elixir (version 0.1.0)

expr_list: Make a list of expressions

Description

Constructs a list of expressions, with support for elixir's special expression syntax (expression literals with {} or ~{}, and alternatives with ?).

Usage

expr_list(..., env = parent.frame())
# S3 method for expr_list
[(xl, i)
# S3 method for expr_list
[(xl, i) <- value

Value

A list of expressions, of class expr_list.

Arguments

...

Expressions to include in the list. If the arguments are named, these will be passed on to the returned list.

env

Environment for injections in ... (see expression).

xl

An expr_list.

i

Index for subsetting the expr_list; an integer, numeric, logical, or character vector (for named expr_lists) interpreted in the usual R way.

value

Replacement; an expr_list, an expression, or a list of expressions.

Details

Be aware that using the [[ indexing operator on an object of class expr_list discards information about whether that element of the list is marked as anchored. In other words, if xl <- expr_list({.A}, ~{.A}), then xl[[1]] and xl[[2]] are both equal to the "bare" symbol .A, so the information that the second element of the list is anchored has been lost. Consequently, in e.g. expr_match(expr, xl[[2]]), it will be as though the tilde isn't there, and xl[[2]] will not just match with the top level of expr as was probably intended. Use the [ operator instead, which retains anchoring information; expr_match(expr, xl[2]) will work as expected.

Note that when you replace part of an expr_list with another expr_list, the anchoring information from the "replacement" expr_list is copied over, while replacing part of an expr_list with an expression or a "plain" list of expressions retains the existing anchoring information.

Examples

expr_list(
   ~{ 1 + 1 = 2 } ? ~{ 2 + 2 = 4 },
   ~{ y = a * x + b },
   { .A }
)

# There is support for rlang's injection operators.
var = as.name("myvar")
expr_list({ 1 }, { !!var }, { (!!var)^2 })