R: Return the head or tail of a call object

lang_head {rlang} | R Documentation

## Return the head or tail of a call object

### Description

These functions return the head or the tail of a call. See section on calls as parse trees in `lang()`. They are equivalent to `node_car()` and `node_cdr()` but support quosures and check that the input is indeed a call before retrieving the head or tail (it is unsafe to do this without type checking).

`lang_head()` returns the head of the call without any conversion, unlike `lang_name()` which checks that the head is a symbol and converts it to a string. `lang_tail()` returns the pairlist of arguments (while `lang_args()` returns the same object converted to a regular list)

### Usage

    lang_head(lang)

    lang_tail(lang)

### Arguments

`lang` | Can be a call (language object), a formula quoting a call in the right-hand side, or a frame object from which to extract the call expression.

### See Also

pairlist, `lang_args()`, `lang()`

### Examples

    lang <- quote(foo(bar, baz))
    lang_head(lang)
    lang_tail(lang)

[Package rlang version 0.1.6 Index]