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The SUB_DEPENDS file is used to make a spell depend on another spell with certain features enabled.
Sub-dependencies can be used in DEPENDS, CONFIGURE or SUB_DEPENDS.

Example: spell "foo" needs spell "bar" with support for "qux".

Syntax in DEPENDS file

In the "foo" spell's DEPENDS file, you'd just change the exsisting depends/optional_depends statements like:

depends -sub qux bar

or for more than one sub-depends target:

depends -sub "qux mux lux" bar

There is also an alternative syntax available:

depends bar && sub_depends bar qux
depends bar && sub_depends bar qux && sub_depends bar mux && sub_depends bar lux

This one is more powerful and is meant to be used to resolve non-trivial cases (which the first syntax can't handle), like if one provider needs a sub-depends and the others don't. Note that sub_depends only takes one sub-depend target, so you need to use more of those calls if you need more special handlers.

Note: optional depends need an additional is_depends_enabled check, so only the enabled ones trigger the sub-depends routines:

optional_depends -sub "apple" pie
optional_depends pie && is_depends_enabled $SPELL pie && sub_depends pie apple

Sub-depending on a provider:

depends -sub FOOBAR X11-LIBS
depends X11-LIBS && sub_depends $(get_spell_provider $SPELL X11-LIBS) FOOBAR

Example with providers that have a different implementation (different matching sub-depends) for the given sub-depends "goo". Some of the providers are even altruistic – they provide what we need, even thought nobody requested it from them, in this case all the ones not specified (*)):

depends X11-LIBS &&
case $(get_spell_provider $SPELL X11-LIBS) in
     xorg) sub_depends xorg fooz ;;
  xfree86) sub_depends xfree86 barz ;;
        *) ;; # do nothing

Note: never call sub_depends without first explicitly depending on the providing spell. That is undefined behavior – it could damage the system or spawn pink killer rabbits (no, you don't want that). It is OK to put just a sub_depends call in CONFIGURE if you already depend on that spell in DEPENDS.

It is also acceptable to mix the two syntax, although I can't imagine why anyone would want to do it.

Syntax in SUB_DEPENDS file

In the "bar" spell's SUB_DEPENDS file you'd say something like:

qux) whatever it is you do to make bar build with qux support ;;
mux) whatever it is you do to make bar build with mux support ;;
lux) whatever it is you do to make bar build with lux support ;;
*) echo unknown sub-depends!!! ; return 1 ;;

The SUB_DEPENDS file can do most of the normal spell activities: they may use the persistent_add/persistent_remove functions, config_query API, they may call depends or optional_depends, and can even request a sub-depends on another spell.

Example usage in spells

  FONT)    depends sdl_ttf;;
  IMAGE)   depends sdl_image;;
  MIXER)   depends sdl_mixer;;
  SMPEG)   depends smpeg;;
  NUMERIC) depends numeric NUMERIC;;
  *)       echo "bogus sub_depends: pygame $THIS_SUB_DEPENDS"; return 1;;
PYTHON) echo "python bindings requested, forcing them."  &&
        DBUS_PYREX="--enable-python"     &&
        depends  glib2  '--enable-glib'  &&
        depends  pyrex
CSHARP) echo "mono bindings requested, forcing them."  &&
        depends  mono  '--enable-mono'
     *) echo "unknown sub-depends!"; return 1;;
  fxscintilla) depends fxscintilla "--with-fxscintilla-include=${INSTALL_ROOT}/usr/include/fxscintilla \
                                    --with-fxscintilla-lib=${INSTALL_ROOT}/usr/lib" ;;
            *) echo unknown sub-depends!!! ; return 1 ;;
GMP_CXX) echo "C++ support requested, forcing it."  &&
         depends  g++  '--enable-cxx'
      *) echo "unknown sub-depends!"; return 1;;

Note: don't forget to add a PRE_SUB_DEPENDS file, these two always come in pairs.