Emacs Lisp - Language Features

Language Features

Notably, the "cl" package implements a fairly large subset of Common Lisp.

Emacs Lisp uses dynamic scope, offering static (or lexical) as an option starting from version 24. It can be activated by setting the file local variable lexical-binding.

In dynamic scoping, if a programmer declares a variable within the scope of a function, it is available to subroutines called from within that function. Originally, this was intended as an optimization; lexical scoping was still uncommon and of uncertain performance. Dynamic scoping was also meant to provide greater flexibility for user customizations. However, dynamic scoping has several disadvantages. Firstly, it can easily lead to bugs in large programs, due to unintended interactions between variables in different functions. Secondly, accessing variables under dynamic scoping is generally slower than under lexical scoping.

Also, the lexical-let macro in the "cl" package does provide effective lexical scope to Emacs Lisp programmers, but while `cl' is widely used, lexical-let is rarely used. Variables bound with lexical-let are never released, even if they are never used.

Emacs Lisp (unlike some other Lisp implementations) does not do tail-call optimization. Without this, tail recursions can eventually lead to stack overflow.

The apel library aids in writing portable Emacs Lisp code, with the help of the polysylabi platform bridge.