Emacs Lisp Cheat Sheet for Clojure Developers

If you're a Clojure developer looking to learn Elisp, you have a great head start since you already know a Lisp dialect. This concise guide focuses on the main differences between the two languages.

Numeric Types:

• Elisp has integers and floats, which can be mixed in arithmetic functions as expected.
• Unlike Clojure, Elisp doesn't have a ratio type, so integer division works similarly to most other languages.
• Integers can be expressed in multiple ways, and the printed representation of REPL output may look odd at first, but it's simply showing alternate representations of the integer.
• Floats can be expressed in scientific notation, similar to Clojure.
• Elisp allows arbitrarily large integers.

Character Types:

• Elisp has strings and characters, represented and escaped in a similar way to Clojure.
• Character literals are displayed using a '?' prefix.
• Characters are actually integers, but not all integers are valid characters.
• String formatting can be done using the format function, similar to Clojure.
• Messages can be displayed in the minibuffer using the message function, which is also helpful for debugging.

Conditional Logic:

• In Elisp, nil and the empty list '()' are falsy, while everything else is truthy.
• Elisp doesn't have a separate Boolean type; instead, the symbol 't' represents true and 'nil' represents false.
• Predicate functions conventionally have a 'p' suffix, unlike Clojure's '?' suffix.
• Comparison functions like '=' and 'string=' are case-sensitive.

Functions:

• Functions are defined using 'defun', with arguments enclosed in parentheses.
• Docstrings are slightly different in Elisp.
• Variadic functions can be created using '&rest'.
• Elisp doesn't support true multi-arity functions, but optional arguments can be used instead.

Let Forms:

• For let bindings, use 'let*'. Each binding pair needs to be placed in its own list.
• 'let' also exists in Elisp, but it doesn't allow the use of earlier bindings within later ones.

Comments, Namespaces, and Access Modifiers:

• Elisp doesn't have namespaces or access modifiers.
• By convention, packages prefix their functions and variables with a package name prefix.

This cheat sheet provides a quick overview of the main differences between Emacs Lisp and Clojure. It can serve as a handy reference for Clojure developers venturing into Elisp development.