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Some small extensions to parsy

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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

yield put_notes(new_notes)

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:

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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