nothing arbitrary going on with the default list — it works for any appropriately defined ADT. We could even do binary trees!

Right away we can write functions to flesh out the API for a mutable linked list. For example, a function to check if a linked list is empty:

-- | Check if a mutable linked list is currently empty
    :: Mutable s a
    => Ref s (List a)
    -> ST s Bool
isEmpty = hasBranch (constrMB #_Nil)

Here is a function to “pop” a mutable linked list, giving us the first value and shifting the rest of the list up.

    :: Mutable s a
    => Ref s (List a)
    -> ST s (Maybe a)
popStack xs = do
    c <- projectBranch (constrMB #_Cons) xs
    forM c $ \(y, ys) -> do
      o <- freezeRef y
      moveRef xs ys
      pure o

And a function to concatenate a second linked list to the end of a first one:

    :: Mutable s a
    => Ref s (List a)
    -> Ref s (List a)
    -> ST s ()
concatLists l1 l2 = do
    c <- projectBranch consBranch l1
    case c of
      Nothing      -> moveRef l1 l2
      Just (_, xs) -> concatLists xs l2

Higher-Kinded Data

I’m rather enamoured by the “higher-kinded data” pattern made popular by Sandy Maguire. It essentially eliminates the need for explicit getters and setters by making the data type itself the thing that offers what you want, and you can get at it by just pattern matching.

Because of this, if your data type is written in the “higher-kinded data” pattern, then MyType f doubles as both the pure type and the mutable type, just by choice of f. MyTypeF Identity would be the pure version, and MyTypeF (RefFor m) would be the mutable version.

data MyTypeF f = MTF
    { mtfInt    :: HKD f Int
    , mtfDouble :: HKD f Double
    , mtfVec    :: HKD f (V.Vector Double)
  deriving Generic

type MyType' = MyTypeF Identity

instance Mutable s MyType' where
    type Ref s MyType' = MyTypeF (RefFor s)

We can directly use it like a normal data type:

MTF 3 4.5 (V.fromList [1..100])
    :: MyType'

But now, MyTypeF (RefFor s) literally has mutable references as its fields. You can pattern match to get rI :: MutVar s Int, rD :: MutVar s Double, and rV :: MVector s Double

MTF rI rD rV :: MyTypeF (RefFor s)

and the accessors work as well:

    :: MyTypeF (RefFor s)
    -> MVector s Double

You can use it like:

runST $ do
    r@(MTF rI rD rV) <- thawRef $ MTF 0 19.3 (V.fromList [1..10])

    replicateM_ 1000 $ do

        -- rI is just the 'Int' ref
        modifyMutVar rI (+ 1)

        -- rV is the 'MVector'
        MV.modify rV (+1) 0

    freezeRef r

-- => MTF 1000 19.3 [1001.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0]

The “mutable version” of a type literally is the ADT, if you use the higher-kinded data pattern!

A Polymorphic Picture

One important thing to note when looking at the actual library — the examples in this post show the provided actions in ST, the mutable actions monad provided by GHC. However, the library provides these actions polymorphic for all PrimMonad m, an abstraction provided by the primitive library to generalize for all “mutable monads” (like IO and monad transformers applied to IO and ST), as long as PrimState m ~ s, so you can run them in whatever useful mutable monads you’d like.

Reflections on Generic

This library is pretty much powered 95% by GHC Generics, as the name GRef implies. GHC Generics is probably one of the single most powerful tools we have in Hasekll-the-language for writing typesafe abstractions and eliminating all the boilerplate.

The structure of the GRef data type is completely determined by using the GHC.Generics Rep of an algebraic data type with a Generic instance. It breaks apart the products and sums and turns them into the mutable references you would normally write by hand.

Writing GRef itself was actually very pleasant: it just involves matching up generic pieces with the references they represent. “What is the reference for a constant value? What is the reference for a product type? What is the reference for a sum type?” And, in the process of answering those questions, I ended up discovering something new (as shown in the section above about mutable linked lists).

Generics also powers the higher-kinded data based systems, which can add a lot of syntactic niceness to everything if you decide to use it.

Still, I understand not everyone wants to restructure their data types in terms of higher-kinded data … there are a lot of practical issues to doing so, and it doesn’t really work well with nested data types. For that, I turned to generic-lens.

generic-lens is what powers the OverloadedLabels-based field accessor methods that let you work with GRefs in a seamless way, by being able to do withField #blah, etc., instead of having to directly match on the GRef value’s internal contents (which can be messy, admittedly). It also allows you to do withPos @2 to get the second item in your GRef, and withTuple to allow you to get the mutable fields in your data type as a tuple.

I was originally going to implement the field accessors myself, looking to generic-lens for inspiration. However, when I looked at the library’s internals, I realized there was a lot more going on than I had originally thought. But, looking at what was exported, I realized that the library was well-designed enough that I could actually directly use its generic implementations for mutable! As a result, the field/position/tuple accessor code actually required no mucking around with generics at all — I could leverage generic-lens, which was powerful enough to allow me to eliminate all of my generics code.

I strongly recommend anyone looking to do things involving generic access to fields to look at generic-lens to see if it can eliminate all your generics code as well!

Unfortunately, I wasn’t able to re-use the code for the “constructor” access (as seen with constrMB #_Cons earlier) — but I could use it as inspiration to write my own. The library offers a very clean and well-written pattern to doing things like this that I probably would have spent a long time trying to figure out, if I had to do it from scratch.

Next Steps

I learned a lot from GHC Generics writing this library — in a sense, the library is pretty much completely an application of GHC Generics, without much new concepts beyond that.

My next step is to equip backprop to use Mutable instead of its Backprop typeclass, so it can do in-place mutation of composite data types for much faster backpropagation.

However, my newly gained experience with generics from writing this library can actually do a lot to improve the ergonomics of backprop as well — in particular, with BVar, which has always been very annoying to work with, even with the lens-based API offered. Working with a BVar as if it were a normal value has always been annoying, especially with product types. There are a lot of ways GHC generics can help this, that I am now only learning about. Check back soon — hopefully I’ll have something to show by then.

Until then, happy mutating! And please let me know if you find any interesting applications of the library :D

  1. Okay so I don’t actually think the library is beautiful, I just like the way that “beautiful mutable values” sounds when you say it out loud.↩︎

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