---
title: "Migration guide"
author: Konrad Rudolph
date: "`r Sys.Date()`"
output:
    rmarkdown::html_vignette:
        toc: true
    md_document:
        variant: gfm
vignette: >
    %\VignetteEngine{knitr::rmarkdown}
    %\VignetteIndexEntry{Migration guide}
    %\VignetteEncoding{UTF-8}
---
## ‘modules’ “v1.0” == ‘box’

‘box’ is the spiritual successor of the [‘modules’
package](https://github.com/klmr/modules), 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

```{r eval = FALSE}
library(modules)

# …

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

With ‘box’, modules and packages are imported via
[`box::use`](https://klmr.me/box/reference/use.html):

```{r eval = FALSE}
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

```{r eval = FALSE}
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:

```{r eval = FALSE}
box::use(mod_alias = prefix/modname)
```

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

```{r eval = FALSE}
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:

```{r eval = FALSE}
# ‘modules’:
import('prefix/mod', attach = c('name1', 'name2'))

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

Wildcard attach lists are also supported:

```{r eval = FALSE}
# ‘modules’:
import('prefix/mod', attach = TRUE)

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

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

```{r eval = FALSE}
# ‘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’:

```{r eval = FALSE}
# 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](https://klmr.me/box/reference/mod-hooks.html), 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`](https://klmr.me/box/reference/file.html). 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`](https://klmr.me/box/reference/name.html). 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`:

```{r eval = FALSE}
# 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:

```{r eval = FALSE}
#' @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:

```{r eval = FALSE}
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’:

```{r eval = FALSE}
box::use(r/core[...])
```

## Loading parents of nested modules

Consider the following file hierarchy defining a nested module:

<pre><code><b>a</b>
├─ <i>__init__.r</i>
╰─ <b>b</b>
   ├─ <i>__init__.r</i>
   ╰─ <i><b>c</b>.r</i></code></pre>

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`](https://klmr.me/box/reference/help.html).

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`:

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

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

- `odd.r`:

  ``` 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`:

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

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