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Attribute calculations can be complicated. Knowing exactly when all the prerequisite calculations have been completed, such that one knows when one can compute any give given attribute is hard. Attribute grammars carefully divide the world into top-down and bottom-up calculations to facilitate organizing the calculations into passes. Compilers generally have many passes, each of which computes a few more attributes, or constructs a new representation of the data (which is typically, another attributed-tree of slightly different shape/design).

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There are a few more details required to deal with errors though. There's a separate wiki page on OOLAG - Object-Oriented Lazy Attribute Grammars, which discusses some of this also.

Above we pointed out one must stick with a pure functional programming style to make this all hang together.  This means we need a technique for dealing with the fact that things can go wrong in a compiler. The input source may have errors in it, and good diagnostic messages must be presented back to the author of that DFDL schema.

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