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Display tree of source references

Source: R/src.R
src.Rd

View source reference metadata attached to R objects in a tree structure. Shows source file information, line/column locations, and lines of source code.

Usage

src(x, max_depth = 5L, max_length = 100L, ...)

Arguments

x

An R object with source references. Can be:

  • A srcref object

  • A list of srcref objects

  • A expression vector with attached source references

  • An evaluated closure with attached source references

  • A quoted call with attached source references

max_depth

Maximum depth to traverse nested structures (default 5)

max_length

Maximum number of srcref nodes to display (default 100)

...

Additional arguments passed to tree()

Value

Returns a structured list containing the source reference information. Print it to view the formatted tree.

Overview

Source references are made of two kinds of objects:

  • srcref objects, which contain information about a specific location within the source file, such as the line and column numbers.

  • srcfile objects, which contain metadata about the source file such as its name, path, and encoding.

Where and when are source references created?

Ultimately the R parser creates source references. The main two entry points to the parser are:

  • The R function parse().

  • The frontend hook ReadConsole, which powers the console input parser in the R CLI and in IDEs. This C-level parser can also be accessed from C code via R_ParseVector().

In principle, anything that calls parse() may create source references, but here are the important direct and indirect callers:

By default source references are not created but can be enabled by:

srcref objects

srcref objects are compact integer vectors describing a character range in a source. It records start/end lines and byte/column positions and, optionally, the parsed-line numbers if #line directives were used.

Lengths of 4, 6, or 8 are allowed:

  • 4: basic (first_line, first_byte, last_line, last_byte). Byte positions are within the line.

  • 6: adds columns in Unicode codepoints (first_col, last_col)

  • 8: adds parsed-line numbers (first_parsed, last_parsed)

The "column" information does not represent grapheme clusters, but Unicode codepoints. The column cursor is incremented at every UTF-8 lead byte and there is no support for encodings other than UTF-8.

The srcref columns are right-boundary positions, meaning that for an expression starting at the start of a line, the column will be 1. wholeSrcref (see below) on the other hand starts at 0, before the first character. It might also end 1 character after the last srcref column.

They are attached as attributes (e.g. attr(x, "srcref") or attr(x, "wholeSrcref")), possibly wrapped in a list, to the following objects:

  • Expression vectors returned by parse() (wrapped in a list)

  • Quoted function calls (unwrapped)

  • Quoted { calls (wrapped in a list). This is crucial for debugging: when R steps through brace lists, the srcref for the current expression is saved to a global variable (R_Srcref) so the IDE knows exactly where execution is paused. See: https://github.com/r-devel/r-svn/blob/fa0b47c5/src/main/eval.c#L2986.

  • Evaluated closures (unwrapped)

They have a srcfile attribute that points to the source file.

Methods:

  • as.character(): Retrieves relevant source lines from the srcfile reference.

wholeSrcref attributes

These are srcref objects stored in the wholeSrcref attributes of:

  • Expression vectors returned by parse(), which seems to be the intended usage.

  • { calls, which seems unintended.

For expression vectors, the wholeSrcref spans from the first position to the last position and represents the entire document. For braces, they span from the first position to the location of the closing brace. There is no way to know the location of the opening brace without reparsing, which seems odd. It's probably an overlook from xxexprlist() calling attachSrcrefs() in https://github.com/r-devel/r-svn/blob/52affc16/src/main/gram.y#L1380. That function is also called at the end of parsing, where it's intended for the wholeSrcref attribute to be attached.

For evaluated closures, the wholeSrcref attribute on the body has the same unreliable start positions as { nodes.

srcfile objects

srcfile objects are environments representing information about a source file that a source reference points to. They typically refer to a file on disk and store the filename, working directory, a timestamp, and encoding information.

While it is possible to create bare srcfile objects, specialized subclasses are much more common.

srcfile

A bare srcfile object does not contain any data apart from the file path. It lazily loads lines from the file on disk, without any caching.

Fields common to all srcfile objects:

  • filename: The filename of the source file. If relative, the path is resolved against wd.

  • wd: The working directory (getwd()) at the time the srcfile was created, generally at the time of parsing).

  • timestamp: The timestamp of the source file. Retrieved from filename with file.mtime().

  • encoding: The encoding of the source file.

  • Enc: The encoding of output lines. Used by getSrcLines(), which calls iconv() when Enc does not match encoding.

  • parseData (optional): Parser information saved when keep.source.data is set to TRUE.

Implementations:

Helpers:

srcfilecopy

A srcfilecopy stores the actual source lines in memory in $lines. srcfilecopy is useful when the original file may change or does not exist, because it preserves the exact text used by the parser.

This type of srcfile is the most common. It's created by:

Fields:

  • filename: The filename of the source file. If isFile is FALSE, the field is non meaningful. For instance parse(text = ) sets it to "<text>", and the console input parser sets it to "".

  • isFile: A logical indicating whether the source file exists.

  • fixedNewlines: If TRUE, lines is a character vector of lines with no embedded \n characters. The getSrcLines() helper regularises lines in this way and sets fixedNewlines to TRUE.

Note that the C-level parser (used directly mainly when parsing console input) does not call the R-level constructor and only instantiates the filename (set to "") and lines fields.

srcfilealias

This object wraps an existing srcfile object (stored in original). It allows exposing a different filename while delegating the open/close/get lines operations to the srcfile stored in original.

The typical way aliases are created is via #line *line* *filename* directives where the optional *filename* argument is supplied. These directives remap the srcref and srcfile of parsed code to a different location, for example from a temporary file or generated file to the original location on disk.

Created by install.packages() when installing a source package with keep.source.pkgs set to TRUE (see https://github.com/r-devel/r-svn/blob/52affc16/src/library/tools/R/install.R#L545), but only when:

  • Encoding was supplied in DESCRIPTION

  • The system locale is not "C" or "POSIX".

The source files are converted to the encoding of the system locale, then collated in a single source file with #line directives mapping them to their original file names (with full paths): https://github.com/r-devel/r-svn/blob/52affc16/src/library/tools/R/admin.R#L342.

Note that the filename of the original srcfile incorrectly points to the package path in the install destination.

Fields:

  • filename: The virtual file name (or full path) of the parsed code.

  • original: The actual srcfile the code was parsed from.

See also

Other object inspectors: ast(), ref(), sxp()