Package files are shell scripts which set variables and call well-defined
functions to describe how to build the package.

## Modules

`chocpkg` is divided into modules which provide specific functionality.
Modules are installed by calling certain dedicated functions.
Usually, your package should install a check module, a fetch module
and a build module.

### Check modules

Check modules are modules which determine whether the package is
installed on the system (may have been installed by `chocpkg` itself
or the system package manager).
If a check module is not installed, the default is to always
assume the package is not installed:

#### check_pkgconfig

`check_pkgconfig` invokes `pkg-config` to determine if the package
is installed. The name of the `pkg-config` .pc file must be provided
as an argument to `check_pkgconfig`; for example:

```shell
check_pkgconfig SDL2
```

#### check_tool

`check_tool` checks if a particular tool is installed in `$PATH`
to determine if the package is installed. The name of the tool must
be provided as an argument to `check_tool`; for example:

```shell
check_tool gnome-terminal
```

### Fetch modules

Fetch modules specify how to retrieve the package from the Internet:

#### fetch_download

`fetch_download` downloads the package from a URL specified as an argument.
The file to be downloaded is assumed to be a well-formed tar.gz file with
all its contents in a directory that matches the package name; if this is
not the case, the variable `$IS_TAR_BOMB` should be set to `true`.

Example use:

```shell
fetch_download http://example.com/example-pkg.tar.gz
IS_TAR_BOMB=true
```

#### fetch_git

`fetch_git` downloads the package from a Git repository at the URL
given as an argument. The branch `master` will be checked out by
default; this can be overriden by providing the branch name as a second
argument to `fetch_git`.

Example use:

```shell
fetch_git http://example.com/example.git my-neato-branch
```

### Build modules

Build modules specify how to build the package after it is fetched:

#### build_autotools

`build_autotools` builds the package assuming that it is laid out as a
standard autotools package (ie. `./configure; make; make install`).
Extra arguments passed to the function will be passed through to
`configure`.

Example use:

```shell
build_autotools --disable-broken-feature
```

### Variants

Variants allow building of different versions of the same package. An
example is "latest stable release" vs. "latest version from source
repository". The default variant is called `stable`. A variant can be
specified on the command line with a suffix, for example, to build the
variant `latest` of package `neato-lib`:

```shell
chocpkg install neato-lib/latest
```

In package files the `variant` function is used for conditional code
that is only executed for a particular variant. Usually this is used
to select a fetch module. For example:

```shell
variant stable fetch_download http://example.com/neato-lib-0.0.1.tar.gz
variant latest fetch_git git://example.com/neato-lib.git
variant frob-branch fetch_git git://example.com/neato-lib.git frob-branch
```

By convention, the `stable` variant is "the most recent stable release
of the package" while the `latest` variant is "the latest version in
the source control repository".

### Other modules and functions

#### dependencies

Arguments provided to the `dependencies` function are the names of other
packages to install before trying to build this one.

Example use:

```shell
dependencies other-package neato-lib
```

#### package_group

`package_group` specifies that this is not really a package that
should be built; rather, it just specifies a number of other packages
to build.

Example use:

```shell
# File contains no other lines
package_group neato-lib dumbo-lib
```

## Advanced builds

Complicated packages can require custom build steps. The following functions
can therefore be overridden in the package file in exceptional circumstances.
These are essentially the functions implemented by the modules described
above.

* `do_fetch`: the function which is invoked to fetch the package from the
  Internet. The fetched package is placed into `$PACKAGE_BUILD_DIR` for
  build.

* `prebuild_setup`: define a function with this name to execute special
  setup commands in the root of a package just before it is built.

* `do_build`: the function which is invoked to build the package.

* `do_install`: the function which is invoked to install the package after
  it has been built. This function should install built files from
  `$PACKAGE_BUILD_DIR` into `$PACKAGE_INSTALL_DIR`.