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Diffstat (limited to 'doc/expert/pre_incl')
| -rw-r--r-- | doc/expert/pre_incl | 23 |
1 files changed, 23 insertions, 0 deletions
diff --git a/doc/expert/pre_incl b/doc/expert/pre_incl index c148453..8a73a49 100644 --- a/doc/expert/pre_incl +++ b/doc/expert/pre_incl @@ -17,4 +17,27 @@ included file may contain arbitrary Lout text, including other directory, then in a sequence of standard places which are not necessarily the same places that databases are searched for. @@SysInclude searches the standard places only. +@PP +From Version 3.27, a special behaviour has been instituted when an +attempt is made to @@Include or @@SysInclude the same file twice. +If a second or subsequent attempt occurs after the end of definitions, +@@Use clauses, and @@Database clauses (i.e. if it occurs within the +content of the document), it will go ahead, thus allowing the +repeated inclusion of files containing objects -- not necessarily +recommended, but certainly one way of repeating information. But +if a second or subsequent attempt occurs within the region of +definitions, @@Use clauses, and @@Database clauses, then that attempt +will be silently ignored. +@PP +This behaviour is useful for situations where two packages +depend on a third, caled, say, {@Code C }. We can then place +@ID @Code "@SysInclude { C }" +at the start of both packages. If neither package is included, then +{@Code C} won't be either. But if one or both is included, then +{@Code C} will be included just once at the start of the first. +Any pattern of acyclic dependencies between packages can be +expressed with this mechanism, just by including every package +that a given package depends on at the start of the file containing +that package. Cyclic dependencies are beyond Lout's one-pass +comprehension anyway. @End @Section |