“Functional programming” has a clear meaning, but does “functional language”?

Question

I understand very clearly the difference between functional and imperative programming techniques. But there\'s a widespread tendency to talk of \"functional languages\

Answer 1

Among people who study programming languages for a living, "functional programming language" is a pretty weakly bound term. There is a strong consensus that:

• Any language that calls itself functional must support first-class, nested functions with lexical scoping rules.

A significant minority also reserve the term "functional language" for languages which are:

• Pure (or side-effect-free, referentially transparent, see also)

as in languages like Agda, Clean, Coq, and Haskell.

Beyond that, what's considered a functional programming language is often a matter of intent, that is, whether is designers want it to be called "functional".

Perl and Smalltalk are examples of languages that support first-class functions but whose designers don't call them functional. Objective Caml is an example of a language that is called functional even though it has a full object system with inheritance and everything.

Languages that are called "functional" will tend to have features like the following (taken from Defining point of functional programming):

• Anonymous functions (lambda expressions)
• Recursion (more prominent as a result of purity)
• Programming with expressions rather than statements (again, from purity)
• Closures
• Currying / partial application
• Lazy evaluation
• Algebraic data types and pattern matching
• Parametric polymorphism (a.k.a. generics)

The more a particular programming language has syntax and constructs tailored to making the various programming features listed above easy/painless to express & implement, the more likely someone will label it a "functional language".