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.

Continue reading