parse: Parse Expressions
In robertzk/monadicbase: The R Base Package

Description Usage Arguments Details Value Partial parsing References See Also Examples

parse returns the parsed but unevaluated expressions in a list.

1
2
3

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

`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`.

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.

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.

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.

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.

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.

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)