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Re: [PATCH] Re: Make peg.el a built-in library?

From: Ihor Radchenko
Subject: Re: [PATCH] Re: Make peg.el a built-in library?
Date: Wed, 16 Nov 2022 06:24:36 +0000

Eric Abrahamsen <eric@ericabrahamsen.net> writes:

> Okay, here's a first stab. I read the paper, and understood about half
> of it, which seemed like enough. It was interesting to see that the
> paper explicitly calls out the exact greedy-matching behavior I'd
> encountered.


> +  Emacs Lisp provide several tools for parsing and matching text, from


> +regular expressions (@pxref{Regular Expressions}) to full @acronym{LL}
> +grammar parsers (@pxref{Top,, Bovine parser development, bovine}).
> +@dfn{Parsing Expression Grammars} (@acronym{PEG}) are another approach
> +to text parsing that offer more structure and composibility than
> +regular expressions, but less complexity than context-free grammars.
> +
> +A @acronym{PEG} parser is defined as a list of named rules, each of
> +which match text patterns, and/or contain references to other rules.
> +Parsing is initiated with the function @code{peg-run} or the macro
> +@code{peg-parse}, and parses text after point in the current buffer,
> +using a given set of rules.
> +
> +The definition of each rule is referred to as a @dfn{parsing
> +expression} (@acronym{PEX}), and can consist of a literal string, a
> +regexp-like character range or set, a peg-specific construct
> +resembling an elisp function call, a reference to another rule, or a
> +combination of any of these.  A grammar is expressed as a set of rules
> +in which one rule is typically treated as a ``top-level'' or
> +``entry-point'' rule.  For instance:
> +
> +@example
> +@group
> +((number sign digit (* digit))
> + (sign   (or "+" "-" ""))
> + (digit  [0-9]))
> +@end group
> +@end example
> +
> +The above grammar could be used directly in a call to
> +@code{peg-parse}, in which the first rule is considered the
> +``entry-point'' rule:
> +
> +@example
> +(peg-parse
> +  ((number sign digit (* digit))
> +   (sign   (or "+" "-" ""))
> +   (digit  [0-9])))
> +@end example
> +
> +Or set as the value of a variable, and the variable used in a
> +combination of calls to @code{with-peg-rules} and @code{peg-run},
> +where the ``entry-point'' rule is given explicitly:
> +
> +@example
> +(defvar number-grammar
> +        '((number sign digit (* digit))
> +          (sign (or "+" "-" ""))
> +          (digit [0-9])))
> +
> +(with-peg-rules number-grammar
> +  (peg-run (peg number)))
> +@end example
> +
> +By default, calls to @code{peg-run} or @code{peg-parse} produce no
> +output: parsing simply moves point.  In order to return or otherwise
> +act upon parsed strings, rules can include @dfn{actions}, see
> +@xref{Parsing Actions} for more information.
> +
> +Individual rules can also be defined using a more @code{defun}-like
> +syntax, using the macro @code{define-peg-rule}:
> +
> +@example
> +(define-peg-rule digit ()
> +  [0-9])
> +@end example
> +
> +This allows the rule to be referred to by name within calls to
> +@code{peg-run} or @code{peg-parse} elsewhere, and also allows the use
> +of function arguments in the rule body.
> +
> +@node PEX Definitions
> +@section PEX Definitions
> +
> +Parsing expressions can be defined using the following syntax:
> +
> +@table @code
> +@item (and E1 E2 ...)
> +A sequence of PEXs that must all be matched.  The @code{and} form is
> +optional and implicit.
> +
> +@item (or E1 E2 ...)
> +Prioritized choices, meaning that, as in Elisp, the choices are tried
> +in order, and the first successful match is used.

It is worth highlighting that it is different from CFGs.

> +@item (* E)
> +Zero or more of an expression, as the regexp ``*''.
> +
> +@item (+ E)
> +One or more of an expression, as the regexp ``+''.

It is worth highlighting the greedy part here and referring to &A and

> +@item SYMBOL
> +A symbol representing a previously-define PEG rule.


> +By default the process of parsing simply moves point in the current
> +buffer, ultimately returning @code{t} if the parsing succeeds, and
> +@code{nil} if it doesn't.  It's also possible to define ``actions''
> +that can run arbitrary Elisp at certain points during parsing.  These
> +actions can affect something called the @dfn{parsing stack}: a list of
> +values built up during the course of parsing.  If the stack is
> +non-@code{nil} at the end of parsing, it is returned as the final
> +value of the parsing process.

Actions are only run when the expression matches; with point moved after
the match, right? What about &A and !A?

> +There must be values on the stack before they can be popped and
> +returned.

What if there is just one value in the stack while the action required two?

> +@item (list E)
> +Match E, collect all values produced by E (and its sub-expressions)
> +into a list, and push that list to the stack.
> +@end table

This one is not very clear. Does it imply that E is recursively wrapped
into substring?

> +It is up to the grammar author to keep track of which rules and
> +sub-rules push values to the stack, and the state of the stack at any
> +given point in the parsing.  If an action pops values from an empty
> +stack, the symbols will be bound to @code{nil}.

The part about popping out of empty stack looks out of scope. Maybe move
it to earlier discussion of variable bindings in actions?

Ihor Radchenko // yantar92,
Org mode contributor,
Learn more about Org mode at <https://orgmode.org/>.
Support Org development at <https://liberapay.com/org-mode>,
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