R: Splice lists

splice {rlang} | R Documentation

## Splice lists

### Description

- `splice` marks an object to be spliced. It is equivalent to using `!!!` in a function with tidy dots semantics.

- `dots_splice()` is like `dots_list()` but automatically splices list inputs.

### Usage

    splice(x)

    is_spliced(x)

    is_spliced_bare(x)

    dots_splice(..., .ignore_empty = c("trailing", "none", "all"),
      .preserve_empty = FALSE, .homonyms = c("keep", "first", "last",
      "error"), .check_assign = FALSE)

### Arguments

`x` | A list to splice.
`...` | Arguments to collect with `!!!` support.
`.ignore_empty` | Whether to ignore empty arguments. Can be one of `"trailing"`, `"none"`, `"all"`. If `"trailing"`, only the last argument is ignored if it is empty.
`.preserve_empty` | Whether to preserve the empty arguments that were not ignored. If `TRUE`, empty arguments are stored with `missing_arg()` values. If `FALSE` (the default) an error is thrown when an empty argument is detected.
`.homonyms` | How to treat arguments with the same name. The default, `"keep"`, preserves these arguments. Set `.homonyms` to `"first"` to only keep the first occurrences, to `"last"` to keep the last occurrences, and to `"error"` to raise an informative error and indicate what arguments have duplicated names.
`.check_assign` | Whether to check for `<-` calls passed in dots. When `TRUE` and a `<-` call is detected, a warning is issued to advise users to use `=` if they meant to match a function parameter, or wrap the `<-` call in braces otherwise. This ensures assignments are explicit.

### Standard splicing versus quoting splicing

The `!!!` operator works differently in standard functions taking dots with `dots_list()` than in quoting functions taking dots with `enexprs()` or `enquos()`.

- In quoting functions `!!!` disaggregates its argument (let's call it `x`) into as many objects as there are elements in `x`. E.g. `quo(foo(!!! c(1, 2)))` is completely equivalent to `quo(foo(1, 2))`. The creation of those separate objects has an overhead but is typically not important when manipulating calls because function calls typically take a small number of arguments.

- In standard functions, disaggregating the spliced collection would have a negative performance impact in cases where `dots_list()` is used to build up data structures from user inputs. To avoid this spliced inputs are marked with `splice()` and the final list is built with (the equivalent of) `flatten_if(dots, is_spliced)`.

Most of the time you should not care about the difference. However if you use a standard function taking tidy dots within a quoting function, the `!!!` operator will disaggregate its argument because the behaviour of the quasiquoting function has priority. You might then observe some performance cost in edge cases. Here is one example where this would happen:

    purrr::rerun(10, dplyr::bind_rows(!!! x))

`purrr::rerun()` is a quoting function and `dplyr::bind_rows()` is a standard function. Because `bind_rows()` is called inside `rerun()`, the list `x` will be disaggregated into a pairlist of arguments. To avoid this you can use `splice()` instead:

    purrr::rerun(10, dplyr::bind_rows(splice(x)))

### Life cycle

- `dots_splice()` is in the questioning stage. It is part of our experiments with dots semantics. Compared to `dots_list()`, `dots_splice()` automatically splices lists. We now lean towards adopting a single type of dots semantics (those of `dots_list()`) where splicing is explicit.

- `splice()` is in the questioning stage. It is not clear whether it is really needed as there are other ways to avoid the performance issue discussed above.

[Package rlang version 0.4.1 Index]