You work for me, Computer.

By Brandon Bloom

Structural and Relational Data

Before reading this post, see the introductory post for this series: Particles and Waves.

Among our goals is the desire to understand the duality of data and code. This duo represents one of the most fundamental pairs of particles and waves in software. However, careful selection of a bounded context can reveal particles and waves at any level. Combined with the observation that particles and space are generally easier to recognize and reason about than waves and time, let’s zoom in on data before tackling code.

Data Structures

Perhaps surprisingly, there is only a small number of foundational abstract data types. Wikipedia lists some common examples: List, Set, Multiset (Bag), Map, Graph, Stack, Queue, Priority Queue, etc. If you squint just right, there are really only two fundamental types of data containers: lists and bags. Every simple data structure can be classified as ordered or unordered. Common modes of use, such as FILO stacks and FIFO queues, are really just specializations of more generalized lists. Similar for maps and graphs; these are specialized bags. More complex data structures can be recognized as a careful composite of ordered and unordered substructures. Even the ubiquitous map can be viewed as a bag of lists, where each list is a key/value pair, constraining keys to be unique.

Individual data structures are the traditional particles we think of when we think about the data in our code. From a typical vantage point, the context that bounds these particles is so far away that we generally don’t ever perceive it. It’s only when we take a step back from a fragment of code and look at a complete system that we can perceive a common and relevant bounded context: a “database”.


Numbers, such as 5, or strings, such as "foo", have sufficient structure to be recognized in our shared global context made up of decimal numbers and unicode. However, 5 is useless on its own. We need to know how to interpret it. For that, we need more shared context. Let’s introduce a database with a fruit table and say that our favorite number is a fruit ID. Now we can lookup up the fruit ID and find out that we’re talking about bananas. It turns out that our number particle is just one of many particles in a banana wave traveling through our database medium.

By changing our perspective, we can study other bounded contexts, revealing more waves. When this happens, there tends to be an inversion: ordered becomes unordered and vice versa. Additional inversions are explored in sections below. For now, let’s choose the perspective of a programming language runtime implementer. From this vantage point, the heap is a database. A simple ordered data structure such as a pair, is actually represented with pointers, no different than our banana ID. The pair structure arrises from the interplay between a ordered structure with two pointers and an unordered structure, a map, encoded as memory itself, that translates addresses to objects. Shift your perspective again, from application user-space to operating system kernel-space and the question of whether memory is ordered or unordered depends on your perspective on virtual memory.

One common form of data warrants a special note: graphs. The easiest representation of a graph in a traditional programming language is to utilize memory pointers. However, this representation is impoverished. It forcibly equates nodes with their graphs that they are part of and sets an implicit boundary equal to all of memory. As much as possible, graphs and nodes should be clearly delimited. A graph should rightfully be considered as a database of nodes.


For the sake of concision, I will refer to data built up from other data in the implicit “global” context as “structural data”, or simply “structures”. Similarly, data derived from the relationship between structures and an explicit context will be referred to as “relational data”, or simply “relations”. These shorthands are not to be confused with the more specific concepts of C-style “structures” or traditional relational database “relations”, despite my abuse of the intentional analogies.

A key distinction and advantage of relational data is its inherit openness. One fruit ID may appear in many tables. Adding a new table to a database extends the meaning of that fruit ID without impacting any existing relations. We can also extend the meaning of that one fruit ID by moving the context boundary: switch databases, connect to a second database, talk to an external fruit service, etc. By contrast, structural data is closed. Structural data can only be “extend” by replacing it with a slightly varied structure; appending an element to a four element list produces a new list of five elements.

Time is the most important dimension of openness. Growth occurs over time as we add tables to a database, mmap additional pages in to virtual memory, or simply add keys to a map. The impact of time on the interpretation of data is a massive topic, to be discussed at another — ahem — time.

Cutting Along The Grain

Much pain in software designs stems from a conflict between the programmer’s intuition and the nature of the data they are working with. Each kind of data “wants” to be used in a particular way. Given the ability to recognize structures and relations, we can choose the most appropriate end of this duality spectrum; then choose strategies for working with such data according to its wants.

Here’s a summary of each kind of data’s respective usual wants:

Structures Relations
Order Ordered Unordered
Substructure Positional Associative
Mutability Immutable Mutable
Hierarchy Contain Children Point To Parents
Construction Bottom-Up Top-Down
Cycles Acyclic Cyclic
Limits Finite Infinite

Each of these wants can be overridden by moving the bounds of your context. Stuff tuples in to the context of a table to exploit associativity; or scope the context of a database to a transaction to build relations out of temporary ids, bottom-up.

Again, time is too vast a topic to discuss in detail here. Suffice to say, time can be bounded by “stopping” it and/or by choosing a time slice. Stopped time turns the mutable in to immutable and a time slice can turn the infinite in to the finite. Conversely, stopped or sliced time can be started and opened again: stick an immutable structure in a mutable box or connect a live pipe to a queue.


This perspective has dramatically hastened and improved how I design software. Carefully considering bounded contexts strongly guides many design choices. In situations where I used to find myself flip-flopping on some tradeoff, I now find myself happily choosing and committing to an appropriate blend of structural and relational data. Future posts in this series will revisit this tradeoff through some more concrete examples.

Particles and Waves

If simpler is better, is fewer better? Is one better than two? When designing programming constructs, there is a tendency towards extremist programming. There is power and beauty in a grand unified approach that has just one class of elements. Alluring as singularity may be, these one-element approaches tend to fail in ways that more complex designs do not. Hybrid approaches tend to win in the end. Why? And how should this reality inform our designs? Can we predict and enable hybrid solutions?

During the 17th century, opposing theories of light were being considered: particles vs waves. A great deal of science was conducted on behalf of either camp, but no clear victor emerged. It wasn’t until the 20th century that the dual nature of light was widely accepted. I’m far from the first to notice the parallels to logic and computing. Similarities are plentiful here. We even use the same word, “duality,” to informally describe the frequent occurrence of this two-element, yin-and-yang pattern in computing.

Where do dualities come from? Why are there so many of them? Why aren’t there more trinities? Quaternities?

Binary classifications arrises when we define a domain of discourse, then segregate that domain by a predicate. That is, some elements of the domain possess some property and the other elements do not. When this property is defined in terms of some bounded context, the binary classifications becomes a metaphysical, particle/wave-style duality. An element of such a domain may or may not satisfy such a predicate depending on your frame of reference!

Consider the famous Rubin’s Vase illusion:

In each image, either a vase or a pair of face profiles can be scene. Consider which subset of pixels satisfies the foreground predicate, and which satisfies its negation or dual, the background predicate. While the “meta-image” has a white background, the left and rightmost sub-images have a black background. Inverting the colors can switch which object appears in the foreground. However, adding a border can achieve the same illusion! Despite the center image being a pixel-perfect, color-matching, sub-rectangle of the rightmost image, the vase and faces appear to have swapped between the foreground and background. The border provides the bounded context that governs the foreground/background duality.

Returning to the duality of light: What property separates particles from waves? Unlike waves, particles possess a definite position in space. Waves have indefinite position, existing only with respect to some medium. A physical medium serves as a bounded context. There’s no splash without a pool of water; just as there is no background without a border.

Faced with duality, there is natural tendency to seek a preference between the two sides. For many binary classifications, there is no clear winner. Heads or tails? White or black? There’s no right answer. However, the equation is not as balanced when it comes to context-bounded dualities. Our preference is necessarily situational. On a white canvas, we paint a black vase in the foreground, but just the opposite on a black canvas. We must consider the context.

Frequently, the “particle” side of a duality is easier to recognize and reason about. The particle theory of light was proposed in the 11th century, nearly 600 years prior to the wave theory. What makes waves more difficult to recognize or reason about? Typically, context is implicit. The choice of context is often already made for you. When I set out to write this blog post, the background of my blog was already white. On a small beach, it’s easy to see the particles of sand, but it’s just as easy to see the waves of the ocean. Our relative scale influences our perspective, and that grants dominance to one of the two perspectives on the duality. Luckily, perspective can be shifted. Placing a handful of sand in to a carefully selected context, such as the surface of a speaker, will instantly reveal a wave. Thinking in terms of bounded contexts can put reasoning about waves on a level playing field with particles.

One last bit of metaphysics is worthy of mention here: spacetime. It is especially difficult to reason about time as a contextual bound on space. Notably, many waves only appear when observing a medium over time, or by projecting the time dimension on to a spatial dimension. Despite being unbounded at a universal scale, time is scarce for us mortals. We can’t capture time, holding on to it for study. At best, we can capture information about the past, or make predictions about the future, effectively transforming time in to the space such information takes up.

A good design takes things apart with an eye towards putting them back together again. However, focusing on the pieces to the exclusion of their contexts will yield a poor design. Even if the parts snap together nicely, a gestalt may not arise, or the composition may clash with its surroundings. The particles and waves or spacetime dualities can be found hiding under every rock in computer science: Data vs interfaces, logic with negation, values and state, imperative vs declarative, structural vs relational, and many more. Learning to recognize these dualities will enable you to design better systems and to produce better compositions within existing systems.

In future posts, I intend to discuss a wide range of programming topics with an eye for context and gestalt. We’ll use the lense of duality to deepen our understanding of the design tradeoffs in systems ranging from calculators to databases and beyond. I’ll then abuse the perspective to argue against the fetishization of modern programming ideals, such as functional purity, or the overuse of constructs such as promises and observables.

Versioning Better Than SemVer

I’ve written before about my dislike for SemVer. Recently, Rich Hickey spoke about this very same issue, and more. I’m currently working on some software that I’m hoping people will use, so I decided to make my thoughts on a better versioning scheme clear. Below you’ll find an exerpt from my new project’s documentation. Each point below could be elaborated on in a full post of it’s own, but this summary has value as a checklist of sorts for those discussions.


Software should make explicit what it promises to provide, as well as make explicit the requirements it relies on. Implicit promises or reliance should be avoided. This is a as much a social contract as it is a technical one.


If software relies on an explicit promise, it should never break.


It is often useful to defer commitments. The default social contract should be one of instability. In the absence of explicit promises, everything should be considered unstable, and subject to change.


It should always be possible to provide more. Making additional promises should never cause breakage.


Occasionally, supporting old contracts becomes burdensome. Parties to these contracts should be provided a path to a new contract without breakage.


Retracting promises should always be preceded by sufficient warning in order to avoid breakage. This entails a new contract which specifies deprecated provisions at the same time as providing new promises to migrate to.


Contracts are identified by a ordinal version number and an instant in time. Any contract may accrete new promises or bug fixes by creating an artifact tagged with a new instant. Successive ordinal versions may retract promises deprecated by previous ordinals.


Even in the presence of migrations, grandfathered contracts should be honored.


A contract timeline may be forked by accreting a new contract with a new name.


Platform systems should facilitate openness such that side-by-side systems may interoperate. Where this is not immediately possible, lower layers should be extended to facilitate this before forking higher layers.

Rarely Reversible

React.js and its “IMGUI” inspired rendering model is exploding in popularity. After my talk at Clojure/West, several folks asked me about two seemingly separate discussion topics: Two-way Bindings and Cursors.

Among those asking me about data binding, there are a few campaigning for a “spreadsheet-like” method of defining user interfaces. On one hand, they’re right to seek a direct-manipulation experience for building artifacts in a fundamentally visual and interactive medium. However, the code-level approach of using spreadsheet-like “equations” is fundamentally flawed.

Meanwhile, folks using Om in practice have expressed their frustrations with cursors. A few suggested using something more principled like Haskell’s lenses. While this may be useful in some situations, it’s fundamentally flawed for the same underlying reasons.

Both designs share a flaw born of a common desire: To automatically map user input back to data sources. When there’s a 1-to-1 mapping from data sources to user interfaces, this is appropriate. However, it’s not sufficient for the general case. In fact, it’s not sufficient for the common case.

Transformations of source data in to views beyond trivial editable fields is almost never reversible or equational.

To substantiate my position, let me demonstrate just how difficult it is to write reversible relationships with some examples in the domain of grammars.

Let’s say you have this simple grammar:

A ::= a+

The production A is formed of 1 or more a characters.

An infinite number of substitutions will satisfy the rule.

a -> A
aaaaaa -> A

Here, the direction of the arrow is critically important.

How can you reverse this production rule?

??? <- A

The only universal way to convert directed rules in to reversible relations is to never discard “information”.

For example, we could parameterize A by a number.

aaaa <-> A(4)

Alternatively, you can specify a general principal for lossy reversal rules, like “always produce the smallest value that would satisfy the rule”.

a <- A

However, this falls flat on its face if you introduce alternation:

AB ::= a | b

a -> AB
b -> AB

??? <- AB

Neither a nor b is “smaller”, so you need a new principle to reverse by. In a traditional context free grammar, | doesn’t imply order, but in a PEG, for example, you have ordered choice. You could say that you reverse to the first choice which satisfies the rule. Let’s use / as the ordered choice operator.

AB ::= a / b

a -> AB
b -> AB

a <- AB

Even in this simplified context, we haven’t even begun to scratch the surface of potential problems with reversibility. Constructs such as binders or loops will cause the reversal principles to explode in complexity.

Grammars offer pretty simple and well defined operations. However, practically every new operation introduces new reversal principles. Once real world business logic gets involved, things get hairy quickly.

In the context of user interfaces, these problems are magnified to be so large that they are practically unrecognizable. However, if you zoom in on individual cases in your own applications, you’ll spot this inherit complexity.

A robust UI solution should not attempt to build upon a foundation of automatic reversibility.

Unsound and Incomplete

The academic programming language community spends lots of time studying and building type systems. It’s widely considered that soundness is the mark of a well designed type system. A sound type system never permits an incorrectly typed program to pass type checking. An ideal type system would also be complete; it would never reject a correctly typed program because it was unable to prove its correctness. If you’re not familiar with this terminology, Ben Karel offers an illuminating analogy to help you.

Recently, a flurry of new, statically typed languages have entered the public stage. Among these are Google’s Dart, as well as Microsoft’s TypeScript. One thing that these languages have in common is that they have unsound type systems. Cue the horrified gasps of the type theorists!

Ignoring the topic of completeness for a moment, since most critics do so themselves (I think “unsound” just sounds scarier than “incomplete”). Surely Google and Microsoft employ some smart folks who have followed the latest research in type systems, so why, in 2014, are we still building languages that have unsound type systems? Well, I’ll let the Googlers and Microsofties explain themselves. In short, it’s difficult to maintain soundness in feature-rich languages. Even when possible, it may cause an astronomical increase in complexity. Achieving soundness requires a trade off against completeness, complexity, toolability, and more. Never mind the fact that being fully sound isn’t even that useful to begin with!

Let’s step back and examine why we even bother with type systems at all. In many cases, static analysis can dramatically reduce programmer errors and opens up significant optimization opportunities. Programming languages that are amenable to formal static analysis are highly correlated with programming languages that are amenable to informal static analysis, i.e. by programmers! Type systems have proven to be a particularly useful category of static analysis; so much so, that many programmers can’t imagine building software without a type checker on hand at all times.

Sadly, type systems fundamentally can’t catch all kinds of programming mistakes, nor will they magically improve the time or space complexity of your algorithms. You’ll have to write tests and benchmarks either way, to say nothing of validating your designs with stakeholders! Still, there are other reasons to have a good type system, like enforcement of type safety (which is a different thing than “type correctness”). However, safety can also be enforced by dynamic means and dynamic mechanisms such as capabilities can enforce security far beyond that of static safety checks. You might even want to intentionally forego safety in order to achieve maximum performance. Furthermore, there’s lots of other kinds of static analysis, both formal and informal, that can be used to find bugs or speed your programs up.

Assuming we want a tool that finds as many errors as possible, we should be willing to forego completeness, instead preferring to have our static analysis be overly cautious, warning us of programs that might be incorrect. We can always suppress such warnings after informal analysis (or alternative formal analysis!). Assuming we’re realistic about the fact that no static analysis tool will ever catch all errors, we should also be willing to tolerate unsoundness in order to spend our mental complexity budgets more wisely. Assuming we want a tool that independently improves with time, we should not create a cyclic dependency between languages and type systems.

There is a class of such tools. They’re called linters.

Slurp & Spit

Does nobody teach fopen anymore?

Many programmers start with “Hello World”. Shortly after that, it’s “Hello $NAME”, with a string read off stdin. But soon, budding engineers get tired of reminding their computer of their name each and every time their code runs. When I was first learning to program, file IO was an early requirement. We were taught how to read and write “data files” using little more than fopen, fscanf, fprintf, and fclose.

Fast forward five to ten years; you find yourself writing your 10 millionth SQL query, wishing for simpler times. Spend some more time with the NoSQL database du jour and the humble fopen function will be but a distant memory. Until that one fateful day arrives where you’ve got a relatively simple program and encounter the need for equally simple durability. Five years ago, you’d have cracked your knuckles and hacked out a pair of “save” and “load” functions. Today, you add a dependency on your favorite database driver, switch to the shell, type createdb myapp and then dutifully begin defining a lovely schema. Of course, now you need to either rework your models to conform to some horrid ORM layer, or write save/load-style “hydrate” and “dehydrate” methods anyway.

Now, at ten years and a day, you decide it’s finally time to learn that hip new programming language that everybody is talking about. You’ve got your book out, you’ve rocked through all your favorite technical interview puzzles, and you’re ready to put together a little web service for personal or small group use. If this was yesterday, you’d know exactly what dependency to add and how to proceed, but that was yesterday. Today, you do some Googling or hop into IRC looking to find out what’s popular.

Why don’t you even consider fopen?

But files aren’t “Web Scale”!

Is that really true? And do you really care? Should you really care?

The answer to all of these questions is “No”. Files can easily be “web scale”. As of 2013, Hacker News is still running as a single process, on a single core, of a single server, backed by a directory structure of simple data files. Nearly 2 million page views are served daily. See this thread and this submission for details.

The bottom line on this subject is that 1) You’re going to need significant cleverness to scale any service regardless of whether you use a database or the file system directly. And 2) You probably won’t need to scale hugely anytime soon, anyway. Being crushed by traffic is a Good Problem To Have, so let’s worry about the Ghost Town problem first. 3) You can always refactor later.

Data Literals: print, read, spit, and slurp

This article was prompted by a reoccurring discussion started by newcomers to the Clojure IRC channel. Having learned the basics, folks want to hit the ground running with stateful and durable services. Coming from a background in some big framework or some hairy application, these folks ask those suggestive questions about databases, driver libraries, and the like. Often, upon interrogation, they only have a couple kilobytes or megabytes of total data to be stored. In such a situation, the advice is always the same: Put that data into an atom, then use spit and slurp to save and load.

(def db (atom {...}))

(defn save-data []
  (spit "somefile" (prn-str @db)))

(defn load-data []
  (reset! db (read-string (slurp "somefile"))))

Because Clojure encourages the use of printable, readable, “pure” data, your save and load functions are virtually free! If you’re not familiar with Clojure, then consider working with pure JSON in Node.js:

var db = {...};

function saveData() {
  fs.writeFileSync("somefile", JSON.serialize(db));

function loadData() {
  db = JSON.parse(fs.readFileSync("somefile"));

Things aren’t quite as easy in languages lacking data literals, but nearly every popular language has some kind of automatic serialization library. However, even if you do need to write your own save and load functions, it’s a pretty straightforward, if somewhat tedious, process.

Even—or especially—experienced programmers are surprised by just how far this brain-dead-simple durability scheme will go.


One other objection to files is that it’s difficult to ensure an application enjoys the same guarantees that a well-tested ACID database affords. Never mind the fact that the majority of Rails applications suffer from dozens of consistency bugs because most developers forget to wrap related updates in transactions; it is true that incorrectly implemented file IO can cause catastrophic data loss.

If you plan to deploy spit and slurp into production, you’d be advised to write to a temporary file and utilize an atomic rename. This ensures that a failure during writing won’t corrupt your database. See man rename for more.

function saveData() {
  fs.writeFileSync("somefile.tmp", JSON.serialize(db));
  fs.renameSync("somefile.tmp", "somefile");

Just this one little tweak and this bad boy is production ready. Clojure programmers can use Java’s File.renameTo method. See below.

Remember to configure your backups!

Don’t Stop The World

Experienced Node.js programmers likely cringed at the “Sync” suffix on the above file operations. These operations will block while data is being read or written. In addition to yielding clearer example code, synchronous operations do not require coordination. If your application only needs to serve a handful of users, you can still write 100 megabytes of data in less than half a second. Even if you write every change to a file on every request, your ten users might not even notice your blocking IO. As you scale up, you’ll need to either split your data files up and/or start writing files asynchronously.

Asynchronous means coordination. Coordination means locking or queues. This stuff really isn’t as scary as it sounds, but it will have to wait; this post has already gotten far too long. However, I don’t want to lead you down the wrong path, so I should mention that, unlike Node.js, synchronous writes on the JVM will not block other requests. You practically don’t have a choice, but to be asynchronous. If two requests both call your save function at the same time, the resulting race condition can lead to incremental data loss. Luckily, Clojure’s agents and persistent data structures provide for a super quick fix:

(import '

(def save-agent (agent nil))

(defn save-data []
  (send-off save-agent
    (fn [_]
      (spit "somefile.tmp" (prn-str @db))
      (.renameTo (File. "somefile.tmp") (File. "somefile")))))

Different web servers across all the different languages have varying concurrency support and configuration defaults. Whether or not you use file IO, you should be aware of the concurrency story for your platform and its impact on your application.

Code Density

I’ve heard from many folks who have had difficulty learning functional programming. In attempting to complete seemingly simple tasks, even the most experienced imperative and object oriented programmers fall flat on their faces when first attempting to program in a functional style. Yet, experienced functional programmers rave about their productivity and their small, beautiful codebases. Are these functional programmers simply that much smarter? Or is there some other force at work?

Even in a familiar language, a small, but powerful library might be described as “write-only code” by those who do not grok the fundamental ideas. Powerful ideas produce powerful programs, but powerful programs don’t always shed light on their underlying powerful ideas. Functional programming is a powerful idea.

A lot of businesses just need somebody to bang out some pretty vanilla solutions to some pretty vanilla problems. If you’re in one of these businesses (and most of us are), then your team’s resident curmudgeon is right: Clever is not a good thing. Familiar and understood beats simple and powerful ideas every time.

However, if you’re looking to up your game; If you want to tackle bigger problems, you need more powerful tools. If you’ve tried to read some Lisp or ML or Haskell, but found your head swimming; If you’ve tried to fight your way through Project Euler with an unfamiliar language, but got stumped by tasks you aced in freshman year; Don’t panic. There’s a good explanation for why these things seem harder. Those functional programmers really aren’t smarter than you. They’re just a little more persistent.

The key issue is one of code density. Powerful ideas can succinctly express solutions to tough problems. The problem isn’t necessarily any easier, nor is expressing the solution necessarily any more difficult. There is simply more meaning per line of code, so reading and writing each line of code takes proportionally longer. If you’ve jumped into a 100,000+ lines-of-code Java project and found your way around like a champ, you shouldn’t feel ashamed by jumping into a 10,000 line Clojure project and being horribly lost. Those 10,000 lines of code might take you five times as long to read per line. These numbers are only backed by anecdotal evidence, but that’s a hypothetical 50% reduction in total code reading time!

When you start writing some object-oriented code, you spend a bunch of time creating classes and defining the shape of your domain. There is some delay between when you begin typing and when you run head first into the problem at hand. During that delay, you have a lot of time to think about the problem. While you’re fingers are hard at work, your mind is too. Your brain needs time for solutions to bake. If you encounter the hard problems as soon as you start typing, you need to walk away from your desk!

Functional programming is a particularly compelling powerful idea because it captures the essence of a lot of computational problems. There is a great deal of complexity that is hidden away in mutable state, implicit context, and abstract interfaces. If you’re used to the constant drip of progress afforded by stitching together class hierarchies, you suffer immediate withdrawals when faced with all of the hidden moving parts laid bare in an individual function’s signature. Suddenly, you need to solve many more problems at once, but you’ve become accustom to having those problems spread out across a wider cross-section of your program.

It takes longer to get started with a powerful idea because you must first understand and internalize that powerful idea. Take a deep breath, lean back in your chair, close your eyes, and think through the problem. Untangle the mess in your head, maybe explore a bit in the REPL, then come back and try the problem again. You might find that you spend half as much time thinking as you used to spend typing.

SemVer: A Technical Solution to a Social Problem

“Semantic Versioning”, commonly called SemVer, is a specification for assigning and reasoning about software version numbers. In short, it codifies some common practices regarding $MAJOR.$MINOR.$REVISION style version numbers, including guarentees about API compatibility and dependency resolution.

Beautifully simple semantics, outlined by a clear and concise specification. What’s not to like? Technically, SemVer is very strong. However, statistically speaking, software developers can not be trusted to maintain the semantics promised by SemVer. A small handful of individuals and projects use SemVer, or something like it, to good effect. The rest can’t be trusted not to introduce major bugs or breaking changes with each and every revision.

Every engineer who has ever worked on a project with more than a handful of external dependencies has had the horrifying experience of deploying some small dependency upgrades only to have their entire house of cards come crashing down. As a result, mature organizations tend to maintain their own package repositories, vendor all dependencies, or otherwise carefully scrutinize and control each and every external component.

Packaging systems such as Ruby’s Bundler and Node’s NPM often default to or otherwise encourage SemVer. Depending on a pessimistic version constraint, such as ~> 1.2, is tantamount to saying “I trust that the maintainer of this project both understands SemVer and is capable of enforcing its guarantees.” Sadly, this is rarely, if ever, true.

Versioning is a social contract. A maintainer makes a promise regarding API stability and versioning policy. Consumers make a judgement call regarding the veracity of the maintainer’s promise. If the README file says “This project uses SemVer” and the project’s maintainer is Tom Preston-Werner, then you can trust that pessimistic version constraint in your dependencies file.

However, the README might say “Releases are only packaged for long-term support versions. Please use the stable branch for new production systems.” In such a case, you might want to consider depending directly on that branch or even on a particular commit hash. Speaking of branches and commits, this calls to mind a far preferable default for packaging and dependency resolution systems: refspecs. It would be great if the SemVer spec was updated to include policies for defining branch and tag names in your version control system of choice. Instead of depending on ~> 1.2, you could depend on branches/v1.2.x. This grants maintainers greater flexibility in making social contracts. It also grants consumers greater control over how much trust they give to maintainers.

Trivial Cycles in Object Oriented Programming

I’m not against objects. I’m against programming in a style oriented by objects.

One of the largest causes of big-ball-of-mud architectures is unnecessary cyclic dependencies. Unfortunately, object-oriented languages allow programmers to trivially create cyclic dependencies without warning or immediate repercussions. Overtime, these cycles become a significant source of complexity and corresponding suffering.

The problem can be summarized in a single line of pseudocode:


This innocuous-looking expression embodies a pleasant interface, but its implementation hides an insidious cyclic relationship between the Person class and the Address class. Worse than that, this cyclic relationship is extended transitively to any other related classes. Consider the following diagram with an additional Company class that may also have an address.

Person / Address / Company ball-of-mud

Any time you add a type to this graph, it is instantly tangled-up with everything else. Compare this with this functional variant:

Person / Address / Company untangled

Now you can analyze, compile, and reason about a subset of this graph. Reasoning about any particular node only requires reasoning about the transitive closure of its dependencies. However, in the presence of ubiquitous cycles, the transitive closure is equivalent to the entire graph.

Unfortunately, we’ve now lost the convenient noun.verb or noun.component syntax. Instead, our expressions are inside out. Here it is in Ruby or Python syntax:


And here it is in Lisp syntax:

(residents (address person))

Luckily, we can recover the pleasant ordering with the help of a macro. Here is an example of Clojure’s threading macro:

(-> person address residents)

In the absense of macros, we can use a binary operator. Here’s an example of F#’s “pipeline” operator:

person |> address |> residents

Both Scala and C# provide tools for directly emulating the dotted access syntax. Respectively, implicit conversions and extension methods leverage the type system to perform dispatch to non-members in a syntactically identical way.

It is unfortunate that it is so easy to create cycles. A small sin at first, but they quickly add up and really start to hurt as a system grows larger.



So here’s my new blog.

