Better smart-assignment with = in R and S, with no underscores.
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Current Version: 0.3.1. This is a major update – indeed a complete rewrite – relative to version 0.2.2, with significant improvements in functionality, flexibility, and performance. This has also been brought up to date with recent versions of ESS.

Assignment in R is syntactically complicated by a few features:

  1. the historical role of '_' (underscore) as an assignment

character in the S language; 2. the somewhat inconvenient-to-type, if conceptually pure, '<-' operator as the preferred assignment operator; 3. the ability to use either an '=', '<-', and a variety of other operators for assignment; and

  1. the multiple roles that '=' can play, including for setting

named arguments in a function call.

This package offers a flexible, context-sensitive assignment key for R and S that is, by default, tied to the '=' key. This key inserts or completes relevant, properly spaced operators (assignment, comparison, etc.) based on the syntactic context in the code. It allows very easy cycling through the possible operators in that context. The contexts, the operators, and their cycling order in each context are customizable.

The package defines a buffer-local minor mode ess-smart-equals-mode, intended for S-language modes (e.g., ess-r-mode, inferior-ess-r-mode, and ess-r-transcript-mode), that when enabled in a buffer activates the '=' key to to handle context-sensitive completion and cycling of relevant operators. When the mode is active and an '=' is pressed:

  1. With a prefix argument or in specified contexts (which for most major modes means in strings or comments), just insert '='.

  2. If an operator relevant to the context lies before point (with optional whitespace), it is replaced, cyclically, by the next operator in the configured list for that context.

  3. Otherwise, if a prefix of an operator relevant to the context lies before point, that operator is completed.

  4. Otherwise, the highest priority relevant operator is inserted with surrounding whitespace (see ess-smart-equals-no-spaces).

Consecutive presses of '=' cycle through the relevant operators. After an '=', a backspace (or other configurable keys) removes the last operator and tab offers a choice of operators by completion. (Shift-backspace will delete one character only and restore the usual maning of backspace.) See ess-smart-equals-cancel-keys.

By default, the minor mode activates the '=' key, but this can be customized by setting the option ess-smart-equals-key before this package is loaded.

The function ess-smart-equals-activate arranges for the minor mode to be activated by mode hooks for any given list of major modes, defaulting to ESS major modes associated with R (ess-r-mode, inferior-ess-r-mode, ess-r-transcript-mode, ess-roxy-mode).


In the left column below, ^ marks the location at which an '=' key is pressed, the remaining columns show the result of consecutive presses of '=' using the package's default settings. position of point.

Before '='         Press '='      Another '='       Another '='
----------         ---------      -----------       -----------
foo^               foo <- ^       foo <<- ^         foo = ^
foo  ^             foo  <- ^      foo  <<- ^        foo  = ^
foo<^              foo <- ^       foo <<- ^         foo = ^
foo=^              foo = ^        foo -> ^          foo ->> ^
foo(a^             foo(a = ^      foo( a == ^       foo( a != ^
if( foo=^          if( foo == ^   if( foo != ^      if( foo <= ^
if( foo<^          if( foo < ^    if( foo > ^       if( foo >= ^
"foo ^             "foo =^        "foo ==^          "foo ===^ ^ =^ ==^ ===^

As a bonus, the value of the variable ess-smart-equals-extra-ops when this package is loaded, determines some other smart operators that may prove useful. Currently, only brace, paren, and percent are supported, causing ess-smart-equals-open-brace, ess-smart-equals-open-paren, and ess-smart-equals-percent to be bound to '{', '(', and '%', respectively. The first two of these configurably places a properly indented and spaced matching pair at point or around the region if active. The paren pair also includes a magic space with a convenient keymap for managing parens. See the readme. See the customizable variable ess-smart-equals-brace-newlines for configuring the newlines around braces. The third operator (ess-smart-equals-percent) performs matching of %-operators analogously to ess-smart-equals. See the Extra Operators section below under Customization for details.

Finally, the primary user facing functions are named with a prefix ess-smart-equals- to avoid conflicts with other packages. Because this is long, the internal functions and objects use a shorter (but still distinctive) prefix essmeq-.

Installation and Initialization

The package can be loaded from MELPA using package-install with

M-x package-install ess-smart-equals

or with the list-packages interface or another Emacs package manager. Alternatively, you can clone or download the source directly from the github repository and put the file ess-smart-equals.el in your Emacs load path.

A variety of activation options is described below, but tl;dr: the recommended way to activate the mode (e.g., in your init file) is either directly with

(setq ess-smart-equals-extra-ops '(brace paren percent))
(with-eval-after-load 'ess-r-mode
  (require 'ess-smart-equals)

or with use-package:

(use-package ess-smart-equals
  :init   (setq ess-smart-equals-extra-ops '(brace paren percent))
  :after  (:any ess-r-mode inferior-ess-r-mode ess-r-transcript-mode)
  :config (ess-smart-equals-activate))

A more detailed description follows, if you want to see variations.

To activate, you need only do

(with-eval-after-load 'ess-r-mode
  (require 'ess-smart-equals)

somewhere in your init file. This will add ess-smart-equals-mode to a prespecified, but customizable, list of mode hooks and activate the mode in already active buffers.

For those who use the outstanding use-package, you can do

(use-package ess-smart-equals
  :after (:any ess-r-mode inferior-ess-r-mode ess-r-transcript-mode)
  :config (ess-smart-equals-activate))

somewhere in your init file. An equivalent but less concise version of this is

(use-package ess-smart-equals
  :after (:any ess-r-mode inferior-ess-r-mode ess-r-transcript-mode)
  :hook ((ess-r-mode . ess-smart-equals-mode)
         (inferior-ess-r-mode . ess-smart-equals-mode)
         (ess-r-transcript-mode . ess-smart-equals-mode)
         (ess-roxy-mode . ess-smart-equals-mode))

To also activate the extra smart operators, which I recommend, and to automatically bind them, you can replace this with

(use-package ess-smart-equals
  :init   (setq ess-smart-equals-extra-ops '(brace paren percent))
  :after  (:any ess-r-mode inferior-ess-r-mode ess-r-transcript-mode)
  :config (ess-smart-equals-activate))

This is the setup that I use. See Extra Smart Operators below.

You can also enable the minor mode in any buffer with

M-x ess-smart-equals-mode

though you will typically want to enable the minor mode in a corresponding mode hook, e.g.,

(add-hook 'foo-mode-hook 'ess-smart-equals-mode)

to enable the mode in foo-mode buffers.


Special Keys

By default ess-smart-equals-mode binds the smart operator to the '=' key, and this is the recommended choice. However, this key can be changed by customizing the variable ess-smart-equals-key. This should be changed either with the customization facility or before the package is loaded, as the key affects several internal keymaps.

When ess-smart-equals-key is pressed, several transient keys are bound. First, the basic key of ess-smart-equals-key (e.g., '=' for '=' or 'C-c ='.) reexecutes ess-smart-equals, cycling the operators according to context. If the smart '%' operator is enabled (see below), then '%' is also bound to ess-smart-equals-percent, which can be interleaved with ess-smart-equals as desired. (Any other key exits the transient keymap.) Second, any key in ess-smart-equals-cancel-keys deletes any inserted operator before point; a shifted version of such a key deletes a single character backwards and thus cancels the transient bindings. Finally, tab allows you to select an operator by completion.

The ess-smart-equals-cancel-keys are by default backspace and DEL, but they can be customized.

Several other useful smart operators can be configured; see Extra Smart Operators below.


The operator inserted or completed by ess-smart-equals is determined by the major mode and the syntactic context at point. The customizable variable ess-smart-equals-contexts specifies the mapping from syntactic contexts to a list of operators to consider in the order specified. This mapping is given for all contexts for the default case (t) along with lists for any major mode that are merged into the default mapping under that mode. In this way, simple modifications can be applied to any relevant mode without repeating all the specifications.

The user can create new contexts by adding additional keys to that mapping and defining ess-smart-equals-context-function. This is called first when the context is determined; if it returns a symbol, that is used as the context; if it returns nil, the built-in context calculation is performed. For specialized purposes, the context can be overridden locally; see ess-smart-equals-set-overriding-context and ess-smart-equals-clear-overriding-context.


Operator padding is controlled by two options: ess-smart-equals-mode-padding-left and ess-smart-equals-mode-padding-right, which control the padding on the left and right side of an inserted operator. If set to the symbol one-space (the default), no-space, or some-space, ess-smart-equals ensures that there is, respectively, exactly one space, no spaces, or at least one space (possibly taken from existing whitespace) on the corresponding side of the operator. If set to the symbol none, no padding adjustment is performed. If set to a string, that string is used as is for the padding on the corresponding side. Finally, this can be set to a function that takes two positions and an optional boolean; this function can adjust the padding in any way desired while also providing a way to compute how much padding has been added for the deletion operator. See the documentation for the padding variables for details.

The customizable variable ess-smart-equals-mode-options is an alist mapping major modes to assignments of minor mode options used locally in each ess-smart-equals-mode buffer. This allows mode-specific configuraiton of this minor mode. The default is an example: in inferior-ess-r-mode, ess-smart-equals-mode uses specialized narrowing so that previous output and commands do not interfere with the context parsing at a given point.

The customizable list ess-smart-equals-default-modes determines the major modes that area affected by ess-smart-equals-activate.


The hook ess-smart-equals-mode-hook is called whenever the minor mode is enabled or disabled. The hooks ess-smart-equals-mode-on-hook and ess-smart-equals-mode-off-hook are called when the mode is enabled or disabled, respectively.

The variable ess-smart-equals-narrow-function is used to narrow the buffer to a specific region where the ESS syntax checking will be valid. This is used primarily in inferior-ess-r-mode to restrict attention to the current prompt line or the zone between prompts because output or erroneous commands can adversely effect the ESS syntax checking. This should not normally be needed otherwise, but it can be set in ess-smart-equals-options if desired.

The customizable variable ess-smart-equals-insertion-hook, if set, allows arbitrary post-processing after an operator insertion. It is passed all the information needed to characterize the insertion; see the documentation for that variable for details.

Extra Smart Operators

If ess-smart-equals-extra-ops is non-nil, it should be a list containing some of the symbols brace, paren, or percent. These settings will cause '{', '(', and '%', respectively, to be bound in the minor mode map to a smart operator with the following features:

  • brace

    Binds '{' to the command ess-smart-equals-open-brace. This inserts a properly spaced and indented pair of braces, wrapping around the region if it is active. The customizable variable ess-smart-equals-brace-newlines controls the placement of newlines before and after each brace. This can be configured separately or added to your ESS style as desired.

  • paren

    Binds '(' to the command ess-smart-equals-open-paren. This inserts a matching pair of parentheses with point on a magic space between them. If region is active, it is wrapped by the parentheses. The magic space has an attached keymap that makes it easy to fill, escape, and expand the parenthesis pair. In particular, when on this space:

    • ')' or ';' eliminates the magic space and exits the parentheses;

    • ',' inserts a spaced comma, leaving point on the magic space;

    • 'C-;' expands the region after the parenthesis pair to encompass an additional balanced expression; and

    • 'M-;' moves the marked region after the pair (e.g., as constructed by 'C-;') inside the parentheses, eliminating leading spaces unless a prefix argument is given.

    Taken together, these make it fast to fill in function calls or conditionals.

  • percent

    Binds '%' to the command ess-smart-equals-percent. This provides expansion, cycling, and completion analogous to ess-smart-equals but for %-operators in R. The ess-smart-equals and ess-smart-equals-percent commands can be interleaved at will; when one follows the other, it will remove operators produced by the preceding command and start cycling anew.

With a prefix argument, all of these insert the literal corresponding character, with repeats if the argument is numeric.

Additional smart operators may be added in future versions.

Note that if you change the setting of ess-smart-equals-extra-ops, you can make it take effect in all relevant buffers by doing M-x ess-smart-equals-refresh-mode.

Change Log

  • 0.3.0: Breaking changes in functionality, design, and configuration. No longer relies on ess-S-assign which was deprecated in ESS. Now provides more powerful context-sensitive, prioritized operator lists with cycling and completion. The mode is now, properly, a local minor mode, which can be added automatically to relevant mode hooks for ESS R modes. Updated required versions of emacs and ESS.

  • 0.2.2: Fix for deprecated ESS variables ess-S-assign and ess-smart-S-assign-key. Thanks to Daniel Gomez (@dangom).

  • 0.2.1: Initial release with simple insertion and completion, with space padding for the operators except for a single '=' used to specify named arguments in function calls. Relies on ESS variables ess-S-assign and ess-smart-S-assign-key to specify preferred operator for standard assignments.

To Do

  • Allow finer control in context operator lists, both in distinguishing cycling from completion and in allowing dynamic operator lists. An example use case would be asking R for the set of current %infix% operators. Some of the infrastructure for this is already in place.

  • Add ess-smart-equals-the-works for simple, full feature setup

  • If it is worthwhile, make contexts sticky over cycling to avoid the context changing during cycling. This does not yet appear to be needed.

  • Add more tests


  • Daniel Gomez (@dangom)