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Efficient Nanopass Compilers using Cats and Matryoshka

↓ Twitter slide ↓

Hi, I’m Greg. I work for SlamData, a small startup in Boulder that works on analytics for NoSQL. The core of our product is basically a compiler from a multidimensional relational model to various NoSQL query APIs. We also expose a frontend that compiles from a generalized SQL variant to our relational model. The techniques I’m going to talk about largely reflect how we do things (although, of course, different parts of the code base are closer or farther from the ideal).

Introduction

First, I want to say that while I’m discussing these things in the context of compilers, it’s applicable in tons of software. For example, anywhere you deal with recursive data structures, like JSON or XML. So, if you don’t have much of an interest in compilers yourself, I hope you still take something useful away from this.

Overview

type Parser[A] = String => List[(A, String)]

type Compiler[A, B] = A => B

type Serializer[B] = B => String
A parser for things is a function from strings to a list of pairs of things and strings.

A compiler’s a function from thing 1 to thing 2

So, these are the three most coarse-grained parts of a compiler – a parser that reads in the syntax, the core of the compiler itself, and a serializer that writes the final code, whether in another source language like JS, some bytecode, etc. And, of course, those first and last parts are also a bit fuzzy. Structure editors are all about skipping the parsing stage, and editing the AST directly. Interpreters replace the serializer with an evaluator (but still usually have some actual compilation stages along the way).

The function in the middle, though, the compiler itself is always there. And boy … it’s pretty generic. Pretty much anything can be a compiler, eh?

Of course, the true type is usually a bit more specific than that, and we’ll see how to expose that.

I’m going to talk about three and a half approaches to compiler design here. And they’re definitely in the order I was introduced to them – although I think people who learn more about compilers in school probably see them in a slightly different way.

There are tradeoffs between them, but I think there’s also a progression that makes sense to the FP mindset, where the benefits of the earlier approaches are perhaps seen as not as important. Or, at least, something to be deprioritized until after the initial design is complete.

The Language

Before we get into the compiler, let’s talk about what we’re going to compile.
sealed trait Lambda
final case class Lam(args: List[String], body: Lambda)
    extends Lambda
final case class App(f: Lambda, expr: List[Lambda])
    extends Lambda
final case class Var[A](name: String) extends Lambda
final case class Let(bindings: List[(String, Lambda)],
                     body: Lambda)
    extends Lambda
final case class If(test: Lambda,
                    consequent: Lambda,
                    alt: Lambda)
    extends Lambda
Not the best language, but pretty simple for our purposes. It’s basically lambda calculus with n-ary functions, let bindings, and an if expression.

The Target

And, of course, we need something to compile to …
sealed trait Assem
Yeah, we don’t really need to know the details of Assem. It’s already a pain to fit a lot of this on a slide, without having to worry about the details of the implementation.

Monolithic

A monolithic compiler is pretty efficient. It trades off readability and maintainability for performance.

For the bulk of my time working in compilers, this is what I was exposed to. I love compilers and language design, but every time I would be introduced to a new compiler, I would have anxiety as I tried to figure out the code base.

This isn’t necessarily one pass, but rather a small number of complicated passes. It gets good performance and pretty compact code in general, but maintainability is … tough.

def compile: Lambda => State[CompState, Assem] = {
  case Var(name) => ???.point[State[CompState, ?]]
  case Lam(args, body) => compile(body).map(???)
  case App(f, exprs) =>
    (compile(f) |@| exprs.traverse(compile))(???)
  case Let(bindings, body) =>
    val (names, exprs) = bindings.unzip
    compile(App(Lam(names, body), exprs))
  case If(test, consequent, alt) =>
    compile(App(test, List(Lam(Nil, consequent), Lam(Nil, alt))))
}
We do everything in a single pass in this compiler, basically compiling bottom-up, with State to carry the information we need between steps. The one interesting bit is where we rewrite the Let into App(Lam), then call compile on that value.

Micropass (multipass)

The name of this style comes from the fact that passes are broken down into smaller pieces, but beyond that there’s a range of possibilities here. EG, do they combine operations on various components that don’t have to be done together? Do they duplicate their datatypes with small changes, or just have one datatype that is a bit sloppy? (usually somewhere in between – “outer” and “inner”)

unsafe

Even at the finest-grained level, this tends to have functions with names like desugar that do a number of transformations at once, basically transforming one largely-duplicated AST into a smaller one (or even using the same AST, and just erroring when one of the cases that shouldn’t exist still does).
val desugar: Lambda => Lambda = {
  case Let(bindings, body) =>
    bindings.unzip((names, exprs) =>
      App(Lam(names, desugar(body)), exprs.map(desugar)))
  case If(test, consequent, alt) =>
    App(desugar(test), List(
      Lam(Nil, desugar(consequent),
      Lam(Nil, desugar(alt))))
  case Lam(args, body) => Lam(args, desugar(body))
  case App(f, exprs) => App(desugar(f), exprs.map(desugar))
  case Var(name) => Var(name)
}

unsafe, cont.

You now have a more restricted structure, but it’s not reflected in the AST. So you end up having to deal with “impossible” cases somehow.
val translate: Lambda => State[CompState, Assem] = {
  case Lam(args, body) => ???
  case App(f, exprs) => ???
  case Var(name) => ???
  case _ => scala.sys.error("invalid node for compilation")
}

val compile: Lambda => State[CompState, Assem] =
  translate.compose(desugar)

duplicated

The other approach in micropass trades off this … unsafety … for a bit of duplication
sealed trait InnerLambda
final case class ILam(args: List[String], body: InnerLambda)
    extends InnerLambda
final case class IApp(f: InnerLambda, expr: InnerLambda)
    extends InnerLambda
final case class IVar[A](name: String) extends InnerLambda

val desugar: Lambda => InnerLambda = ???

val translate: Lambda => State[CompState, Assem] = {
  case ILam(args, body) => ???
  case IApp(f, exprs) => ???
  case IVar(name) => ???
}
Now we’ve duplicated a subset of Lambda as InnerLambda, but we no longer have (im)possible failure in translate.

Again – these are very simple examples. The pain doesn’t scale linearly – there are often many analyses that happen in a compiler, and with the duplication, there is way more code. And also, duplication means keeping things in sync. If you change one AST, you have to change others, and all the operations that touch them.

Nanopass

This tightens up the micropass, generally with more peformance cost – Coproducts, Prisms, etc. help us in the Typelevel world.

a digression: fixed point data types

Up to this point, we’ve been writing our transformations pretty directly. But for this next step, we’re going to have to change that a bit.
sealed trait InnerLambda[A]
final case class Lam[A](args: List[String], body: A)
    extends InnerLambda[A]
final case class App[A](f: A, expr: List[A])
    extends InnerLambda[A]
final case class Var[A](name: String) extends InnerLambda[A]
case class Fix[F[_]](unFix: F[Fix[F]])

Fix[InnerLambda]

Coproduct and Inject (Cats)

The first thing we’ll use Fix for is composing smaller data types, so we can incrementally change from one type to another without a lot of duplication.
final case class Let[A](bindings: List[(String, A)], body: A)
final case class If[A](test: A, consequent: A, alt: A)

def expandLet[Lambda :<: F]: Fix[Let :+: F] => Fix[F] =
  _.unFix match {
    case Let(bindings, body) =>
      bindings.unzip((names, exprs) =>
        Fix(App(Fix(Lam(names, expandLet(body)).inject),
                exprs.map(expandLet)).inject))
    // and don’t forget the other cases
  }

def expandIf[Lambda :<: F]: Fix[If :+: F] => Fix[F] =
  _.unFix match {
    case If(test, consequent, alt) =>
      Fix(App(expandIf(test), List(
        Fix(Lam(Nil, expandIf(consequent))),
        Fix(Lam(Nil, expandIf(alt))))))
    // seriously, still gotta handle the other cases
  }
What this gives us is fine-grained control over our data types without duplicating them. We spilt it into “atomic” pieces and only have to be aware of the pieces we care about in a particular transformation. That allows us to do either expandIf <<< expandLet or expandLet <<< expandIf whenever we want. In the former, the types go

Fix[If :+: Let :+: Lambda]Fix[If :+: Lambda]Fix[Lambda]

and in the latter

Fix[If :+: Let :+: Lambda]Fix[Let :+: Lambda]Fix[Lambda]

So, you can see that even though we diverge in the middle, we go from the same initial type to the same final type.

regaining performance … through further abstraction

Turning our various transformations from those on recursive structures (granted, already fixpoint, thanks to Coproducts) to folds, natural transformations, etc. allows us to compose these operations again.

Up to now, we’ve been defining our transformations like Mu[F] => Mu[F]. But there is a way that is both easier and more efficient. We’ll start with something familiar.

Matryoshkahttps://github.com/slamdata/matryoshka

  • 1.0 coming soon (~1 month)
  • (hopefully) Typelevel incubator status soon

folds, unfolds, and transformations

def cata[F[_]: Functor](t: Fix[F])(φ: F[A] => A): A

def ana[F[_]: Functor](a: A)(ψ: A => F[A]): T[F]

val primeFactors: Int => ListF[Int, ?]
val inferType: (Type, Fix[Lambda]) => Lambda[(Type, Fix[Lambda])]

def transCata[F[_]: Functor, G[_]: Functor](
  t: Fix[F])(f: F[Fix[G]] => G[Fix[G]]):
    T[G]
Does that look familiar? If you’ve used something by the same name in Scalaz, you might be onto me. That function is foldRight generalized to any functor. And what does foldRight generally do for us? It allows us to specify single-step semantics for a list and it handles the recursion for us.

algebras

So, ok, we don’t need to worry about recursion ourselves, but what else does that buy us? It allows us to eliminate multiple passes over our data. Using things like cata, we can write various algebras (that’s what they’re called), and compose them efficiently.
def expandLet[Lambda :<: F]: (Let :+: F)[Fix[F]] => F[Fix[F]] = {
  case Let(bindings, body) =>
    bindings.unzip((names, exprs) =>
      Fix(App(Fix(Lam(names, body).inject), exprs).inject))
  case x => x
}

def expandIf[Lambda :<: F]: (If :+: F)[Fix[F]] => F[Fix[F]] = {
  case If(test, consequent, alt) =>
    Fix(App(test,
      List(Fix(Lam(Nil, consequent)), Fix(Lam(Nil, alt)))))
  case x => x
}
There are a few things to note here: First, we now take the “unwrapped” functor as the argument. So we don’t need to unFix prior to matching, and the type itself tells us that the recursive calls have already been made, so we don’t need to do that explicitly either. And since we don’t need to handle the recursive calls, we can handle all the cases we don’t care about with a simple identity.

And, like when we first introduced Cat’s Inject above, we can compose our functions …

composition

expandIf <<< expandLet
But now, rather than that operating over trees, it operates over a single node of the tree.
val desugar: Fix[Let :+: If :+: Lambda] => Fix[Lambda] =
  _.transCata(expandIf <<< expandLet)
So, now, rather than traversing the tree and building intermediate trees for every step of the way, we only traverse once (like we did in the micropass version), but still have the nano-level operations.

Also, when you traverse a tree, how does it work? A bottom-up traversal first walks to the leaves, then applies some operation on the way back up. A top-down traversal performs some operation at the root, then performs it on the branches, etc. and pops back out at the end. So … in both cases, you go to the leaves, then back to the root, but you only perform the operation in one direction.

Matryoshka gives us another way to compose recursive operations that takes advantage of this:

other combinations

up and down

x.hylo(bottomUp, topDown)

x.ana(topDown).cata(bottomUp)
Now, in a single pass we apply a top-down transformation (which may be a composition of smaller transformations) as we move toward the leaves, then a bottom-up transformation as we move back to the root. So, even though those two operations aren’t directly composable, we’ve still managed to perform both of them in a single traversal of the tree.

Matryoshka has a bunch of variants on this theme. Applied in the right places, we can take advantage of the small, clear nanopass style while still maintaining the performance and relatively small code size of the monolithic style.

zipping

val pprint: Lambda[String] => String
val eval: Lambda[Int] => Int

lam.cata(pprint zip eval): (String, Int)

annotations

val buInferType: Lambda[Type] => Type

lam.cata(buInferType.attribute): Cofree[Lambda, Type]

val useType1: Lambda[(Type, Value)] => Value
lam.zygo(inferType, useType1): Value

val tdInferType: (Type, Fix[Lambda]) => Lambda[(Type, Fix[Lambda])]
val useType2: (Type, Lambda[Value]) => Value
lam.coelgot(useType2, tdInferType): Value

Questions?

  • I’ll probably be a bit scarce tomorrow, but I’m happy to talk about this more in the hallways today or during both days of NEScala.
  • I’m happy to unconference about more of this stuff tomorrow or Saturday (on NEScala’s unconference day) – specifically, go into depth on Matryoshka
  • Rob Norris (@tpolecat) is planning an unconference session on Doobie that goes into some other fixpoint topics
  • SlamData will be hiring soon, so if any of this sounded interesting (or if it didn’t, but other aspects of our company – NoSQL, analytics, etc. do), then talk to me about that as well.
  • convince Rob Norris (@tpolecat) to talk about some Doobie-related fixed point stuff tomorrow
  • more about Matryoshka in Saturday’s unconference?
  • SlamData (http://slamdata.com/) is hiring
  • LambdaConf on Memorial Day weekend in Boulder, with a one-day Typelevel Summit

Greg Pfeil greg@slamdata.com