Anil Madhavapeddy – Unikernels: Functional Infrastructure with Mirage OS


Anil captivates the crowd with his talk on Mirage OS.

Last week, we had the privilege of hosting a very special guest speaker, Anil Madhavapeddy. You already know him as one of the authors behind “Real World OCaml: Functional programming for the masses”.

Anil showed us some neat developments from the Mirage OS project. Topics of interest included: an informal demonstration of OCaml Unikernels, a tutorial about operating system aspects, and a walkthrough of the TLS stack. Magnus Skjegstad gave us a demo of Jitsu which can spin up servers within milliseconds just-in-time as a request arrives. Anil also showed us the Bitcoin Piñata and invites programmers to tackle this challenge for a chance at free Bitcoins!

Screen Shot 2015-05-12 at 2.59.14 PM

Anil talks about Unikernels.

Below is a video of the entire talk for your enjoyment.

Those who came out to our event also had the chance to meet other OCaml enthusiasts, munch on some food, partake in some drinks, and bask in our uniquely painted officeWe’d like give a big thanks to Anil once again for making the long trip and being awesome as usual.

Thanks to everybody who came out and hope to see you at the next OCaml Meetup.

Programming challenges

Maze navigation challenge

We’ll be running this friendly programming challenge continuously for hopefully many months or years. Keep your submissions coming!

At Esper we’re always looking to challenge ourselves in new ways. Some of us find that writing programs and running them is more fun than a bunch of other activities, including but not limited to watching tv and talking to people; although we sometimes do. Anyway. Maybe your current task is a bit boring and you’re looking for new ways to procrastinate. Seek no further, we’ve created this little programming challenge just for you. If you succeed, you’ll win this one amazing mug, which undoubtedly will make all your friends and colleagues jealous of your advanced computer hacking skills.



This is one mug, not two. Sorry, we will no longer ship the mug to destinations outside of the United States. We’re really sorry and we’re working on an alternate prize we can send to everyone.


You must write a program for a robot that navigates a closed maze in search for letters that were printed on the floor of the maze. The solution consists of the list of all letters, ordered alphabetically. The letters are capitals (A-Z) and the same letter may exist in multiple locations.


The solution for the following maze is AABQ:

#   A   #   #
# ## ## # # #
# Q# #   B  #
#  #A# # ####

# symbols indicate walls, which are illegal positions. Your robot can navigate on all other cells vertically or horizontally but not diagonally. You can see the 4 neighbors cells (up, down, left, right) around your current cell.

The initial position of the robot can be any non-wall cell (empty or letter), inside the maze.

Navigation API

Your program will read cell information from stdin, then write a command on stdout, and loop.

Cell information is given as one line of 5 characters indicating the type of the current cell and the type of each of the neighbor cells in the following order: current, up, down, left, right. In the example, if the current position is the position of the letter Q, cell information will be given as:

Q#  #

A command is either one of the 4 navigation commands indicating which neighbor cell to move to (U: up, D: down, L: left, R: right) or the key for escaping the maze starting with the K prefix, e.g. KAABQ which terminates the game. Commands are terminated by a newline character (LF).

An attempt to move into a wall or giving a wrong key terminates the life of the poor robot.

Here is a very simple maze:


A successful session for this maze, assuming a start position on cell marked with A is:

 input: A###B
output: R
 input: B##A#
output: KAB


The number of moves is limited to at most twice the number of moves required by our reference solution. This limit as well as the size of the maze remain undisclosed (edited 2015-03-17).

Your program may not consult files or network resources other than stdin and stdout as specified above. It must consist in human-readable source code written by yourself. We will build it and run it on Ubuntu 14.04 (64 bits). Please provide:

  • the list of packages needed to build and run your solution or your best guess
  • a shell script to build your solution in one shot
  • the command to run your solution


Make sure your solution is successful on the sample mazes on this page, and make sure to flush stdout after printing each command when your program runs non-interactively.


Please send us your solution to programming@esper.com. All successful entries win the exclusive trophy mug!

About us

We are Esper, a still small company based in the San Francisco Bay Area, and we’re building a service to empower executives with professional executive assistants 24/7. If you share our passion and standards in software engineering and want to make a difference in the world, you should talk to us!


Algebraic Data Types

Algebraic data types are the fundamental building blocks of programs in ML-style languages like Haskell and OCaml. Since they play such an important role in these languages, it is well worth understanding how they work and where they come from—at first, the design may feel a bit arbitrary, but in reality it flows naturally from a reasonable starting point.

Algebraic data types are made up of two components: products (also known as structs, records or tuples) and variants (also known as tagged unions, sums or coproducts). Let’s take a look at each of these in turn, how they can be combined and how they’re related.


Products are a construct that appears in most programming languages: it’s a type that contains multiple values. The simplest example in OCaml would be a pair (just like an ordered pair in mathematics):

# let pair = (1, 2);;
val pair : int * int = (1, 2)

This pair has the type int * int which means it has two members, both integers. In prose rather than code, this is often written as int × int because it corresponds to a cross-product of sets. Fundamentally, we can do two interesting things with a pair: get the first element or get the second element:

# let first (x, y) = x;;
val first : 'a * 'b -> 'a = <fun>
# let second (x, y) = y;;
val second : 'a * 'b -> 'b = <fun>

In OCaml, pattern matching makes the definitions pretty visual. The functions first and second are often abbreviated as fst and snd or π₁ and π₂ (π is used because these are projection functions).

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Esper!, OCaml

Why We Use OCaml


According to the TIOBE Programming Community index for June 2014, the following nine programming languages make up over 60% of the total popularity ratings: C, Java, Objective-C, C++, C#, PHP, Python, JavaScript, and Ruby.

Here at Esper, we have chosen to use the OCaml language for the server-side backend that implements most of our application’s functionality. OCaml includes many features that are not available in the more mainstream programming languages listed above, and we believe this gives us a competitive advantage. But even among Silicon Valley startups, OCaml is an unusual choice. The purpose of this post is to explain the benefits of OCaml and compare it to other languages. We hope to convince readers, especially other developers, to consider adopting OCaml for their projects as well.



Here are some of the most important features of the OCaml language that we consider useful, or even essential, for building high-quality, reliable software:

First-class functions and immutable values

It is not always clear what people mean when they say “functional programming”, so we avoid this term and state directly what we want. Functions should be full-fledged values that can be created within other functions, stored inside data structures, passed as arguments, and returned as results. A function should automatically capture the relevant parts of its environment at the time of creation into a closure.

Most recent programming languages support these features, but they may be little used in idiomatic programs. One reason is that a function’s behavior is more difficult to predict when the values in its environment can be modified after its definition. Such modification could cause the function to produce different results when called with the same arguments, contrary to the expected behavior of a function. Mutability can be a useful tool in some situations, but it is often unnecessary and potentially harmful.

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Meet Oblivion: Inline HTML templates for JavaScript

So at Esper we made this tool to make our lives easier.

Oblivion is a preprocessor for JavaScript source files. It allows the programmer to embed raw HTML code into JavaScript with very few modifications. Only local variable names need to be inserted, allowing these HTML elements to be modified programmatically – and safely.

Have you ever wanted to copy-paste HTML code directly into a JavaScript string?

var s = '
  <span class="example">Welcome!</span>

Oops, that’s invalid. JavaScript doesn’t support line breaks in string literals. Instead you would have to write something like:

var s =
+'  <span class="example">Welcome!</span>'

This required more edits than we wanted, but why not. Now something we often do is modify child nodes based one some conditions. We use jQuery to produce a dom node from a string, maybe as follows:

var view = $(
+'  <span class="example" id="msg"></span>'

var text = isNewUser(user) ? "Welcome!" : "Hello!";

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