Some small extensions to parsy

# Some small extensions to parsy

This offers a small extension to the behaviour of Parsy.

## Note-keeping

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:

parsy_extn.monkeypatch_parsy()
result = grammar_entry_point.parse(original_input)


## Motivation

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.

## Project details

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