The Functional Programming Language Universe

  • “This is the Dawn of the Age of FP”, there are now many choices of languages
  • There are both viable functional-focused languages as well as FP extensions to existing languages.

Functional-focused languages

  • These are languages designed with FP style in mind from the start: immutable variables and data structures, full higher-order functions, currying, etc.
  • There are generally two “schools”
    • ML school: static types, type inference, polymorphism, pattern matching (OCaml, Standard ML, Haskell, F#, etc)
    • Lisp school: dynamically typed (Lisp, Scheme, Clojure, etc)

OCaml and Standard ML

  • Of course we have to include OCaml!
  • Standard ML is another variant of ML which has limited popularity these days


  • F# is Microsoft’s ML-style language, it has all the main features of OCaml
  • It integrates well with the MSFT toolchain
  • Here is an example from their tutorial; looks familiar, eh?
let square x = x * x
let isOdd x = x % 2 <> 0

let sumOfOddSquares nums =
    |> List.filter isOdd
    |> List.sumBy square

let numbers = [1; 2; 3; 4; 5]
let sum = sumOfOddSquares numbers
printfn "The sum of the odd squares in %A is %d" numbers sum

type Shape =
    | Square of side: double
    | Rectangle of width: double * length: double

let getArea shape =
    match shape with
    | Square side -> side * side
    | Rectangle (width, length) -> width * length

let square = Square 2.0
printfn "The area of the square is %f" (getArea square)


  • Reason is an interesting beast, it has exactly the features of OCaml
    • but with a somewhat different syntax not so loaded with historical oddities and kludges
    • You can convert .rs to .ml and vice-versa very cleanly and it also translates to JavaScript for browser code
    • The Overview provides a glimpse of the syntax
      • it is not hard to see how it is a minor variation on OCaml
    • We will play with this cool live demo which shows code in all three languages at once


  • The main thrust behind Reason is use of FP in web UI programming
  • ReasonReact is the Reason version of Facebook’s excellent React UI library
  • We will take a peek at a Simple To Do app written in ReasonReact


  • Elm is an ML-school language
  • Designed for writing web apps, it runs in the browser via translation to JS.
    • Similar to Reason in goal


  • Scala is a hybrid of Java and ML which runs on the JVM so can link with Java libraries
  • It is easier to do FP in compared to Java since it was built-in from the start: pattern matching, type inference, etc.


Haskell is in the ML school; Devin will present Haskell in detail.

Lisp / Scheme / Racket

  • Lisp was the very first functional programming language, from the late 50’s
    • it was dynamically typed, there are no type declarations or inference and errors caught at runtime.
  • Scheme was a clean-up of Lisp in the 70’s-80’s, there were several errors in the Lisp design
    • e.g. dynamic scoping – closures were not computed in Lisp.
  • Racket is a popular modern dialect of Scheme with many added features including types
  • Additionally, Smalltalk, Ruby, Python, and JavaScript are dynamically-typed and are thus descended from Lisp.


Clojure is in the Lisp school; Kelvin will present Clojure in detail.

FP Extensions to YourFavoriteLang

  • It is now possible to do FP-style programming in Java, C++, Python, JavaScript, etc.
  • All of these languages support higher-order functions with Currying, etc.
  • But, there is not necessarily good library support or integration
  • Also, to “really do” FP you need immutable data structures and variables
    • limited support for that in most of these languages currently

FP in Java

Java 8+ has Lambdas

  • Lambdas are a bit clunky to use compared to OCaml due to how they were patched in.
  • Java 8 higher-order functions “pun” as an interface with only one method in it.
    • the function is taken to be the body of that single method, no need to write the method name when declaring the function then.
  • There is also some (limited) type inference for Lambda parameters.
  • Here is a tutorial
  • Currying in Java, somewhat painfully: Gist currying example. For that example here is the Function and BiFunction type.
  • Use final to declare variables immutable in Java - use it!
  • There are no immutable data structures in the Java standard library unfortunately

Terminology aside: closure

  • A closure is just a higher-order function return value
  • The term “closure” comes from how they are implemented – all variables not local to the function must be remembered
  • OCaml example:
# let f = (fun x -> fun y -> x + y) 4;;
val f : int -> int = <fun> (* f is at runtime the closure <fun y code, {x |-> 4}> *)
# f 3;;
- : int = 7
  • Note how x is a function parameter and is remembered in spite of function returning, means x needs to be copied into the closure.

FP in C++

  • Everyone is joining the Lambda party!
  • Details here.
  • Closures are a headache in C++ due to different low-level ways data can be accessed in C++.
  • Use const declarations to get immutable variables
  • C++ also has some type inference a la OCaml C++ local type inference
    • e.g. auto mydata = 22;. auto is like var in Java.
  • C++-14 adds generic lambdas which look like the polymorphic types of OCaml/Java but are really just fancy macros.

FP In Python

  • Python “already has” FP including closures and Currying.
def adder(x):
     def uni_adder(y):
         return x + y
     return uni_adder
print adder(4)(18)
  • The above is a bit clunky having to name the intermediate function
  • map, filter, reduce etc are already in the core language
  • In addition, the functools standard library supports other convenience higher-order function operations
  • Plus, if you want even more FP-ism, there are additional libraries such as PyToolz
from toolz import curry
 def add(x, y):
     return x + y

plusfour = add(4)     
  • PyToolz is basically a port of the Clojure FP libraries to Python
  • Python is weak on immutable variables, there is no const/final
    • but the tuples and frozenset are immutable data structures
    • and Python 3.8 finally has final

FP In JavaScript

  • JavaScript is similar to Python, it has the basics built-in already
function adder(a) 
{ return function uni_adder(b) 
    { return a+b;
  • const declarations bring the FP immutable variable default to JS - use it!
  • JavaScript has no immutable data structures however
    • means e.g. lists won’t be able to share sub-structures so “FP programming” will be less efficient in JS.