Use setopt for user options (defcustom)
Introduced in Emacs 29. Preferred for all defcustom variables in your init file.
setopt runs the :set callback defined by defcustom and validates the value against its :type. Using setq on a defcustom variable bypasses both, which can silently break packages that depend on the setter to update internal state.
Caveat: slower than setq because it goes through the Customize machinery. Don’t use it for regular variables in hot paths.
Works in early-init.el: setopt is autoloaded (via loaddefs.el, which is part of the Emacs dump), so it is available during early-init.el. The first use triggers loading cus-edit.el slightly earlier than normal — negligible overhead for a few variables.
Use setq for regular variables
The standard way to set any variable. Use for internal/non-customizable variables.
setq sets the current binding (lexical or dynamic). Inside a let, it modifies the local binding, not the global one.
Use setq-default for buffer-local defaults
Sets the default value for buffer-local variables. The value applies to buffers that haven’t set their own local value.
Use setq-local for buffer-local values
Sets a variable’s value in the current buffer only.
Use set when the variable name is computed
A function (not a special form) — evaluates its first argument to get the symbol to set.
Quick decision table
| Variable kind | Use |
|---|---|
defcustom user option | setopt |
| Regular variable | setq |
| Buffer-local default | setq-default |
| Buffer-local (this buffer) | setq-local |
| Dynamic symbol name | set |