base (version 3.4.3)

parse: Parse Expressions

Description

parse returns the parsed but unevaluated expressions in a list.

Usage

parse(file = "", n = NULL, text = NULL, prompt = "?",
      keep.source = getOption("keep.source"), srcfile,
      encoding = "unknown")

Arguments

file

a connection, or a character string giving the name of a file or a URL to read the expressions from. If file is "" and text is missing or NULL then input is taken from the console.

n

integer (or coerced to integer). The maximum number of expressions to parse. If n is NULL or negative or NA the input is parsed in its entirety.

text

character vector. The text to parse. Elements are treated as if they were lines of a file. Other R objects will be coerced to character if possible.

prompt

the prompt to print when parsing from the keyboard. NULL means to use R's prompt, getOption("prompt").

keep.source

a logical value; if TRUE, keep source reference information.

srcfile

NULL, a character vector, or a srcfile object. See the ‘Details’ section.

encoding

encoding to be assumed for input strings. If the value is "latin1" or "UTF-8" it is used to mark character strings as known to be in Latin-1 or UTF-8: it is not used to re-encode the input. To do the latter, specify the encoding as part of the connection con or via options(encoding=): see the example under file.

Value

An object of type "expression", with up to n elements if specified as a non-negative integer.

When srcfile is non-NULL, a "srcref" attribute will be attached to the result containing a list of srcref records corresponding to each element, a "srcfile" attribute will be attached containing a copy of srcfile, and a "wholeSrcref" attribute will be attached containing a srcref record corresponding to all of the parsed text. Detailed parse information will be stored in the "srcfile" attribute, to be retrieved by getParseData.

A syntax error (including an incomplete expression) will throw an error.

Character strings in the result will have a declared encoding if encoding is "latin1" or "UTF-8", or if text is supplied with every element of known encoding in a Latin-1 or UTF-8 locale.

Partial parsing

When a syntax error occurs during parsing, parse signals an error. The partial parse data will be stored in the srcfile argument if it is a srcfile object and the text argument was used to supply the text. In other cases it will be lost when the error is triggered.

The partial parse data can be retrieved using getParseData applied to the srcfile object. Because parsing was incomplete, it will typically include references to "parent" entries that are not present.

Details

If text has length greater than zero (after coercion) it is used in preference to file.

All versions of R accept input from a connection with end of line marked by LF (as used on Unix), CRLF (as used on DOS/Windows) or CR (as used on classic Mac OS). The final line can be incomplete, that is missing the final EOL marker.

When input is taken from the console, n = NULL is equivalent to n = 1, and n < 0 will read until an EOF character is read. (The EOF character is Ctrl-Z for the Windows front-ends.) The line-length limit is 4095 bytes when reading from the console (which may impose a lower limit: see ‘An Introduction to R’).

The default for srcfile is set as follows. If keep.source is not TRUE, srcfile defaults to a character string, either "<text>" or one derived from file. When keep.source is TRUE, if text is used, srcfile will be set to a srcfilecopy containing the text. If a character string is used for file, a srcfile object referring to that file will be used.

When srcfile is a character string, error messages will include the name, but source reference information will not be added to the result. When srcfile is a srcfile object, source reference information will be retained.

References

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

Murdoch, D. (2010). Source References. The R Journal 2/2, 16-19.

See Also

scan, source, eval, deparse.

The source reference information can be used for debugging (see e.g.setBreakpoint) and profiling (see Rprof). It can be examined by getSrcref and related functions. More detailed information is available through getParseData.

Examples

# NOT RUN {
cat("x <- c(1, 4)\n  x ^ 3 -10 ; outer(1:7, 5:9)\n", file = "xyz.Rdmped")
# parse 3 statements from the file "xyz.Rdmped"
parse(file = "xyz.Rdmped", n = 3)
unlink("xyz.Rdmped")

# A partial parse with a syntax error
txt <- "
x <- 1
an error
"
sf <- srcfile("txt")
try(parse(text = txt, srcfile = sf))
getParseData(sf)
# }