elixir (version 0.1.0)

expr_apply: Apply a function over expressions

Description

Recursively apply a function over an expression, or any expression elements of a list, and optionally the subexpressions within any expressions.

Usage

expr_apply(
  x,
  f,
  depth = Inf,
  into = FALSE,
  order = c("pre", "post"),
  how = c("replace", "unlist", "unique"),
  env = parent.frame()
)

Value

If how = "replace" (the default), the original object x with f

applied to expressions within it. If how = "unlist", the same but with unlist() applied to it. If how = "unique", first unlist() then unique() are applied.

Arguments

x

The R object; can an expression, or a list of arbitrary nestedness potentially containing expressions.

f

Function to apply to all expressions within x; takes 1 to 3 arguments.

depth

How many levels to recurse into lists; default is Inf.

into

Whether to recurse into expressions. Can be TRUE to visit all subexpressions, FALSE to not recurse, or "leaves" to recurse and only apply f to terminal nodes of expressions (i.e. the symbols and syntactic literals comprising the expressions).

order

Whether a parent node is visited before ("pre") or after ("post") its children (the terminology comes from pre-order and post-order depth-first search). This only has an effect if into == TRUE.

how

How to structure the result.

env

Environment for injections in x (see expression).

Details

The function f can take one to three arguments. The first argument is the expression itself for f to apply to, and f should return some kind of replacement for, or modified version of, this argument.

The second argument is a list with information about the name of the expression in the list x and of its parents. Specifically, the first element of the list is the name of the expression, the second element of the list is the name of the "parent" of the expression, and so on. If any elements in this chain are unnamed, an integer is provided as the name. If the expression is within another expression (which only happens with into = TRUE), this is signalled as a NULL at the top of the list, one for each level of recursion into the expression.

The third argument is an integer vector, the index into x where f is currently operating. This is suitable for use with expr_sub().

Examples

expr_apply(list(quote(a + b), quote(c)), function(x) all.vars(x), how = "unlist")