Enso: Simplicity and Correctness

Enso is an award-winning, general-purpose, purely functional programming language with equivalent, first-class visual and textual representations and a highly expressive and novel type system.

Enso has been designed from the ground up to be easy to use for people of all experience levels, providing power to those that want it while keeping it hidden from those that do not. Its type rich type system helps users to catch their mistakes before they happen, which combines with its performant runtime to drastically improve the quality of the end result. Enso is very much intended to be a production language (rather than a research project), and the user-focused development philosophy reflects that.

From a technical point of view, Enso incorporates many of the recent innovations in the design and development of programming languages to improve the user experience. It provides higher-order functions, strict semantics with opt-in laziness, and opt-in algebraic data types, all under the auspices of a novel type system that merges dependent typing with a great user experience. Enso is the culmination of many years of research into functional programming languages, consolidating the work trail-blazed by Haskell, Idris and Agda, but also improvements in user-experience and ergonomics.

All in all, Enso is a programming language unlike any other before it, a seamless blend of code and visual communication that can span organisations.

“The compiler is your assistant, and you interact with it to arrive at a working program via a conversation, with information going both ways.”

Edwin Brady

Why a New Programming Language?

Since the 1980s, the way that programmers work and the tools that they use have changed remarkably little. However, there is a small but growing chorus that worries that the status quo is unsustainable: programmers are having problems keeping up with their own creations. “Even good programmers are struggling to make sense of the systems that they are working with,” says Chris Granger, a software developer who worked as a lead at Microsoft on Visual Studio.

We believe, without a drastic change to how software is created, that humanity’s progress is going to be stymied. However, in contrast to many approaches to finding the next-generation interfaces for human-computer interaction, we believe that textual code should not be replaced, but instead enhanced. In the same way that writing and speaking coexist, there are times where code is more convenient than any other approach.

Software Correctness Matters

“In September 2007, Jean Bookout was driving on the highway with her best friend in a Toyota Camry when the accelerator seemed to get stuck. When she took her foot off the pedal, the car didn’t slow down. She tried the brakes but they seemed to have lost their power. As she swerved toward an off-ramp going 50 miles per hour, she pulled the emergency brake. The car left a skid mark 150 feet long before running into an embankment by the side of the road. The passenger was killed. Bookout woke up in a hospital a month later.

“The incident was one of many in a nearly decade-long investigation into claims of so-called unintended acceleration in Toyota cars. Toyota blamed the incidents on poorly designed floor mats, “sticky” pedals, and driver error, but outsiders suspected that faulty software might be responsible. The National Highway Traffic Safety Administration enlisted software experts from NASA to perform an intensive review of Toyota’s code. After nearly 10 months, the NASA team hadn’t found evidence that software was the cause—but said they couldn’t prove it wasn’t.

“It was during litigation of the Bookout accident that someone finally found a convincing connection. Michael Barr, an expert witness for the plaintiff, had a team of software experts spend 18 months with the Toyota code, picking up where NASA left off. Using the same model as the Camry involved in the accident, Barr’s team demonstrated that there were more than 10 million ways for key tasks on the on-board computer to fail, potentially leading to unintended acceleration. They showed that as little as a single bit flip could make a car run out of control.”

The above text is part of an amazing article The Coming Software Apocalypse by James Somers, we strongly encourage you to read it all in order to understand the reasoning behind much of Enso’s design principles.

We believe that everyone should be able to process data and create software. Thus, we strongly disagree with the assumption that developers should learn how to formally prove properties about their programs, as it requires a very rare theoretical background. We believe that it’s the responsibility of the language and its tooling to prove the correctness of the users’ creation. “Human intuition is poor at estimating the true probability of supposedly ‘extremely rare’ combinations of events in systems operating at a scale of millions of requests per second. That human fallibility means that some of the more subtle, dangerous bugs turn out to be errors in design; the code faithfully implements the intended design, but the design fails to correctly handle a particular ‘rare’ scenario.”, wrote Chris Newcombe, who was a leader on Amazon Web Services team and one of Steam’s creators. Enso was designed to prove the correctness and provide as much valuable information to the user as possible in as automated and intuitive a fashion as possible.

Connections to your Data

Software creation is a very creative process. However, while using conventional languages, programmers are like chess players trying to play with a blindfold on

  • so much of their mental energy is spent just trying to picture where the pieces are that there’s hardly any left over to think about the game itself.

Enso was designed around the idea that people need an immediate connection to what they are making, as described by by Brett Victor in his amazing talk Inventing on Principle. Any violation of this principle alienates users from the actual problems they are trying to solve, which consequently decreases their understanding and increases the number of mistakes they make.

Enso’s visual representation targets domains where data processing is the primary focus, including data science, machine learning, IoT, bioinformatics, predictive maintenance, computer graphics, sound processing and so on. Each domain requires a highly tailored toolbox for working with such data, and Enso provides a coherent and unified foundation for building such toolboxes, on top of a growing library of existing ones. At its core, Enso delivers a powerful environment for the modelling of data flows, with extensive inbuilt capabilities for data visualisation and manipulation.

Tenets of Enso

The design of a tool can have a drastic effect on the cognitive load experienced by users. As we design Enso, we rely on a small set of principles to guide the design and evolution of the language, with the aim of making it easier to both express thoughts and understand existing logic.

Enso’s design is based on a small set of principles that all focus on minimising the cognitive load for users. They are elucidated below

  • Visual and Textual: The visual and textual syntaxes are both first-class. One is just as a canonical representation of the program as the other. New features must work across both.
  • Unified Syntax: The language syntax should be simple, concise, and be usable on both the type and term levels.
  • Visual Communication: A pure and functional language that lends itself easily to visualisation of data-flows.
  • One Right Way: There should, overwhelmingly, be only one way to perform a given task. Limited choice breeds simplicity.
  • Help the User: Enso should do its utmost to make things easier for the user, even if that involves accepting additional complexity in the implementation. This does not come at the exclusion of letting users access that power.
  • Simple Complexity: Though the language is backed by powerful functionality and compiler smarts, but this should be invisible to the users until they care to engage with it.
  • Powerful When Necessary: Designed to employ powerful techniques that confer safety and speed, allowing the users to write correct programs.
  • Performance and Predictability: Predictable performance, with integrated debugging and profiling features to help users diagnose their problems.
  • Explicit Over Implicit: The design of Enso and its libraries should always account for making all behaviour explicit.
  • Guidance Over On-Boarding: While an on-boarding process can help new users get up to speed, the language itself should endeavour to be as helpful as possible.

Visual and Textual

While Enso is a programming language, the visual environment with which it ships is just as important. This means that when designing the language, and any new feature for the language, we need to explicitly account for how it works in both the visual and textual syntaxes.

  • Necessity: Is this functionality necessary in both environments, or is it compensating for a deficiency in one environment.
  • Simplicity: Features should seem uniform between the two environments as much as possible.
  • Portability: One environment should not expose any features not exposed by the other.

Both syntaxes are of equal importance when thinking about designing Enso, and so any functionality from one syntax that doesn’t fit in the other should be rejected.

Unified Syntax

With the syntax being the primary mode of interaction between users and Enso, whether it be visual or textual, it is important that it is consistent within itself. This is quite a nebulous concept, but it boils down to a few main factors:

  • The syntax should have a minimal number of constructs in it.
  • The syntax should use a coherent visual language in both its forms.
  • The type-level syntax should equal the value-level syntax, allowing a natural blend of programming with both types and terms.
  • The behaviour of the program should be immediately evident from the syntax (and so the syntax should not embody any context-sensitive behaviour).
  • The language syntax should be amenable to providing clear and descriptive diagnostics to users.
  • Special cases in the syntax should be avoided

A key point of design for Enso’s syntax is the notion that syntactic rules need to be remembered, so it is incredibly important to find a happy middle ground between too many and not enough syntactic constructs.

Visual Communication

Humans are inherently visual creatures, and Enso is designed to enable a visual style of communication. When designing new language features, it is overwhelmingly important to consider their impact on Enso’s visualisation functionality.

  • How they impact the existing visualisation capabilities.
  • How they can be visualised themselves.

One Right Way

Choice in a language inherently creates myriad ways to do things. This is singularly unhelpful in any programming language, let alone one intended to span all experience levels, as it increases the cognitive burden of using the language.

To that end, it is significantly important to not introduce elements to the design that can already be achieved another way, or that can be achieved simply through a combination of existing features.

However, there are sometimes cases where the duplication of functionality may be introduced while providing a significant ergonomic benefit to users. Under such circumstances, the trade-offs of the design decision must be very carefully weighted, and a decision made based on the balance between complexity and ergonomics.

Help the User

Features with significant usability benefits for users should be weighted very highly in Enso’s design. This is true even when adding such a feature may bring significant complexity to the implementation of the language or tooling, as the user experience is paramount.

That is not to say, however, that any and all complexity should be accepted in the aid of user experience. If the maintenance burden is too great, then it is likely that introducing such a photo will impact future UX improvements. To this end, it is always a careful balancing act, even if we tilt the scales heavily in favour of UX over simplicity of the implementation.

Simple Complexity

A big part of Enso’s design is the notion of employing sophisticated and cutting-edge techniques in programming language design. However, many languages that employ these techniques do so to the detriment of the user experience.

While this increased power may allow the system to guarantee more things, or perform better, it often means that the user has to contend with additional complexity. A huge part of the design of Enso is hiding this complexity from the user as much as is possible, ensuring that the focus of the language design is on a good user experience.

Nevertheless, we do not intend to bar users from this complexity. As much as Enso should provide a good user experience, it should also not stand in the way of power users.

Powerful When Necessary

While we want to provide a simple tool to our users, this should not induce the language to shy away from necessary power.

Sometimes, the expression of an idea in its most succinct requires powerful language constructs. Given that Enso doesn’t want to limit the ability of its users to express things, it needs to provide this power. However, this power should be opt-in, with users not wanting the enhancements to expressiveness not having to know or care about it.

Performance and Predictability

There is little that is more frustrating than the code you write running slowly for no explicable reason.

To that end, Enso aims not only to be fast, meaning that users will not have to rely on other languages or tools for performance, but it also aims to be predictable. With Enso, ‘predictable’ means that it should be easy for advanced users to reason about the performance of their code. As a result, we aim for a sensible mapping from source code to machine execution, and aim to ensure that seemingly innocuous changes do not impact Enso’s performance.

Explicit Over Implicit

When designing Enso and its libraries, we don’t want to have any behaviour of a function that is not recorded in its type, or its defaults. This gives rise to two main principles for designing Enso’s APIs:

  1. Use the correct types to inform both users and the tools about the behaviour of the function.
  2. Use the inbuilt capabilities for named and default arguments to provide sensible defaults, for an API, without hiding behaviour.

An example of this trade-off is reading a opening a file handle with openHandle.

Unlike more specialised functions such as readFile and writeFile, openHandle is much more flexible about how it opens the file. In such cases, users making use of this file function can generally be assumed to want to open the file for both reading and writing.

This is the sensible, default, but it is made properly explicit by inclusion as a defaulted keyword argument to the function:

type File.Mode :
    type Read
    type Write
    type RW
    type Append
    type RWA

openHandle : File.Path -> File.Mode -> File.Handle
openHandle path -> fileMode = RW -> ...

In doing so, the design of the function tells the user the following things:

  • It takes a file path, which represents the location of a file on the user’s local system (as opposed to a Resource.Path, which is a location for a generic resource).
  • It has an explicit default behaviour that it opens the file for both reading and writing, that is defaulted as part of the definition, but can be overridden if necessary.

As a result, this design allows for explicit communication of the behaviour of the function, both under default, and non-default circumstances. Hence, the user who would like to open a file for appending (via appendHandle), as well as reading and writing, can call it as follows: openHandle path (fileMode = RWA), or just openHandle path RWA.

Guidance Over On-Boarding

While it is important to have an on-boarding process to help bring new users up to speed, that process cannot be the only guidance that users of Enso get.

To this end, it is overwhelmingly important that the language is helpful and informative as much as is possible. This means things like the provision of intelligent suggestions to users based on the kinds of program they are writing. It means things like helpful and informative error messages that explain problems to the user in an intuitive fashion whilst not hiding the details.

In essence, it means designing for the enduring user experience of Enso, rather than purely the initial user experience.

Designing Enso

As Enso’s design and functionality evolves, we have to take the utmost care to ensure that it doesn’t balloon beyond control. As a result, every new feature that we contemplate adding to the language should advance these core tenets, and thereby ensure that it matches with the overall vision for Enso.

We stick to the above principles when we’re building the compiler as well, with code being liberally commented with need-to-know information, as well as for clean and clear design documents to accompany it.

You can find out more about the various components that make up Enso by having a read of the design documents listed below. These documents are highly technical, and are not intended to be user-facing documentation.

  • Semantics: A description of Enso’s semantics, with a focus on both the main principles and the details.
  • Syntax: A description of Enso’s syntax, with a focus on both the main principles behind its design and its nitty gritty details.
  • The Type System: A description of Enso’s type system, starting at a very high level and slowly getting into more detail as the design evolves.
  • The Runtime: A description of the design for the runtime that is evolving in tandem with the runtime’s development.