PRE_SUB_DEPENDS

Part of the sub-depends specification is that spells must have a PRE_SUB_DEPENDS file, which tells Sorcery whether or not the sub-dependee is providing the given sub-depends or not. If the spell isn't providing the sub-dependency, or the file doesn't exist, sorcery must cast the sub-dependee. Unnecessary casts can be avoided with a proper PRE_SUB_DEPENDS file, this is regardless of prior dependency or sub-dependency history.

Some rules are applied to this file:

PRE_SUB_DEPENDS is sourced in the same manner as SUB_DEPENDS – that is, once for each sub-dependency, with the variable $THIS_SUB_DEPENDS set to the sub-dependency currently in consideration (not sure about $PROCESSED_SUB_DEPENDS, but it would stand to reason).
PRE_SUB_DEPENDS should return 0 if the sub-dependency is already in effect, otherwise it should return 1.
PRE_SUB_DEPENDS is sourced from the tablet, so make sure you add a repair file for it.

Example usage in spells

"pygame" spell:

case $THIS_SUB_DEPENDS in
  FONT)    is_depends_enabled $SPELL sdl_ttf   && return 0;;
  IMAGE)   is_depends_enabled $SPELL sdl_image && return 0;;
  MIXER)   is_depends_enabled $SPELL sdl_mixer && return 0;;
  SMPEG)   is_depends_enabled $SPELL smpeg     && return 0;;
  NUMERIC) is_depends_enabled $SPELL numeric   && return 0;;
esac
return 1

"fxruby" spell:

case $THIS_SUB_DEPENDS in
  fxscintilla) if is_depends_enabled $SPELL fxscintilla; then return 0; fi;;
            *) return 1 ;;
esac

checking for an optional provider ("SSL" in this case):

case $THIS_SUB_DEPENDS in
    SSL) if is_depends_enabled $SPELL $(get_spell_provider $SPELL SSL) || is_depends_enabled $SPELL gnutls; then
         return 0
         fi
         ;;
      *) echo "unknown sub_depends!" ;;
esac
return 1

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PRE_SUB_DEPENDS

Example usage in spells