[Lilypond-auto] Issue 4085 in lilypond: Redo the whole pure/unpure folderol/caching

[lilypond]

Status: Accepted
Owner: ----
Labels: Type-Maintainability

New issue 4085 by address@hidden: Redo the whole pure/unpure folderol/caching
https://code.google.com/p/lilypond/issues/detail?id=4085

The whole pure/unpure calculation and its effects on cross-staff and  
whatever else is pretty shaky.

Here is a sketch of how it may be possible to redo a considerable part of  
the whole: start and end, the indexes into line-breaking for the "pure"  
calculations are no longer passed as arguments to callbacks.  Instead they  
are available on demand by calling a parameter-like function (see srfi-39  
for a description of parameter).  The actual parameter call would be  
additionally rigged in order to record whether the line ends have been  
queried and what value resulted.  The set of queried parameters as well as  
the resulting value will be cached in something more general than an  
unpure/pure container so that any query with the same settings will be  
answered without repeating the calculation.

In that manner, the effort for cross-staff-like redoings of calculations of  
usually-but-not-quite-line-break-independent properties would be reduced to  
the necessary level.  Also users would not need to be aware of the details  
of call chain purities and stuff: the calculations would be cached and  
redone based on the amount of settings that actually were involved.   
Also "pure" calculations would be cached as well: I assume that the amount  
of stored parameter/value pairings would be limited.

At the current point of time, interactions between properties tend to fall  
apart and need fixing at the rate they are added: that does not bode well  
for making LilyPond extensible in that area.  Tracking the dependencies  
dynamically would make it a lot safer to add new stuff without breaking  
existing use.

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