> ## Documentation Index
> Fetch the complete documentation index at: https://docs.jylhis.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Setting Variables in Emacs Lisp

> Quick reference for choosing the right way to set variables in your init file

## Use `setopt` for user options (defcustom)

Introduced in **Emacs 29**. Preferred for all `defcustom` variables in your init file.

```elisp theme={}
(setopt display-line-numbers-type 'relative)
(setopt copilot-idle-delay 0.5)
```

**Why:** `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.

```elisp theme={}
(setq gc-cons-threshold (* 50 1000 1000))
(setq x 1
      y 2)
```

**Note:** `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.

```elisp theme={}
(setq-default indent-tabs-mode nil)
(setq-default fill-column 80)
```

## Use `setq-local` for buffer-local values

Sets a variable's value in the current buffer only.

```elisp theme={}
(add-hook 'org-mode-hook
  (lambda () (setq-local fill-column 100)))
```

## Use `set` when the variable name is computed

A function (not a special form) -- evaluates its first argument to get the symbol to set.

```elisp theme={}
(set 'my-var 42)          ; equivalent to (setq my-var 42)
(set (intern name) value)  ; dynamic symbol lookup
```

Rarely needed in init files.

## 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`          |

## Sources

* [GNU Elisp Manual -- Setting Variables](https://www.gnu.org/software/emacs/manual/html_node/elisp/Setting-Variables.html)
* [Emacs Redux -- Goodbye setq, Hello setopt](https://emacsredux.com/blog/2025/04/06/goodbye-setq-hello-setopt/)