rlang (version 0.1)

parse_expr: Parse R code

Description

These functions parse and transform text into R expressions. This is the first step to interpret or evaluate a piece of R code written by a programmer.

Usage

parse_expr(x)
parse_exprs(x)
parse_quosure(x, env = caller_env())
parse_quosures(x, env = caller_env())

Arguments

x

Text containing expressions to parse_expr for parse_expr() and parse_exprs(). Can also be an R connection, for instance to a file. If the supplied connection is not open, it will be automatically closed and destroyed.

env

The environment for the formulas. Defaults to the context in which the parse_expr function was called. Can be any object with a as_env() method.

Value

parse_expr() returns a formula, parse_exprs() returns a list of formulas.

Details

parse_expr() returns one expression. If the text contains more than one expression (separated by colons or new lines), an error is issued. On the other hand parse_exprs() can handle multiple expressions. It always returns a list of expressions (compare to base::parse() which returns an base::expression vector). All functions also support R connections.

The versions prefixed with f_ return expressions quoted in formulas rather than raw expressions.

See Also

base::parse()

Examples

# NOT RUN {
# parse_expr() can parse_expr any R expression:
parse_expr("mtcars %>% dplyr::mutate(cyl_prime = cyl / sd(cyl))")

# A string can contain several expressions separated by ; or \n
parse_exprs("NULL; list()\n foo(bar)")

# The versions suffixed with _f return formulas:
parse_quosure("foo %>% bar()")
parse_quosures("1; 2; mtcars")

# The env argument is passed to as_env(). It can be e.g. a string
# representing a scoped package environment:
parse_quosure("identity(letters)", env = empty_env())
parse_quosures("identity(letters); mtcars", env = "base")


# You can also parse source files by passing a R connection. Let's
# create a file containing R code:
path <- tempfile("my-file.R")
cat("1; 2; mtcars", file = path)

# We can now parse it by supplying a connection:
parse_exprs(file(path))
# }