Category: Uncategorized

A Complex World

In Hindsight

Photo by David Kovalenko on Unsplash

As I shared before, my primary principle is: We want to thrive in a complex world. In this post I will elaborate on what I mean by complex.

How to Organize a Children’s Party

First, a three minute introduction to complexity by Dave Snowden:

I want to discuss systems in the world. For this particular discussion, I want to highlight a specific framing through which I will analyze these systems: what is the relationship between cause and effect?1 Specifically, are we able to determine causes for effects we observe?

Non-Complex Systems

Dave Snowden highlighted three types of systems in the Children’s Party video: Chaotic Systems, Ordered Systems, and Complex Systems. I’ll start with Ordered Systems, which are most familiar. 

In the framing of cause and effect, an Ordered system is one where the relationship between cause and effect can be determined. This relationship is either clear, or can be discovered through analysis. 

An example of a clear relationship would be you clearly knowing that if I hold out an apple in my hand and let it go, it will fall to the ground. It is clear that gravity (or “things fall”) combined with me letting go is the cause of the apple hitting the ground. The relationship between my letting go and the apple hitting the ground is clear to any reasonable person.

An example of a relationship requiring analysis would be me going to a car mechanic to figure out what’s wrong with my car. If the problem is not clear to the mechanic, they would run some tests, analyze the problem, and likely determine the cause of the problem, thus discovering the cause and effect relationship in the system (my car).

For Chaotic Systems, a relationship cannot be determined between cause and effect. Imagine that I am walking in the park and the trees around me start exploding, this would be an example of a chaotic system. I don’t know why there are explosions, and it does not matter. I must act to extract myself from the situation.


In a Complex System, the relationship between cause and effect has a fascinating property, it is only knowable in hindsight. This may be difficult to imagine the first time, but I think you’ll notice it is quite common once pointed out.

If one year ago, I were to ask you what you would be doing right now (reading this), would you be able to predict it? What will you be doing exactly one year from now? That is an impossible question to answer. Now, if instead I were to ask you about the various things over the past year that led you to this moment, you’d be able to tell me (I assume you have a really good memory 😄). This is an example of a relationship between cause and effect that is only knowable in hindsight. This is the nature of cause and effect in a complex world.

What Do You Think?

Do you have examples of complex systems in your life? Do you have examples of systems that are not complex? Let me know in the comments.

Next Up

In this post, I described complexity through the framing of cause and effect. Next, I’ll introduce other frames to help distinguish complexity: Cynefin Complexity.

1 I am sharing small pieces of Dave Snowden’s Cynefin framework, which I’ll write about and link to more directly in the future.

Optimize For Learning…

Principles and Values: Secondary and Tertiary Principles

In the last post, I shared my primary principle: We want to thrive in a complex world. While I found this to be a great overall orienting principle, it is only an aspirational statement. As such, it is a few steps removed from actionable advice. 

Optimize For Learning

One effective way of making sense of a complex1 world so that we can act in it2 is to optimize for learning, my secondary principle. A complex world changes from moment to moment.  Taking appropriate actions in such a world requires constant learning. It may be beneficial to learn stable patterns or how things generally work. At the same time, we need to keep learning if those stable patterns we learned are now changing into different patterns. Learning never stops. How do we optimize for it?

Tertiary Principles

Effective communication, short feedback loops, systems awareness, and diversity of perspectives are the four tertiary principles that inform how I optimize for learning. 

I strive for effective communication because we’re in this together. As the primary principle states, for me to thrive, we must thrive. I believe we need to effectively communicate to coordinate between each other.

The speed/duration of feedback loops constrains how fast I can learn. The shorter the feedback loop, the faster I can learn if my actions had desired outcomes. The longer the feedback loop, the longer it takes to learn. If the feedback loop is too long, learning may not occur at all.

By systems awareness, I intend the need for understanding that we operate within multiple interacting systems which continually modify our constraints and actions available to us. My actions may have no consequence, some consequence, or vast cascading unforeseen consequences good, bad, or otherwise. I believe having this awareness facilitates learning and reduces the mystery of the world a bit.

Diversity of perspectives is needed for my learning to not get stuck in some local optimum. Each of us takes a different path through life. We are each on our very own epic journey. You all know things that I will never experience. If I am to make any claim of optimizing for learning, a diversity of experiences and perspectives is a must.

People Are the Ones Who Act

Because we are people, I believe it is important to always remember that people are the ones who act in the world. In the military, there is a saying: “people first, mission always”. It is far too easy to get stuck behind a facade of numbers on a spreadsheet and forget that there are people on the other side. Let’s never forget.3

What Do You Think?

Do these principles make sense to you? Am I missing a principle that I should include? Let me know in the comments.

Next Up

I’ll start the journey into elaborating on what I mean by complex: A Complex World.

1 I will elaborate on what I mean by complex in future posts.

2 Making sense of the world so we can act in it is a definition of sense-making, by Dave Snowden, “Trespassers W” (blog post, 28 Feb 2021), , accessed on 1 Mar 2021.

3 This is a reason why I dislike the term “resources” when applied to people. I borrow from the military vocabulary of personnel/materiel and always strive to distinguish people/personnel/staff from resources.

We Want to Thrive in a Complex World

Principles and Values: Primary Principle

A principle is “a fundamental truth or proposition that serves as the foundation for a system of belief or behavior or for a chain of reasoning.”1 My primary principle is:

We want to thrive in a complex world.

What is primary about it? I mean primary in the sense that this is the first principle. I take this principle as an axiom. I can think of no firster [sic 😉] principles than this. 

One consequence of this being the primary principle is that I cannot provide an explanation for it. It just is.2

We Want

This used to be a self-centered principle. It used to state I want instead of We want. I always acknowledged that for me to thrive, others must also thrive, but I felt it was less presumptuous to speak only for myself than all of us. Ultimately, we communicates the principle and its intent better and serves as a reminder that we’re in this together.

to Thrive

The word thrive is very intentional here. I believe it is not sufficient to only persist or endure. Please do not misunderstand. It may be the case that the best we can do at times is persist or endure. This is not a judgement that people doing that are somehow lesser. Thriving is an aspirational statement.

in a Complex World

This is such a simple phrase, but here be multitudes and here be dragons. I found that complex means a lot of things to a lot of people. What I mean by complex is very specific and it will take me multiple posts to communicate the exact essence of the concept. The picture of a jungle is a hint. I’ll return to complexity in depth later on, after going through the remaining principles and values.

What Do You Think?

Do you have a primary principle? Let me know in the comments.

Next Up

Optimize For Learning…

1 Google Dictionary (“define principle”), accessed on 27 Feb 2021.

2 While I cannot provide a cause for a primary principle, there was a process that allowed me to stumble across it and articulate it to myself. In the first part of How to Do Things, I mention that “I want to thrive in a complex world” resulted from answering the question of “how should I do things?”.

Onboarding to a Software Team: In Three Parts

Photo by Duncan Meyer on Unsplash

I usually work in software development, or “tech”. Specifically, I work in web services type of software development. This is an important highlight, because it is a different environment from, for example, designing software to specification to go on a space probe to another planet. Mistakes in web services type of software are not desirable, but they’re also not catastrophic.

This onboarding series is a summary of the things I found effective for me and (maybe?) the teams I participate in. The format is the way I usually present these things, which is, onboarding a new team member onto the team. It consists of three sections: Principles and Values, Team Processes, and How To Deliver Work.

Principles and Values

My friend and colleague Leora Pearson and I had the opportunity to create a team from scratch. This gave us the chance to sit down and think through our principles and values. It was a hugely valuable exercise to think from first principles as to why we were going to build a team and engage in the work. This is a lot of mental models, abstractions, framing, and techniques that help to align the team: complexity, Cynefin, how systems fail, black swans, metaphors, finite and infinite games, Wardley Mapping and doctrine, and Universal Scalability Law.  I’ll share these Principles and Values in future posts.

Team Processes

As part of creating the team, we agreed on Team Processes. There are two processes that I find fundamental: the Advice Process, and the Conflict Resolution Process. I’ll share more on these in future posts as well.

How To Deliver Work

Lastly, having Principles and Values and Team Processes in place, I’ll share the ideas around how we organize to deliver work. These will include minimizing cycle time, setting commitment expectations, defining work items, defining failures, tracking ideas, system to human communication patterns, feedback loops, and learning documents.

Next Up

More posts on Principles and Values, coming soon.

Microdoctrine: Wardley Doctrine Piece by Piece

Wardley Doctrine is a doctrine developed by Simon Wardley within Wardley Mapping.

“Doctrine are the basic universal principles that are applicable to all industries regardless of the landscape and its context.”

There is a lot of doctrine. It consists of 44 principles, many of which are entire topics onto themselves. Many writings on doctrine exist. Simon provided these under the Creative Commons Attribution-ShareAlike 4.0 International License. There are many ways to learn doctrine.

I wanted to experiment with the structure of doctrine. I hoped that I could structure it to make it easier to learn and adopt. I want to be able to assess my level of doctrine adoption. And, if I am adopting doctrine, I want to know what I should adopt next. With these goals in mind, I set out to create a doctrine format that breaks up doctrine into small pieces. I call this microdoctrine.

Microdoctrine takes inspiration from a pattern language. It organizes around the principles outlined by Simon. It breaks up those principles into specific practices for individual learning. For example:

Phase: Stop Self Harm
Category: Development
Principle: Focus On User Needs
Practice: Examine Transactions

Any value we create is through meeting the needs of others. A mantra of “not sucking as much as the competitors” is not acceptable. We must be the best we can be.

Consider these first:
Know Your Users

Illustrative description:
Look at the transactions that an organisation makes with the outside world. Examine the customer journey when interacting with those transactions.

Detailed description:
Look at the transactions that an organisation makes with the outside world. This will tend to give you an idea of what it provides and what is important. Next, examine the customer journey when interacting with those transactions. Question this journey and talk with customers. You will often find pointless steps or unmet needs or unnecessary needs.

Another mechanism, if you adopted Wardley Mapping, is to map out the user’s landscape. By mapping out their landscape, you can often clarify what the user needs. You can also find entire new opportunities for business.

Consider next:Align Value Generation With User Needs or Consider Stage of Evolution

“Consider these first” and “Consider next” links express sequencing. This is to answer what principle to adopt next. This also intends to provide immediate benefit. At the same time, it intends to build up ability to adopt future practices.

I completed the first nine doctrine principles in microdoctrine format.  These make up Phase I: Stop Self Harm. They are all listed on the Wardley Mapping community site. The starting place is Phase I : Development : Know Your Users.

I can imagine us, doctrine practitioners, organizing around the microdoctrine structure. Each of us could contribute our specific expertise. For example, Phase I : Communication : Challenge Assumptions practices all deal with Spend Control. An expert on Cynefin could contribute a Phase I : Communication : Challenge Assumptions : Ritual Dissent practice for us to consider. My hope is that microdoctrine provides minimal scaffolding for us to organize around.

What do you think? Is this structure useful for learning or assessing doctrine? Are the existing patterns valid? Do you want to add patterns you know about?

Introducing Phase Line Mapping

TL;DR Phase Line Mapping is like Wardley Mapping, but at a smaller scale, the scale of projects or initiatives. Instead of the evolution axis, we have a phase line completion axis.

Phase Line Mapping attempts to bring topographical intelligence to project management, similarly as to how Wardley Mapping brought topographical intelligence to business strategy. Taking the classic tea shop Wardley Map, let’s assume we decided to replace the custom kettles we’re building with a supply of commodity kettles.

Wardley Map

Phase Line Mapping

A phase line is a synchronization mechanism that is similar to a milestone but different from a due date. It depicts changes in the phase of an operation without using dates, therefore, making it possible to coordinate without coupling to the calendar.

Phase Line Mapping retains the value chain scaffolding as the y-axis, in order to keep the project anchored to the outcome it is supposed to provide, while substituting phase line completion axis for the evolution axis:

Phase Line Map

We conduct vendor selection by trying out a few vendors (SuperKettles, HotKettle, and battery powered KettleGo):

We determined that HotKettle has the right stuff and select them as our vendor. We go through the purchasing process. Notice that we continue to use our custom kettles throughout:

We are hitting some snags with purchasing, but we’re almost there:

Purchasing complete, awaiting delivery:

New HotKettle kettles delivered:

We started our replacement in-place process and are now using HotKettles as well as custom kettles:

Our adoption of HotKettles is complete:

And at Wardley Mapping scale, we completed our transition to commodity:

Phase Line Mapping retains the required elements of a basic map. It is visualcontext specific, position has meaning, it is anchored in user needs, movement is present (across phase lines, instead of evolution), and it has components.

Phase Line Mapping has at least two of the three elements of an advanced map. It demonstrates flow between components and can represent different types of things. I am uncertain about what the project level climate looks like or if a stable climate exists. That remains to be seen.


I have made multiple attempts to share Wardley Mapping. Wardley Maps were intuitive and obvious to me (once I saw them), due to my life experience with topographical maps. However, my experience is not a shared context, and I found it difficult to communicate the value to someone who didn’t immediately “get it”.

The thinking with Phase Line Mapping is that many organizations are executing many projects and programs and are communicating progress in multiple custom ways. Phase Line Mapping is yet another (custom) way to demonstrate project progress across milestones. However, the hope here is to demonstrate mapping that is coherent with Wardley Mapping (same elements, things still move to the right), but where the feedback cycle is much quicker than your typical Wardley Map time scale. It can take years for things to move on a Wardley Map. By introducing a phase line axis, we can generate more feedback loops quicker while training people in Wardley Map intuitions.

My hope is that by being able to demonstrate Phase Line Mapping more broadly in projects, perhaps then, it will be easier to introduce a Wardley Map by saying: it’s like Phase Line Mapping, but at a larger scale, the scale of business. Instead of the phase line completion axis, we have an evolution axis.

An Experiment with Corporate RFCs

I was searching for an RFC-like or an ISO-like structure that defines a particular type of organizational processes. I did not find one, so the Corporate RFC (CRFC) (for example: CRFC2) is an experiment to see if structured specifications like that would be useful.

In software development, I came across RFCs and found them surprisingly effective in communicating protocol specifications. At the same time, being part of a large-enough organization, I find myself in need of being able to communicate heuristics and approaches to organizational practices that I found useful over time, for example: PRFAQ, Toyota A3, OKRs (still unsure about the utility of this one). PRFAQ, popularized by Amazon, doesn’t even have a Wikipedia entry at the time of this writing. 

Introducing a new organizational process takes time and lots of mentorship. However, part of the work to introduce a new process is all of the documentation required to communicate and establish the process. It seems to me that each one of us attempting this, is building custom documentation for a supposedly well-known process we are attempting to introduce. This is what I was searching for, some sort of standard documentation of a well-known process that I wouldn’t have to extract out of a series of blog posts, books, or courses. This is where my experience reading and using RFCs pointed at a possible approach.
One of the things that I find useful about RFC-like structure is that it seems to function toward the commodity end of the Wardley Evolution axis.

Another thing I find useful about RFC-like structure is that it is not a regulatory standard, and therefore not subject to licensing or certifications that I know of.

As mentioned and depicted on the image above, I understand the current state of the art for describing organizational processes to consist of blog posts, ad hoc agreements within organizations on an organizational standard (for example: standard way to do design reviews). Additionally, there exist certifications and licensed frameworks; the ones that come to mind are commercializations of Agile, but surely there are others. Then there are regulatory standards that are the cost of doing business like PCI, GDPR, etc.

I’m thinking that CRFC could be a way to provide RFC-like commodity specifications that we can share for the types of organizational processes that are not regulatory, but that summarize good or best practices within organizations. Their intended use would be as references to specific protocols that an organization wants to implement. Their specific scope would be somewhere between saying the phrase “PRFAQ” and writing down explicit patterns one can find for business processes in

Perhaps the best way to illustrate where a CRFC would fit in is by example, so there exist two initial examples for reference: CRFC2 and CRFC1.

If you’re interested in these types of specifications, the list of existing CRFCs is available at

If you’re interested in contributing, the initial thoughts on contributions are available in

Map/Serverless/DevOps DaysATL 2019 Things Learned

There are lots of things I learned at Map/Serverless/DevOpsDaysATL 2019 that I will probably not mention here, but I do want to share a random set of highlights that I’ll probably want to reference myself in the future.

What Wardley Maps really look like

It turns out, that most practicing mappers, including Simon Wardley himself, use maps in a way where the generated artifact looks like this:

This wasn’t at all obvious to me when reading the mapping book and learning about mapping in general. The pretty versions of maps in the book, in presentations, etc.. are that way mostly* to teach others about mapping. After all, the picture above would be a difficult pedagogical tool.

* some maps are worth presenting or keeping around and iteratively come back to them; also, digital maps allow for long-distance collaboration; however, most maps probably look like the picture.

Vertical axis of a Wardley Map is… kinda there

There is a lot that goes into explaining the horizontal evolution axis of a Wardley Map. Multiple adoption cycles, diffusion of innovation, all sorts of things come into play. It takes a lot of research to determine the stage of evolution, and we usually bypass it via crowdsourcing of people’s opinions. I even created to help me figure out where things ought to go. The value chain vertical axis, on the other hand, is there as “scaffolding”. People kept asking Simon what is the vertical axis so it is what it is. Movement on the vertical axis, while meaningful, seems to me to be much less meaningful that movement on the horizontal axis. However, Jabe Bloom framed the vertical axis in an interesting way using Regimes, Strategies, and Niches.

I haven’t fully grokked this framing yet, so I’m uncertain if it is useful. However, I am intrigued.

Burja Mapping

Tasshin Fogleman introduced Samo Burja’s Empire Theory and created a sort of mapping for power structures in human organizations.

I have more reading to do, but Burja’s ideas on top of some sort of Tasshin’s mapping are intriguing.

Spatial visualization of framing

As always, Jabe explains some philosophy in a very accessible way. In this case, I really enjoyed his visualization of framing. It is also the first time in my mind it clicked that framing can be interpreted as figuratively putting a picture frame on some part of reality and considering what is in the frame. I always understood framing abstractly, but not this spatially.

Output and Outcomes

Also from Jabe’s presentation, this was an important highlight, that both outputs and outcomes should be on the map.

Cost of Change

Also also from Jabe’s presentation, he highlighted three types of change to keep track of on a Wardley Map. In particular, not only the change of a component on a map, or the change due to movement of component on a map, but the change due to changing relationship between components, the changes in lines on a map and what components the lines link.


I got to sit out of frame and hear Claire Moss talk about contracts and pacts. I’ve heard of these before, but she highlighted multiple nuances that I wasn’t aware of. Definitely provided additional context for my thoughts on the boundary between Complicated and Complex. You can find the discussion on Twitter.

And lots of other things…

Aside from some of the highlights above, I was fortunate to meet a bunch of people who I only ever saw on video and have conversations with them. That was definitely the highlight of the conf.

Now, I have some reading and thinking to do…

The Hidden Cost of Collaboration -Resource Contention

This post explores why the speed of software development can slow to a crawl inside large organizations. In particular, we will consider software services and their customers from the perspective of resource contention. It turns out that this framing highlights a major contributing factor that slows software development. We will conclude with what can be done about it.

Consider the problem of resource contention. Software services require resources to operate, and those resources are finite. When users use a software service, resource contention problem occurs as soon as there is more than one user. The key question to consider is, who is responsible for managing the resource contention problem? We have two possibilities. The service itself can manage the problem in a centralized way or the users can manage the problem in a decentralized way. This is the key insight to extract from this framing.

The service itself can manage the problem in a centralized way or the users can manage the problem in a decentralized way.

The resource contention problem can be managed by the service or by the users. In the case of the service, the management can be centralized within the service. In the case of the users, the management must be distributed.

There is a cost associated with managing the resource contention problem.

When the service bears the cost, it bears the entire cost of managing the problem. A typical solution to the resource contention problem is for the service to become multi-tenant. To every user, it seems as they are the sole tenant of the service and need not worry about managing resource contention.

When the users collectively bear the cost, the cost for any single user is mostly small, most of the time. A typical solution takes the form of an ad hoc, distributed, and probably incorrect, consensus protocol. This protocol is typically referred to as  “careful” or “being a good citizen.” Every user knows that there are other users who at any point can do something that severely degrades their own use of the service, or worse, they’re unaware of it. Typically, users coordinate with each other in order to manage the shared service and not exhaust its resources.

Consider what happens when a user uses multiple services for which they have to manage resource contention with other users. Every new service requires adoption of a new, ad hoc, distributed, and incorrect, consensus protocol. While the cost of coordination may be low initially, the cost, per user, grows super-linearly with the number of services used, since all of the users of each new service, some of them unknown, must be coordinated with.

Alternatively, when each service manages the resource contention problem by offering multi-tenancy, every new service requires no coordination from the user. The cost, per user, increases linearly with the number of services used and is limited to learning how to use the service.

When each service manages the resource contention problem by offering multi-tenancy, every new service requires no coordination from the user.

Another relevant concept in the dynamic between services and users is the notion of internal and external users.

Users external to the business, tend to have other options, and therefore are less likely to put up with additional cost of coordination required for using the service. On the other hand, internal users typically have no such luxury, are cost insensitive, and must use the service designated for them. This tends to lead to a pathology where the service seems justified in not paying the additional cost of providing multi-tenancy, as the internal users have to use the service no matter the cost of coordination. But, what is often missed, is the super-linear cost the internal users must bear for their distributed management of resource contention. Fortunately, there are things we can do to avoid this situation.

Service seems justified in not paying the cost of providing multi-tenancy, as the internal users have to use the service no matter the cost of coordination.

We can demand that all services manage the resource contention problem by being multi-tenant. Alternatively, we can ensure that internal customers have a choice of using or creating another service that is multi-tenant, which may eventually lead to multi-tenant services as the cost of their adoption is lower.

Lastly, I want to highlight a pathological move not to make.

Forcing the use of a single service is the right move if and only if that service is multi-tenant.

There is great efficiency to be gained by removing duplication of effort. Forcing the use of a single service is the right move if and only if that service is multi-tenant. Otherwise, the organization is placed in a configuration where internal users must bear the cost of managing resource contention and cannot improve their situation by using a multi-tenant alternative.

Explain Types (In Programming) As If I Was A Normal Person

What is a type?

Something, something, computer programming…

What I find interesting about types is that they enable me to think in terms of patterns as opposed to in terms of specific examples. A type system, allows me to express something in terms of patterns, and the patterns can be arbitrarily abstract. Ironically, let’s look at some examples of patterns, i.e. types.

Unit type

A Unit type can be thought of as a pattern of “something”. It conveys the notion that “something” exists. There is an instance of “something”.

It may help to think in terms of receiving an email in your inbox, but that you only saw the number of new messages increase by one. You haven’t read the email. You know nothing about it. You just know that you have another email in your inbox. That’s like Unit type. It conveys that “something” (some email) exists.

I tend to think of Unit type as a “signal”. If you imagine a light switch, a “signal” is not whether or not the light is on or off. A “signal” would be the flipping of the switch, the flip itself. Imagine you can’t see the light, you just hear the switch flipping. flip flip flip… three signals, three Unit types.

Ok, that might still be fairly abstract. Let’s contrast this with something more familiar, but let’s name it something really weird, like, the Sum type.

Sum type

A Sum type can be thought of as “exclusive or” pattern. In other words, it can be this “something”, or that other “something”, but not both. For example, consider the notions of True and False. We say that something can be True or False but not both.

In fact, True or False (but not both) is of the type Sum with the shape of Unit + Unit. “Unit + Unit” means that the Sum type has space for two Units, but the fact that it’s exclusive or, means that it will only accept one Unit. True is defined by putting “something” (of type Unit) into the first space of Unit + Unit. False is defined by putting “something” (of type Unit) into the second space of Unit + Unit. What if you want to put “something” into both? You can’t, because by definition, we say that you can only put “something” into one of the spaces. Why is True putting “something” into the first space and not the second? The answer is that that’s the convention that most people who use Sum types use. You can use any convention you want, but it may be more difficult to understand what you’re communicating.

Remembering that Sum type describes a pattern of “exclusive or” helps me to remember how it works.

Going back to our light switch example, and to illustrate the difference between Sum and Unit, imagine that we now can tell whether the light is on or off. We can represent the pattern of knowing whether the light is on or off by Sum type with the shape Unit + Unit. If light is on, we will put “something” into the first space. If light is off, we will put “something” into the second space. It can’t be both on and off. All we need to put into one of the spaces is of Unit type, a “signal”. Remember that it is not the “signal” that tells us the light is on. The space the “signal” is in is what tells us whether the light is on or off (first space means light on, second space means light off). The nature of the “signal” itself is immaterial, we only care that “something” is in the space.

Why is it called “Sum” (as in “summation”) type? The name comes from how one would calculate the number of unique things that one can represent using a Sum type. For example, the Sum type Unit + Unit, can represent only one plus one, that is, two things. This is why it’s used for representing True and False, as those are exactly two things. If, for some reason, we wanted to represent four things, for example: Spring, Summer, Fall, Winter, I could use a Sum type of Unit + Unit + Unit + Unit. One plus one plus one plus one is four. And a season can (for our illustration purposes here) be either Spring, or Summer, or Fall, or Winter, but not more than one of those.

Product type

A Product type can be thought of as “and” pattern. In other words, it can be this “something” and that other “something” together.

A Product type that has two “somethings” would be Unit x Unit. “Unit x Unit” means that the Product type has space for two Units (“Unit x Unit x Unit” would mean that the Product type has space for three Units). For example, a weekend is Saturday and Sunday. We can represent Saturday by putting “something” into the first space and Sunday by putting “something” into the second space. Now, this is a somewhat not useful example of a Product. Let’s come up with a better example.

Remember our Sum type of Unit + Unit where we defined True and False? Let’s name that particular Sum type shape of Unit + Unit a Boolean type (it’s named after George Boole). Now that we have our Boolean type (which represents the notions of True and False), let’s define a more useful Product of the shape Boolean x Boolean. “Boolean x Boolean” means that the Product type has space for two Booleans. We’ll still think about the weekend, but this time, the first space will represent whether we are working on Saturday, and the second space will represent whether we are working on Sunday. So, if I’m working on Saturday and Sunday, I would represent that as True x True. If I’m working on Saturday, but not working on Sunday, I would represent that as True x False. Not working on Saturday, but working Sunday would be False x True. And, lastly, not working all weekend would be False x False.

Why is it called “Product” (as in “multiplication”) type? That’s because to calculate the number of unique things that one can represent using a Product type, we multiply the number of things that can be in the first space by the number of things that can be in the second space and so on. Notice, in our weekend representation of Unit + Unit, we could only put one thing in each space (Saturday and Sunday), so the number of things we could represent was one times one is one, the weekend. However, once we could put two things into each space, as in our example of whether we are working on the weekend, we could put two things into first space (True, False), and two things into second space (True, False). Two times two is four, and the Product type of Boolean x Boolean could represent four different work schedules over the weekend.

Void type

A Void type can be thought of as a “nothing” pattern. This pattern is either obvious to people, or very difficult to understand.

In the email inbox example, a Void type means that an email hasn’t arrived. You received no signal, “nothing” happened, no change at all.

In the light switch example, a Void type means that you can’t see if the light is on or off, and you can’t hear the flipping of the switch. It’s not that you will eventually hear or see something, but not yet. It’s that you will never hear or see anything. “Nothing” will happen. Void is the absence of any signal.

We now have some understanding of other types that can help us understand the nature of Void type. Imagine I have a Sum type with the shape of Void + Unit. “Void + Unit” means that the Sum type has only one space, and it is only the second space. There is no first space in Void + Unit type. How many things can you represent using Void + Unit type? It is zero plus one. You can only put zero things into the first space, because there is no first space. There is only second space, into which you can put one thing. Void type is analogous to zero.

To see this another way, consider a Product type of Void x Unit. How many things can you represent using Void x Unit type? It is zero times one, which would be zero. The first space doesn’t exist, it is of type Void, and therefore we have no way of constructing something that fits the pattern of “nothing and something”. The problem is that we cannot construct a something that fits the pattern of “nothing”, so we can never construct a something of Void x Unit type.

Arrow type

An arrow type is a pattern of “how things on the left side of the arrow relate to the things on the right side of the arrow” (but not the other way around). This sounds fairly abstract, let’s dive into an example.

Previously, we used the example type Product of Boolean x Boolean to describe a weekend work schedule. Let’s call this Product type a Schedule type. To build our example, we’ll also consider a Sum type of Unit + Unit, where putting Unit into the first space will mean worker Tristan, and putting Unit into the second space will mean worker Dale. Let’s call this Sum type a Worker type. Now, we can describe an Arrow type of Worker -> Schedule which is a pattern of “how Workers on the left side of the arrow relate to the Schedules on the right side of the arrow”.

Other common names for Arrow type are Exponential type, or Function type. The reason for “Exponential” name, is the same as for Sum and Product types, that is, it describes a way of how to count how many number of unique things one can represent using an Arrow type. Remember that our Arrow type is Worker -> Schedule. Worker type is a Sum type of Unit + Unit, which can represent, one plus one, so two things. Schedule type is a Product type of Boolean x Boolean, which can represent, two times two, so four things. The Arrow (“exponential”) type can represent Schedule ^ Worker number of things, or four to the power of two things, so 16 things. Let’s count them:

  1. Tristan -> (True, True), Dale -> (True, True)
  2. Tristan -> (True, True), Dale -> (True, False)
  3. Tristan -> (True, True), Dale -> (False, True)
  4. Tristan -> (True, True), Dale -> (False, False)
  5. Tristan -> (True, False), Dale -> (True, True)
  6. Tristan -> (True, False), Dale -> (True, False)
  7. Tristan -> (True, False), Dale -> (False, True)
  8. Tristan -> (True, False), Dale -> (False, False)
  9. Tristan -> (False, True), Dale -> (True, True)
  10. Tristan -> (False, True), Dale -> (True, False)
  11. Tristan -> (False, True), Dale -> (False, True)
  12. Tristan -> (False, True), Dale -> (False, False)
  13. Tristan -> (False, False), Dale -> (True, True)
  14. Tristan -> (False, False), Dale -> (True, False)
  15. Tristan -> (False, False), Dale -> (False, True)
  16. Tristan -> (False, False), Dale -> (False, False)

The reason for Arrow type to be called Function type is that Arrow type corresponds to what people mean by “function” in mathematics. If I have a thing of Arrow type, for instance, Tristan -> (True, True), Dale -> (True, True), then if I want to find out Tristan’s schedule, I would provide Tristan as input to the function, and the function would return the result (True, True).

Why call it an Arrow type then? There is a thing in mathematics called “up-arrow notation”, and it so happens that a single up-arrow in up-arrow notation corresponds to “exponential”. Discussing multiple arrows is out of scope of this post, but mentioned here for the curious.

Value type

A Value type can be thought of as a pattern in contrast to the Unit type pattern. Where Unit type was a “something” pattern, Value type is “this particular thing” pattern.

In the email inbox example, again, by contrast, where Unit type would be a signal that some new email arrived and we only care about the signal. Value type would be saying that a particular email arrived, and while we care that email arrived, we also care about the value, the particular contents of that particular email.

Another way of phrasing this, is that for a Unit type we only care about the signal. In the sentence “This thing exists”, what we focus on in Unit type is exists. For Value type, we focus on the entire sentence this thing exists, because we are trying to express the pattern that particular thing not only exists, but that it is a particular thing.

For example, think of the boolean True. Looking at True through the lens of “this particular thing”, we care that it is True, and that it is not False. The value of True is True.

Type type

Time to get weird.

Type type expresses the pattern of “a pattern” 😬. We covered multiple examples of Type type. Unit is of type Type. Sum is of type Type. Product is of type Type.

There is an important concept to highlight. Earlier, we defined True and False as being of the type Boolean. What’s worth highlighting is that Boolean is of type Type, but True is of type Boolean.

Also, notice that the type Type is of type Type. This is because the pattern of “a pattern” fits the pattern of being “a pattern”.

Everything is a Value


Recall that when we talked about the Value type, we were expressing the pattern of “this particular thing”. If we have the type Boolean, and we have a particular boolean, say True, then the particular boolean True is of type Boolean, but it is also of type Value. This is because the boolean True fits the pattern of “booleans” and it fits the pattern of “particular thing”. “Fitting a pattern” is referred to as “inhabiting a type”. So, the boolean True inhabits the type Boolean and it inhabits the type Value. This is because it “fits the pattern of booleans” and it “fits the pattern of particular thing”.

Types are Values and Values are Types


Types are Values. This is because Type type (a pattern of “a pattern”) fits the pattern of being “a particular thing”. The type Type inhabits the type Value.

Values are Types. This is because Value type (a pattern of “a particular thing”) fits the pattern of being “a pattern”. The type Value inhabits the type Type.

Everything inhabits Unit

Notice that the Unit type is the pattern of “something”. This means that everything that exists fits the pattern of being “something”, therefore everything that exists inhabits the type Unit.

It is worth highlighting the interplay of Void type and Unit type. The Void type itself inhabits type Type, inhabits type Value, and inhabits type Unit, because the pattern of “nothing” fits the pattern of being “a pattern” (Type), fits the pattern of being “a particular thing” (Value), and fits the pattern of being “something” (Unit). However, notice that there is nothing that can inhabit the type Void. This is because to fit the pattern of “nothing”, there can be nothing there. If there was something there, it wouldn’t be nothing.

That’s all for now…

Let’s stop before it gets weirder (like thinking about Arrow types with Void types), but this should be a fair introduction to the basic concepts with hints at where things start to get out of hand and we might need something more sophisticated than the english language.

While what I’ve described here (in english) is a description of a type system, there are different type systems that can be described, and they can differ from each other in subtle ways. The differences between them doesn’t make any of the other systems incorrect, nor does it make this description correct. But, hopefully, I managed to communicate some intuition about one kind of type system to you.

If something isn’t clear, please comment/respond and we’ll talk about it.

Also, thank you Dale Schumacher for pointing out errors in the early drafts and thinking through all this stuff with me.