A Completion Generator for R

This page documents a mechanism to generate relevant completions from a partially completed command line. It is not intended to be useful by itself, but rather in conjunction with other mechanisms that use it as a backend. The functions listed in the usage section provide a simple control and query mechanism. The actual interface consists of a few unexported functions described further down.

utilities, IO
rc.settings(ops, ns, args, func, ipck, S3, data, help,
            argdb, fuzzy, quotes, files)

rc.status() rc.getOption(name) rc.options(...)

.DollarNames(x, pattern)

## S3 method for class 'default': .DollarNames(x, pattern = "") ## S3 method for class 'list': .DollarNames(x, pattern = "") ## S3 method for class 'environment': .DollarNames(x, pattern = "")

ops, ns, args, func, ipck, S3, data, help, argdb, fuzzy, quotes, files
logical, turning some optional completion features on and off.

[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

All settings are turned on by default except ipck, func, and fuzzy. Turn more off if your CPU cycles are valuable; you will still retain basic completion on names of objects in the search list. See below for additional details.

name, ...
user-settable options. Currently valid names are [object Object],[object Object],[object Object] Usage is similar to that of options.
An R object for which valid names after "$" are computed and returned.
A regular expression. Only matching names are returned.

There are several types of completion, some of which can be disabled using rc.settings. The most basic level, which can not be turned off once the completion functionality is activated, provides completion on names visible on the search path, along with a few special keywords (e.g., TRUE). This type of completion is not attempted if the partial word (a.k.a. token) being completed is empty (since there would be too many completions). The more advanced types of completion are described below.

[object Object],[object Object],[object Object],[object Object],[object Object]


  • If rc.settings is called without any arguments, it returns the current settings as a named logical vector. Otherwise, it returns NULL invisibly. rc.status returns, as a list, the contents of an internal (unexported) environment that is used to record the results of the last completion attempt. This can be useful for debugging. For such use, one must resist the temptation to use completion when typing the call to rc.status itself, as that then becomes the last attempt by the time the call is executed.

    The items of primary interest in the returned list are:

  • compsThe possible completions generated by the last call to .completeToken, as a character vector.
  • tokenThe token that was (or, is to be) completed, as set by the last call to .assignToken (possibly inside a call to .guessTokenFromLine).
  • linebufferThe full line, as set by the last call to .assignLinebuffer.
  • startThe start position of the token in the line buffer, as set by the last call to .assignStart.
  • endThe end position of the token in the line buffer, as set by the last call to .assignEnd.
  • fileNameLogical, indicating whether the cursor is currently inside quotes.
  • fguessThe name of the function the cursor is currently inside.
  • isFirstArgLogical. If cursor is inside a function, is it the first argument?
  • In addition, the components settings and options give the current values of settings and options respectively.

    rc.getOption and rc.options behave much like getOption and options respectively.


If you are uncomfortable with unsolicited evaluation of pieces of code, you should set ops = FALSE. Otherwise, trying to complete foo@ba will evaluate foo, trying to complete foo[i, 1:10]$ba will evaluate foo[i, 1:10], etc. This should not be too bad, as explicit function calls (involving parentheses) are not evaluated in this manner. However, this will affect promises and lazy loaded symbols.

Unexported API

There are several unexported functions in the package. Of these, a few are special because they provide the API through which other mechanisms can make use of the facilities provided by this package (they are unexported because they are not meant to be called directly by users). The usage of these functions are:

.assignToken(text) .assignLinebuffer(line) .assignStart(start) .assignEnd(end)

.completeToken() .retrieveCompletions() .getFileComp()

.guessTokenFromLine() .win32consoleCompletion(linebuffer, cursorPosition, check.repeat = TRUE, minlength = -1)


The first four functions set up a completion attempt by specifying the token to be completed (text), and indicating where (start and end, which should be integers) the token is placed within the complete line typed so far (line).

Potential completions of the token are generated by .completeToken, and the completions can be retrieved as an Rcharacter vector using .retrieveCompletions. It is possible for the user to specify a replacement for this function by setting rc.options("custom.completer"); if not NULL, this function is called to compute potential completions. This facility is meant to help in situations where completing as R code is not appropriate. See source code for more details.

If the cursor is inside quotes, completion may be suppressed. The function .getFileComp can be used after a call to .completeToken to determine if this is the case (returns TRUE), and alternative completions generated as deemed useful. In most cases, filename completion is a reasonable fallback.

The .guessTokenFromLine function is provided for use with backends that do not already break a line into tokens. It requires the linebuffer and endpoint (cursor position) to be already set, and itself sets the token and the start position. It returns the token as a character string.

The .win32consoleCompletion is similar in spirit, but is more geared towards the Windows GUI (or rather, any front-end that has no completion facilities of its own). It requires the linebuffer and cursor position as arguments, and returns a list with three components, addition, possible and comps. If there is an unambiguous extension at the current position, addition contains the additional text that should be inserted at the cursor. If there is more than one possibility, these are available either as a character vector of preformatted strings in possible, or as a single string in comps. possible consists of lines formatted using the current width option, so that printing them on the console one line at a time will be a reasonable way to list them. comps is a space separated (collapsed) list of the same completions, in case the front-end wishes to display it in some other fashion.

The minlength argument can be used to suppress completion when the token is too short (which can be useful if the front-end is set up to try completion on every keypress). If check.repeat is TRUE, it is detected if the same completion is being requested more than once in a row, and ambiguous completions are returned only in that case. This is an attempt to emulate GNU Readline behaviour, where a single TAB completes up to any unambiguous part, and multiple possibilities are reported only on two consecutive TABs.

As the various front-end interfaces evolve, the details of these functions are likely to change as well.

The function .addFunctionInfo can be used to add information about the permitted argument names for specific functions. Multiple named arguments are allowed in calls to it, where the tags are names of functions and values are character vectors representing valid arguments. When the argdb setting is TRUE, these are used as a source of valid argument names for the relevant functions.

  • rcompgen
  • rc.settings
  • rc.status
  • rc.getOption
  • rc.options
  • .DollarNames
  • .DollarNames.default
  • .DollarNames.list
  • .DollarNames.environment
  • completion
Documentation reproduced from package utils, version 3.3, License: Part of R 3.3

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