Library vs Language extension

Question

What is the difference between a Library and a Language extension? Specifically in Scala.

Answer 1

This is the Scala language specification. If you can write it with the language described by this specification, then it is a library. If you make changes to the language described by this document that cannot be written in the language itself, then it is a language extension.

Answer 2

This is valid both for Scala and for Java, a library could be defined as:

In computer science, a library is a collection of resources used to develop software. These may include pre-written code and subroutines, classes, values or type specifications.

It means that (I know, I simplify a lot) a library is a collection of routines you'll use in your code to implement an algorithm. They save you to write the same code again and again (for example you do not need to reinvent the wheel every time you have to print a string to console).

A language extension is not code that you'll call (or use) directly from your code but something that will change how you write your programs. It may change (and usually it does) the syntax of the language itself and often it's a plug-in for the compiler. Many of the features added to the Java language are extension (generics, for example).

For example Session-Scala is an extensions made of both:

• a library to manage parallel programming (code you can call directly from your code).
• a language extension: to make the code you write more clear (what sometimes is called syntactic sugar), it changes the syntax of the language to make the use of its library functions more easy (should I say nice?)

For an example take a look to this page.

That said, often with Scala a language extension isn't really needed (even if it's so easy to write, more than for .NET languages, for example) because of its syntax. First thing that comes to my mind are infix operators but in general all its syntax for methods invocation makes everything simple to use it as DSL without a DSL.