OCaml

We begin this course by studying OCaml for that very reason: it's a vastly different perspective from what most of you will have seen in previous programming courses. Since you've already taken 1110 and 2110, you have learned how to program. This course gives you the opportunity to now learn a new language from scratch and reflect along the way about the difference between programming and programming in a language.

"A language that doesn't affect the way you think about programming is not worth knowing." —Alan J. Perlis (1922-1990), first recipient of the Turing Award

OCaml will change the way you think about programming.

OCaml is a functional programming language. The key linguistic abstraction of functional languages is the mathematical function. A function maps an input to an output; for the same input, it always produces the same output. That is, mathematical functions are stateless: they do not maintain any extra information or state that persists between usages of the function. Functions are first-class: you can use them as input to other functions, and produce functions as output. Expressing everything in terms of functions enables a uniform and simple programming model that is easier to reason about than the procedures and methods found in other families of languages.

OCaml supports a number of advanced features, some of which you will have encountered before, and some of which are likely to be new:

  • Algebraic datatypes: You can build sophisticated data structures in OCaml easily, without fussing with pointers and memory management. Pattern matching makes them even more convenient.

  • Type inference: You do not have to write type information down everywhere. The compiler automatically figures out most types. This can make the code easier to read and maintain.

  • Parametric polymorphism: Functions and data structures can be parameterized over types. This is crucial for being able to re-use code.

  • Garbage collection: Automatic memory management relieves you from the burden of memory allocation and deallocation, a common source of bugs in languages such as C.

  • Modules: OCaml makes it easy to structure large systems through the use of modules. Modules (called structures) are used to encapsulate implementations behind interfaces (called signatures). OCaml goes well beyond the functionality of most languages with modules by providing functions (called functors) that manipulate modules.

OCaml is a statically-typed and type-safe programming language. A statically-typed language detects type errors at compile time, so that programs with type errors cannot be executed. A type-safe language limits which kinds of operations can be performed on which kinds of data. In practice, this prevents a lot of silly errors (e.g., treating an integer as a function) and also prevents a lot of security problems: over half of the reported break-ins at the Computer Emergency Response Team (CERT, a US government agency tasked with cybersecurity) were due to buffer overflows, something that's impossible in a type-safe language.

Some languages, like Scheme and Lisp, are type-safe but dynamically typed. That is, type errors are caught only at run time. Other languages, like C and C++, are statically typed but not type safe. There's no guarantee that a type error won't occur.

Genealogically, OCaml comes from the line of programming languages whose grandfather is Lisp and includes modern languages such as Clojure, F#, Haskell, and Racket. Functional languages have a surprising tendency to predict the future of more mainstream languages. Java brought garbage collection into the mainstream in 1995; Lisp had it in 1958. Java didn't have generics until version 5 in 2004; the ML family had it in 1990. First-class functions and type inference have been incorporated into mainstream languages like Java, C#, and C++ over the last 10 years, long after functional languages introduced them. By studying functional programming, you get a taste of what might be coming down the pipe next. Who knows what it might be? (My bet would be pattern matching.)

A digression on the history of OCaml

Robin Milner and others at the Edinburgh Laboratory for Computer Science in Scotland were working on theorem provers in the late '70s and early '80s. Traditionally, theorem provers were implemented in languages such as Lisp. Milner kept running into the problem that the theorem provers would sometimes put incorrect "proofs" (i.e., non-proofs) together and claim that they were valid. So he tried to develop a language that only allowed you to construct valid proofs. ML, which stands for "Meta Language", was the result of that work. The type system of ML was carefully constructed so that you could only construct valid proofs in the language. A theorem prover was then written as a program that constructed a proof. Eventually, this "Classic ML" evolved into a full-fledged programming language.

In the early '80s, there was a schism in the ML community with the French on one side and the British and US on another. The French went on to develop CAML and later Objective CAML (OCaml) while the Brits and Americans developed Standard ML. The two dialects are quite similar. Microsoft introduced its own variant of OCaml called F# in 2005.

Milner received the Turing Award in 1991 in large part for his work on ML. The award citation includes this praise: "ML was way ahead of its time. It is built on clean and well-articulated mathematical ideas, teased apart so that they can be studied independently and relatively easily remixed and reused. ML has influenced many practical languages, including Java, Scala, and Microsoft's F#. Indeed, no serious language designer should ignore this example of good design."

Industry

OCaml and other functional languages are nowhere near as popular as C, C++, and Java. OCaml's real strength lies in language manipulation (i.e., compilers, analyzers, verifiers, provers, etc.). This is not surprising, because OCaml evolved from the domain of theorem proving.

That's not to say that functional languages aren't used in industry. There are many industry projects using OCaml and Haskell, among other languages. A Cornellian, Yaron Minsky (PhD '02), wrote a paper about using OCaml in the financial industry (that link must be accessed from inside Cornell's network). It explains how the features of OCaml make it a good choice for quickly building complex software that works.

But ultimately this course is about your education as a programmer, not about finding you a job.

"Education is what remains after one has forgotten everything one learned in school." —Albert Einstein

OCaml does a great job of clarifying and simplifying the essence of functional programming in a way that other languages that blend functional and imperative programming (like Scala) or take functional programming to the extreme (like Haskell) do not. Having learned OCaml, you'll be well equipped to teach yourself any other functional(-inspired) language.

Beauty

A non-scientific, subjective reason to study OCaml that I will put forth as my own opinion: OCaml is beautiful.

"Beauty is our Business" —title of a book in honor of Edsger W. Dijkstra

(Dijkstra was the recipient of the Turing award in 1972 for "fundamental contributions to programming." David Gries was an editor of the book.)

OCaml is elegant, simple, and graceful. The code you write can be stylish and tasteful. At first, this might not be apparent. You are learning a new language after all—you wouldn't expect to appreciate Sanskrit poetry on day 1 of SANSK 1131. In fact, you'll likely feel frustrated for awhile as you struggle to express yourself in a new language. So give it some time. I've lost track of how many students have come back to tell me in future semesters how "ugly" other languages felt after they went back to writing in them after 3110.

Aesthetics do matter. Code isn't written just to be executed by machines. It's also written to communicate to humans. Elegant code is easier to read and maintain. It isn't necessarily easier to write.

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