Small extensions to Parsy
# Some small extensions to parsy
This offers a small extension to the behaviour of Parsy.
Input streams gain the ability to keep notes. These are additional information associated with a particular index in the stream. As the parsing process proceeds, new notes may be written. An attempt to fetch the most recent set of notes can be performed by:
recent_notes = yield parsy_extn.get_notes
and notes at the current point in parsing may be written by
As the parser backtracks, older sets of notes may be thrown away.
### Notes on the mutability of notes
Notes are kept in a dictionary. get_notes performs a shallow copy of this dictionary; however, references to embedded objects are identical.
This is intentional; the utility of notes is in allowing a parser to back-fill already-generated placeholder values with actual content.
However, care should be taken when writing new sets of notes that references to other mutable objects (such as lists) are not duplicated accidentally; this will result in the backtracking process losing the immutability of previous notes.
## Handling augmentation
### Manually augmenting the stream
An input str or list may be augmented before being passed to a parser. This should be done at the initial point of ingestion into the grammar:
result = grammar_entry_point.parse(parsy_extn.Noted.augment(original_input))
### Wrapping a single entry-point’s parse method
Alternatively, a grammar’s entry-point may be decorated to automatically wrap values as they are passed to the parse method:
grammar_entry_point = parsy_ext.keeps_notes(grammar_entry_point) result = grammar_entry_point.parse(original_input)
### Monkey-patching parsy.Parser
Finally, the parsy.Parser object itself may be monkey-patched to wrap any input on entry to parse or parse_partial:
parsy_extn.monkeypatch_parsy() result = grammar_entry_point.parse(original_input)
The motivation for this is to handle constructs like shell ‘heredocs’ with a single pass of a parser. They are otherwise notoriously difficult to handle; even with this facility, the resulting grammars are fiddly.
A trivial example is given in the test_parsy_extn module.
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