# data_source

From rlang v0.0.0.9000
by Hadley Wickham

##### Evaluate a formula

`f_eval_rhs`

evaluates the RHS of a formula and `f_eval_lhs`

evaluates the LHS. `f_eval`

is a shortcut for `f_eval_rhs`

since
that is what you most commonly need.

##### Usage

```
data_source(x, lookup_msg = NULL)
f_eval_rhs(f, data = NULL)
f_eval_lhs(f, data = NULL)
f_eval(f, data = NULL)
```

##### Arguments

- x
- An object for which you want to find associated data.
- lookup_msg
- An error message when your data source is accessed inappropriately (by position rather than name).
- f
- A formula. Any expressions wrapped in
`UQ()`

will will be "unquoted", i.e. they will be evaluated, and the results inserted back into the formula. See`f_quote()`

for more details. - data
- A list (or data frame).
`data_source`

is a generic used to find the data associated with a given object. If you want to make`f_eval`

work for your own objects, you can define a method for this generic.

##### Details

If `data`

is specified, variables will be looked for first in this
object, and if not found in the environment of the formula.

##### Pronouns

When used with `data`

, `f_eval`

provides two pronouns to make it
possible to be explicit about where you want values to come from:
`.env`

and `.data`

. These are thin wrappers around `.data`

and `.env`

that throw errors if you try to access non-existent values.

##### Examples

```
f_eval(~ 1 + 2 + 3)
# formulas automatically capture their enclosing environment
foo <- function(x) {
y <- 10
~ x + y
}
f <- foo(1)
f
f_eval(f)
# If you supply data, f_eval will look their first:
f_eval(~ cyl, mtcars)
# To avoid ambiguity, you can use .env and .data pronouns to be
# explicit:
cyl <- 10
f_eval(~ .data$cyl, mtcars)
f_eval(~ .env$cyl, mtcars)
# Imagine you are computing the mean of a variable:
f_eval(~ mean(cyl), mtcars)
# How can you change the variable that's being computed?
# The easiest way is "unquote" with !!
# See ?f_quote for more details
var <- ~ cyl
f_eval(f_quote(mean( !!var )), mtcars)
```

*Documentation reproduced from package rlang, version 0.0.0.9000, License: GPL-3*

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