Author: Konrad Rudolph
Date: 2024-02-07

# Migration guide

#### Konrad Rudolph

#### 2024-02-07

## ‘modules’ “v1.0” == ‘box’

‘box’ is the spiritual successor of the ‘modules’ package, which will forever remain at version 0.9.x. However, the API of ‘box’ intentionally breaks backwards compatibility, and module code written for use with ‘modules’ will no longer work with ‘box’.

The following guide is intended to ease migration to ‘box’. Migration is strongly recommended, since the authors believe that ‘modules’ had conceptual shortcomings that have been fixed in ‘box’, and to keep the ecosystem consolidated. For the time being, ‘modules’ will receive support (only) in the form of critical bug fixes.

## Importing modules and packages

### General syntax

With the ‘modules’ package, modules and packages were imported via

    library(modules)

    # …

    modname = import('prefix/modname')
    pkgname = import_package('pkgname')

With ‘box’, modules and packages are imported via `box::use`:

    box::use(prefix/modname)
    box::use(pkgname)

Notably, module and package names in a use declaration in ‘box’ are unquoted, unevaluated expressions; and the ‘box’ package is never loaded via `library`. In fact, `library(box)` raises an error.

Furthermore, with ‘box’, module names must be fully qualified. The equivalent of the ‘modules’ code `mod = import('modname')` no longer exists. To import a local module without namespace prefix, instead use

    box::use(./modname)

`box::use` has no return value. Instead, it automatically creates an alias with the module/package name in the calling scope if no names are attached (see below). To override the alias name, specify it as as a named argument:

    box::use(mod_alias = prefix/modname)

Unlike with `modules::import`, `box::use` allows (and encourages) multiple use declarations at once:

    box::use(
        prefix/mod1,
        mod = prefix/mod2,
        pkg
    )

### Attaching names

In ‘modules’, the `attach` and `attach_operators` parameters controlled if and which names were attached. In ‘box’, attachment is controlled via attach list declarations:

    # ‘modules’:
    import('prefix/mod', attach = c('name1', 'name2'))

    # ‘box’:
    box::use(prefix/mod[name1, name2])

Wildcard attach lists are also supported:

    # ‘modules’:
    import('prefix/mod', attach = TRUE)

    # ‘box’:
    box::use(prefix/mod[...])

To introduce a module/package alias when attaching names, specify an alias name:

    # ‘modules’:
    mod = import('prefix/mod', attach = c('name1', 'name2'))

    # ‘box’:
    box::use(mod = prefix/mod[name1, name2])

‘box’ also allows declaring aliases for attached names; this feature did not exist in ‘modules’:

    # Declare alias for one name, attach other name unchanged:
    box::use(prefix/mod[name_alias = name1, name2])

    # Declare alias for two names, attach all other exported names unchanged:
    box::use(prefix/mod[name_alias1 = name1, name_alias2 = name2, ...])

The `attach_operators` option from ‘modules’ has been dropped. If users require operators, they need to explicitly attach them when using ‘box’.

### Loading documentation

The `doc` option from `modules::import` has been dropped without replacement. ‘box’ loads documentation lazily only when it is requested via `box::help` (see below).

## Executing code during module loading

The ‘modules’ package treated modules as regular R source code files: upon importing them, the entire code inside a module file was executed. ‘box’ conceptually no longer does this. It regards module source code as declarative: the module source code defines a number of names to be exported. However, code with side-effects on the module file level is no longer guaranteed to execute.

Instead, ‘box’ introduces module event hooks, in particular `.on_load`, which is a function that gets executed whenever a module is first loaded inside an R session.

## Changed options

In ‘modules’, the module search path was set via `options('import.path')`. In ‘box’, use `options('box.path')` instead.

The ‘modules’ options `options('import.attach')` and `options('warn.conflicts')` no longer exist. In particular, ‘box’ no longer warns of name conflicts when attaching names. Instead, it encourages consciously choosing which names to attach.

## Changed function names

The ‘modules’ function `module_file` is now called `box::file`. Its semantics have also changed: it no longer cares whether files relative to the module exist or not; it merely constructs appropriate path strings.

The ‘module’ function `module_name` is now called `box::name`. It no longer has any arguments.

## Exporting names from modules

With the ‘modules’ packages, module source files exported all non-hidden names; that is, all names that didn’t start with a dot (`.`).

‘box’ makes exporting explicit. By default, no names are exported from a module, unless they are marked with the directive comment `#' @export`:

    # This function is not exported:
    f1 = function () {}

    # This nested module is not exported
    box::use(./nested1)

    # This function is exported:
    #' @export
    f2 = function () {}

    # This nested module is exported
    #' @export
    box::use(./nested2)

`@export` directives that decorate `box::use` declarations apply to all names declared in it:

    #' @export
    box::use(
        pkg_alias = pkg,
        prefix/mod,
        prefix/mod2[a, b, c]
    )

The above code will export the names `pkg_alias`, `mod`, `a`, `b` and `c`. This replaces the function `export_submodule` from ‘modules’, which no longer exists.

There’s one exception to this: if a module contains no declared export, ‘box’ assumes that it is a plain R script, and treats it as a legacy module. This causes ‘box’ to revert to the export behaviour of ‘modules’. To suppress this behaviour (that is, to create a module which explicitly doesn’t export any names), module authors can add the following declaration to their module source code, which explicitly declares that the source file is to be treated as a module with no exports:

    box::export()

## Accessing default packages

In ‘modules’, module source code “sees” the package search path at the time it is loaded. That is, modules have implicit access to all names in packages that were attached when the module was loaded. In particular, this means that legacy modules could access names in the default packages (in non-interactive sessions, that’s typically ‘datasets’, ‘utils’, ‘grDevices’, ‘graphics’ and ‘stats’; in interactive sessions, the package ‘methods’ is added).

‘box’ modules no longer attach any packages by default, except ‘base’. If a module needs to use functions from those other packages, it needs to declare them explicitly (e.g. via `box::use(stats)`). If users want to import all default R packages, they can import the module `r/core` for this purpose. The following declaration approximates the behaviour of ‘modules’:

    box::use(r/core[...])

## Loading parents of nested modules

Consider the following file hierarchy defining a nested module:

    a
    ├─ __init__.r
    ╰─ b
       ├─ __init__.r
       ╰─ c.r

In ‘modules’, the declaration `import(a/b/c)` would import the module `c`, but this would first execute the code of the modules `a` and `a/b`. In other words it would source the files `a/__init__.r`, `a/b/__init__.r`, and `a/b/c.r`, in this order.

‘box’ no longer loads the full module hierarchy: `box::use(a/b/c)` loads only the module defined by `a/b/c.r`. Likewise, `box::use(a/b)` does not automatically load `a/b/c.r` (but the same behaviour was already present in ‘modules’).

## Displaying documentation

‘modules’ overrode the `help` function and the `?` operator to allow displaying module documentation. Since ‘box’ is no longer attached, these functions no longer display module documentation. Instead, the documentation of anything imported via ‘box’ (both modules and packages!) can be queried via `box::help`.

Unlike ‘modules’, ‘box’ also supports displaying the documentation of nested module names, e.g. `box::help(a$b$c)`.

## Cyclic imports

Cyclic/circular imports are supported by both ‘modules’ and ‘box’. However, the level of support differs. The details are complicated, and it is generally recommended to avoid cyclic modules. However, there are some situations where circularity in the dependencies makes sense, and ‘box’ strives to make this work, where technically possible.

For instance, consider the following (nonsensical) working, circular definition of the functions `even` and `odd`, which determine whether a non-negative integer is even or odd:

- `even.r`:

      box::use(./odd[...])

      #' @export
      even = function (n) {
          n == 0L || ! odd(n)
      }

- `odd.r`:

      box::use(./even[...])

      #' @export
      odd = function (n) {
          n != 0L && even(n - 1L)
      }

These modules compute `even` in terms of `odd`, and vice-versa. Yet ‘box’ has no trouble importing and using both these modules. However, this no longer works once we attempt to export imported submodules themselves. That is, the following version of `odd.r` would cause an error:

- `odd.r`:

      #' @export
      box::use(./even[...])

      #' @export
      odd = function (n) {
          n != 0L && even(n - 1L)
      }