R: Environment Browser

browser {base} | R Documentation

## Environment Browser

### Description

Interrupt the execution of an expression and allow the inspection of the environment where `browser` was called from.

### Usage

    browser(text = "", condition = NULL, expr = TRUE, skipCalls = 0L)

### Arguments

`text` | a text string that can be retrieved once the browser is invoked.
`condition` | a condition that can be retrieved once the browser is invoked.
`expr` | An expression, which if it evaluates to `TRUE` the debugger will invoked, otherwise control is returned directly.
`skipCalls` | how many previous calls to skip when reporting the calling context.

### Details

A call to `browser` can be included in the body of a function. When reached, this causes a pause in the execution of the current expression and allows access to the R interpreter.

The purpose of the `text` and `condition` arguments are to allow helper programs (e.g., external debuggers) to insert specific values here, so that the specific call to browser (perhaps its location in a source file) can be identified and special processing can be achieved. The values can be retrieved by calling `browserText` and `browserCondition`.

The purpose of the `expr` argument is to allow for the illusion of conditional debugging. It is an illusion, because execution is always paused at the call to browser, but control is only passed to the evaluator described below if `expr` evaluates to `TRUE`. In most cases it is going to be more efficient to use an `if` statement in the calling program, but in some cases using this argument will be simpler.

The `skipCalls` argument should be used when the `browser()` call is nested within another debugging function: it will look further up the call stack to report its location.

At the browser prompt the user can enter commands or R expressions, followed by a newline. The commands are

`c`

  exit the browser and continue execution at the next statement.

`cont`

  synonym for `c`.

`f`

  finish execution of the current loop or function

`help`

  print this list of commands

`n`

  evaluate the next statement, stepping over function calls. For byte compiled functions interrupted by `browser` calls, `n` is equivalent to `c`.

`s`

  evaluate the next statement, stepping into function calls. Again, byte compiled functions make `s` equivalent to `c`.

`where`

  print a stack trace of all active function calls.

`r`

  invoke a `"resume"` restart if one is available; interpreted as an R expression otherwise. Typically `"resume"` restarts are established for continuing from user interrupts.

`Q`

  exit the browser and the current evaluation and return to the top-level prompt.

Leading and trailing whitespace is ignored, except for an empty line. Handling of empty lines depends on the `"browserNLdisabled"` option; if it is `TRUE`, empty lines are ignored. If not, an empty line is the same as `n` (or `s`, if it was used most recently).

Anything else entered at the browser prompt is interpreted as an R expression to be evaluated in the calling environment: in particular typing an object name will cause the object to be printed, and `ls()` lists the objects in the calling frame. (If you want to look at an object with a name such as `n`, print it explicitly, or use autoprint via `(n)`.

The number of lines printed for the deparsed call can be limited by setting `options(deparse.max.lines)`.

The browser prompt is of the form `Browse[n]>`: here `var{n}` indicates the ‘browser level’. The browser can be called when browsing (and often is when `debug` is in use), and each recursive call increases the number. (The actual number is the number of ‘contexts’ on the context stack: this is usually `2` for the outer level of browsing and `1` when examining dumps in `debugger`.)

This is a primitive function but does argument matching in the standard way.

### References

Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.

Chambers, J. M. (1998) Programming with Data. A Guide to the S Language. Springer.

### See Also

`debug`, and `traceback` for the stack on error. `browserText` for how to retrieve the text and condition.

[Package base version 4.0.5 Index]