Compilers are hard


I’ve often heard that writing a compiler is hard, and different than writing other kinds of software. Some recent experience hsa provided me insight as to why this is the case and proved quite interesting!

I recently completed work on a new big feature in ShipReq. I’d been working on it for ~2 months and it ended up being the hardest code that I’d ever written in my life. I’ve been coding for decades and have worked on so many different projects, and for some very big organisations; I tell you this for context. When I say this is the hardest code I’ve ever written, it had a lot of competition. Concurrent systems usually has the reputation of being very difficult and I’ve designed and written a massively concurrent project from the ground up in my OO days, included writing all the scheduling and concurrency primitives, and managing all the thread-safety myself (those were different times). I found this piece of work an order of magnitude harder than that.

I say in the ShipReq About page that ShipReq is like a compiler. The big feature that I completed recently was the most compiler-like feature yet, and to my surprise it’s specifically that property that made it so deceptively hard. What is the feature? Basically when you have a bunch of requirements / issues / design documents / whatever, this feature adds automation to populate certain fields according to their relationships and related fields so that users don’t have to manually edit or maintain it all themselves, which is both tedious and error-prone. An example would be if someone raises a bug which requires a few dev tasks to fix, one could setup their project so that when all those dev marks are marked as “complete” the bug itself automatically changes to “ready for testing”. This feature doesn’t have any knowledge of “completion” or “testing”, but allows users to define whatever states and rules they want. That might not sound like it relates to writing a compiler. (It probably doesn’t even sound hard.) Nor do requirements, bug reports, project documents seem very relatable to code. But there are surprising similarities. Let’s take a look…

Similarity #1: Relationships

Nearly all data relationships in ShipReq are many-to-many / DAGs. Some views in ShipReq present data as flat lists but under-the-hood it’s very rare that anything is stored as a flat list, and there’s nearly always a way to view the same data as a graph or tree.

If you squint a bit, source code is kind of like that too. Packages have multiple classes / objects which have multiple methods / fields / inner classes that have multiple statements that have multiple clauses, etc. In terms of declaration, it’s more one-to-many than many-to-many but it’s still all a big DAG. When we start considering relationships like imports and methods calling other methods, then it becomes many-to-many too and we get a cyclic graph.

Similarity #2: Interactions

Secondly, this new feature in ShipReq is very configurable by users. It’s a major goal of ShipReq to avoid hardcoding implicit assumptions and find appropriate and configurable abstractions. Obviously having users need to reconfigure everything all the time would be a terrible burden so we provide pre-configured complete setups with best-practices which users can (optionally) use, and can reconfigure without restriction to best serve their needs. What that means from a coding point of view, is that we never know the automation rules at compile-time. The code needs to understand and apply all the rules at runtime, and it needs to be able to recognise and handle data and logic combinations that invalidate invariants. That might not sound like a big deal but there are many invariants in a complex system and invariants don’t stop at their immediate domain boundary - they propagate though all relevant graphs and compose, with various levels of complexity, with other invariants.

For example you may have 10 simple invariants in 10 isolated domains, but when they’re all combined the number of states to consider becomes 10,000,000,000 and there’s going to be a very significant percentage of those states that you’d consider illegal (i.e. bugs).

This too is like writing a compiler. For example, the Java compiler doesn’t hardcode the privacy level of your classes; it allows you to set it yourself. Where as ShipReq might have a UI page for config, in Java you have modifier keywords such that public class Hello is different than private class Hello. From the perspective of writing a compiler, these are runtime values with runtime rules. When we consider additional keywords like final and abstract we start getting into what I meant about combinational explosions. The Java compiler, at runtime, has to ensure all possible combinations are valid. For example, it will reject the following code:

final public class Hello {
  public abstract void doSomething(); // abstract method in final class not allowed


abstract class Hello {
  private abstract void doSomething(); // private + abstract not allowed

Imagine you were writing your own Java compiler. It’s entirely up to you to think about those illegal combinations and then write the code to catch them. I think we all take for granted things like marking a method as private and then knowing it’s impossible to call externally, but under-the-hood that it’s not impossible at all. You’re literally relying on someone’s if statement in the right place. Just like my ShipReq example, it’s all value inspection at runtime, with no guarantees about how all the features will interact. The class/method privacy feature interacting with final and abstract features is mentally easy enough but every single feature has the ability to interact badly with something else and it’s not always easy to identify those combinations.

I recently saw a PR to Scala that’s a great demonstration of exactly this. The private and sealed keywords have been in Scala forever and someone only just recently came up with the realisation that private classes are effectively sealed. Scala’s been in existence for 16 years! When you have hundreds of features it doesn’t seem feasible to manually consider each possible combination. 100 features yields 4950 unique pairs. There’s nothing special about pairs though, some issues may only manifest when three specific features intersect. For 100 features there are:

  • 4950 unique sets of 2
  • 161,700 unique sets of 3
  • 3,921,225 unique sets of 4
  • 17,310,309,456,440 unique sets of 10

It’s not unlikely to have a bad interaction between 10 features out of a possible 100. It’s just impossible to manually consider.

Similarity #3: Type-Unsafety

Type-safety is a pillar that I depend on all the time. And no please don’t think of Java’s type system when you think type-safety, that’s not really what I mean, although it’s a start. I mean more the type systems in languages like Scala and Haskell, think features like existential and dependent types. Type systems like Java’s and Typescript’s are great! They add value! But you can be an order of magnitude or two more safe with more advanced type systems and it’s profoundly beneficial when you learn how to make good use of them.

Unfortunately, type-safety didn’t help me very much when developing my big feature. Say you’ve got one data location than can accept a set of values. At compile-time, every runtime value is acceptable because there always exists a configuration that would allow each possible value to be legal. It’s like having a ‘Name’ field of type String, and then a runtime rule dictating the required first letter of the name. (Unrealistic but it’s the concept I’m highlighting.) The configuration might be firstNameLetter: Option[Char]. In such a case you have to have the name field be something accommodating of all possibilities, like name: String. You could be tricky an create a type that encodes a proof of the name such that it adheres to the rule, but that’s only a derivation of (name: String, firstNameLetter: Option[Char]). Unless the name and the configuration and atomically modifiable as a single, composite value, you still need to store both values independently before such a derivation. The first-letter of a name restriction is a toy example but it demonstrates the fact that you have runtime configuration in one location, runtime data in the other, and no type-safety at compile-time to enforce that the right thing happens at runtime.

Compilers have the same problem. They maintain a huge repository of runtime data, and it’s entirely up to the compiler authors to apply all of the rules, move data in and out of scope depending on other runtime-only conditions, etc, etc. And unlike us compiler users, they don’t have the benefit of being able to rely on (much of) a type system. They’re on their own and can only use type safety to guard the bottom layers of their very-complex domain.

Similarity #4: Feedback

It’s a necessary part of the process that things will go wrong, and users need to be provided meaningful feedback.

In the case of ShipReq, it will…

  • detect changes that are always wrong, and prevent the user from making those changes. These are errors.
  • detect data/config that has become wrong, modify derivation accordingly, and present them to users in a fully-comprehensible manner. These are warnings.
  • collect all info required to explain to users exactly how, why and from where, derived data came to be. Automatedly populating/amending users’ data is one thing, but I believe users should be able to inspect and understand things like what steps were performed, and what data were factors. This is provenance. It doesn’t sound that hard but in practice it was surprisingly challenging to implement.

Compilers do the same thing. They…

  • detect illegal data and combinations of data at runtime, and explain them to users. Those are errors.
  • detect data that is acceptably or potentially wrong, handle them, maybe by ignoring the statement, or modifying the generated code somehow, and then finally, they flag this to users. These are warnings.
  • some compilers calculate how something has come to be and present it, usually in a compilation error. Some compilers have error messages that run for 10-30 lines because they provide so much information about the context and calculations up to that point. This is a form of provenance.

Similar to the point above, this is all just dedicated people writing quadrillions of if conditions (not literally), collecting all kinds of runtime data in very strategic locations, and doing lots of conditional data transformation to be able to concisely explain a much larger underlying dataset.

Similarity #5: Performance

Slow compilers frustrate masses. Every two years or so, Scala compilation speed increases very significantly and will continue to do so, yet the bad reputation of being slow still persists in a lot of people’s minds. I think the Rust community is starting to accrue this reputation right now. Proponents of Go proudly declare super-fast compilation speed as one of the major selling points of the language. Speed is very important when you’re writing a compiler.

The feature that I wrote has the same requirement. It has the potential to require recalculation across the board on every single user change! Anything that happens on every change needs to be fast.

I’ve been a strong proponent of FP (functional programming) for around 9 years now but for this feature, I had to use a large number of variables and mutability. The fantastic thing about FP is that you can encapsulate mutable code in an immutable and referentially-transparent way such that the “atom” of work is itself FP, despite all the mutability under-the-hood. That’s exactly what I did and I was able to have my cake and eat it: the code is super fast and the entire “compilation” step is atomic and pure, meaning I don’t sacrifice FP. In other words, same immutable data in; same immutable result, but I just happen to use lots of mutability to arrive at that immutable result. This is a fantastic property of Scala and it’s multi-paradigm nature by the way. I’m sure that evangelists of Haskell (which is strictly an FP-only language) would argue that you could have the same speed in Haskell, but I’m not sure about the veracity of that statement, nor the means by which one would accomplish it. Would the code just do the exact same thing but with more boilerplate using IORefs or huge StateT stacks? Would Haskell be so efficient in its optimisation that a more straight-forward FP approach would be sufficiently fast? I don’t actually know but it’d very interesting to find out. I imagine that looking at the internals of the Haskell compiler would be a treasure trove of profoundly awesome insight. But I’ve digressed.

With the possible exception of Haskell, writing a fast compiler means having lots of mutable state; and mutate state is exponentially harder to reason about and keep correct. With a tear in my eye, it’s something I had to reach for to have lightning-fast performance (and this is after lots of very strategic caching), and as far as I know, it’s in nearly every compiler out there.

Solutions (1/3)

We’ve talked a lot about the difficulties, but what kind of solutions are there? How on earth do you ensure code with this type of complexity actually works? This is my approach.

First things first is to make your tests, and specifically your test data representations, as easy to read and write as possible.

For example, say your expectations look like this:

final case class Result(accounts: Set[Account])

final case class Account(id     : Long,
                         name   : String,
                         roles  : Set[Role],
                         aliases: Set[String])

sealed trait Role
object Role {
  case object Admin   extends Role
  case object User    extends Role
  case object Offline extends Role

You might be inclined to start writing tests like this:

val result = doStuff()

val accounts = result.accounts.toVector.sortBy(
assertEqual(accounts.length, 3)

  Account(2, "David",
    Set(Role.Admin, Role.User),
    Set("Baz", "Bazza", "Dave", "Davo")))

  Account(7, "Bob",
    Set(Role.User), Set.empty))

  Account(13, "John",
    Set(Role.User, Role.Offline),
    Set("Jono", "Johnny")))

Don’t do it!

  • It’s painful to write
  • You only get a subset of failure at a time. Maybe “4 is not 3”, maybe one account failure when you’ve got multiple
  • If “Davo” is missing it’s pretty hard to tell from the results exactly what is wrong with the Account, even more so for larger data
  • If you change the data structure you have to change every single test. Imagine when writing your 51st test case that you need to change Set[Role] to a new class called Roles. That’s 50 tests with who-knows-how-many occurrences you need to manually update.

Instead pamper yourself and your team. Invest an hour or two in being nice to yourself. You’re going to be writing lots of tests so make it as easy as possible for yourself to:

  • write new tests
  • modify existing tests
  • comprehend test failures
  • read and comprehend test cases

In our example above, we’ll write a function that converts a Result to a String by doing the following:

  • Sort accounts by id
  • Represent a Set[Role] as auo flags for Admin,User and Offline; just like in Linux how you have rwx permissions
  • When aliases are present, sort and print them like YAML

Also if your testing library doesn’t handle multi-line strings well, find one that does. Ideally you’d want to see coloured side-by-side comparison when two multi-line strings don’t match.

Our new test would look like this:

def assertResult(actual: Result, expect: String): Unit =
  assertEqual(formatForTests(actual), expect.trim)

val result = doStuff()

def expect =
 """#2) David [au-]
   |    Aliases:
   |      - Baz
   |      - Bazza
   |      - Dave
   |      - Davo
   |#7) Bob [-u-]
   |#13) John [-uo]
   |     Aliases:
   |       - Jono
   |       - Johnny

assertResult(result, expect)

Much better!

  • Faster to write
  • Faster to modify
  • Failures are easy to comprehend (especially with coloured side-by-side comparison)
  • We can change the data structure without changing all the tests
  • When a test fails, we see everything at once in a concise way

Now go forth and unit test! Write all the tests you normally would. Put similar effort to make generating test inputs as easy as possible if you need to. Write a test for every problematic combination of data that you can think of. Make sure to add comments describing why the combination is problematic as it’s not always going to be obvious.

Solutions (2/3)

Once you’ve gone through all the pain and suffering to make all your unit tests pass, it’s time for the next step: property testing.

If you don’t know what “property testing” is, give it a search it because it’s an invaluable tool. It’s life-changing.

Think of all the properties (i.e. invariants) that resulting data should satisfy. I find that thinking of properties for compiler-like code is much easier than usual run-of-the-mill code, probably because the domain is so heavily about data and data relationships.

Even with our toy accounts example above, you could come up with properties like:

  • ids should be unique
  • names should be unique
  • aliases must never be the same as another user’s name (maybe the code was supposed to filter those out)

If you’re dealing with graphs instead of lists there are usually properties that each node should adhere to, in relation to it’s children or parents.

Make sure that your property testing library is able to reproduce its tests. Each time it finds a failure, extract the data and add it as a unit test. My OSS Nyaya library has a feature called a bug-hunt that allows me to tell it “hey, run n tests across m threads starting at seed x and one test per seed”. When I was working on aforementioned ShipReq feature it found many issues, usually within 30 seconds, until it ended up running for an hour or two without finding a failure. I can’t guarantee that my code doesn’t still have some crazy bug in it but with all of my unit tests and over a few hours of systematic testing, I can at least rest assured that it’s extremely mathematically improbable. That’s the best you’re going to get for a reasonable amount of effort.

Solutions (3/3)

You can also invest an unreasonable amount of effort and go further if you like! You could attempt to write a formal, machine-verifiable proof. Once you have a verified proof, you have a guarantee that your logic will work in all cases without any unexpected bugs (assuming you implement the proven logic correctly). Whilst this is the outcome we’d all love, it’s rarely feasible because it can takes years of work to complete.

An interesting example of this is Scala. It’s been around for ages and over the years many bugs were encountered by the community. These days, it’s pretty rare to run up against a compiler bug because there’s been so much continual effort to fix them. But one of the most exciting things about the upcoming Scala 3, is that after nearly 10 years of multiple PhD students’ work, the foundations of the Scala language have been formally proven. That’s not even the full language. The interaction between higher-kinded types and subtyping is still an open research question, for example. This isn’t Scala-specific by the way; any language that attempts to include both of those features will have the problem that we’ll never really know if there are any remaining bugs, and until we get a proof, we’ll only know that everything added to the language test suites work. I love that Scala has what it calls a “Community Build” that compiles and tests hundreds (or so) of the most popular OSS Scala libraries and frameworks. This is a great way to catch regressions. But I’ve really digressed now. The point is: formal proof = very hard.

I should also mention that you can use TLA+ to obtain a proof but with much less effort. You basically build up a very abstract state machine and it will brute-force a proof by testing all possible combinations. In very specific cases, it’s a godsend! No matter how rare the edge case, TLA+ will find it; usually within seconds. I’ve used it with ShipReq to guarantee things like eventual consistency, and making sure that users never see stale data. You could spend a few days and end up with a guarantee that your logic holds in all possible cases. For verifying compiler-like code though, I haven’t tried it and you’d likely have to sit down and think very, very, very hard about how to model your logic in a compatible and efficient way. The state space is likely going to be too large. Depending on exactly what you’re doing, it might be feasible though, so I at least recommend considering it.

Am I missing anything else? Sorry if the solutions I’m presenting are a bit anti-climactic. If you can think of other solutions I’d really love to know so please let me and other readers know in the Reddit or HackerNews comments!


There are a few reasons I wanted to write this. I’ve heard compiler authors say that writing a compiler is very different than writing anything else but I didn’t properly understand why. Sadly I’ve also seen many instances of people being rude and derisive towards certain compiler authors and languages because of tradeoffs or bugs. Some more mild-mannered people have said things like “wouldn’t FP solve problem X so easily?”, “why so much mutability?! It’s so obviously a recipe for disaster”, etc. and whilst they seem like reasonable positions, I’d often find myself thinking that these compiler authors who’re smart enough to produce such complex systems surely already know about these tactics and their benefits. So what’s really going on?

I think I have my answer now, and I hope this article and my reflections provide you some insight as well. The work that I did pales in comparison to the complexity of a full language compiler, and yet it was literally the hardest code I’ve ever written. (And I’ve been coding over 30 years since a kid!)

I hope that you found these insights as interesting as I did. I think I further understand now the statement that writing a compiler is very different than writing anything else. I would actually go a step further back in abstraction though, and say that writing a highly-configurable and highly-flexible system is very hard.

Thanks, Giants!

We’re all standing on the shoulders of giants. Remember to stop and appreciate the work and effort that’s gone in the tools that we all rely on every single day. Think of all the if statements that must be in your language’s compiler that make it mirror what we all think is so obvious. All that sophistication we rely on is built on layers of primitive tedium. So maybe buy your everyday-language’s authors a coffee if you can, or show your appreciation with just a nice thank you message.

I’m so grateful to all the people upon whose work I base my own. Part of what drives me personally, is that I hope that I can produce a new generation of benefit and quality in ShipReq that my users will just take for granted and think to themselves “yep, tech’s evolved again, this 2021 tech is my baseline now”, then go on to build what they think is revolutionary. It’s a beautiful cycle.

Avatar of David Barri
Written by David Barri
Hi! I'm the founder and creator of ShipReq. I've been coding since I was a kid, and have been euphorically doing full-stack functional programming in Scala for 11 years and counting. I love to create that which sparks joy in others!