R: Parse R code

parse_expr {rlang} | R Documentation

## 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)

### 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.

### Details

`parse_expr()` returns one expression. If the text contains more than one expression (separated by semicolons 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 a base::expression vector). All functions also support R connections.

### Value

`parse_expr()` returns an expression, `parse_exprs()` returns a list of expressions. Note that for the plural variants the length of the output may be greater than the length of the input. This would happen is one of the strings contain several expressions (such as `"foo; bar"`). The names of `x` are preserved (and recycled in case of multiple expressions).

### See Also

`base::parse()`

### Examples

    # parse_expr() can parse 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)")

    # Use names to figure out which input produced an expression:
    parse_exprs(c(foo = "1; 2", bar = "3"))

    # 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))

[Package rlang version 0.4.7 Index]