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# luaepnf -- Extended PEG Notation Format # ## Introduction ## The [LPeg][1] library is a powerful tool to parse text and extract parts of it using captures. It even provides grammars, which can be used to parse non-regular languages, but the complexer the language gets, the more difficult error handling and keeping track of captured information becomes. `luaepnf` enhances usage of LPeg grammars by building an abstract syntax tree (AST) for the input and providing tools for error reporting, as well as offering syntax sugar and shortcuts for accessing LPeg's features. ## Basic Usage ## The `luaepnf` module provides syntax sugar for defining [LPeg][1] grammars with error handling and AST building: $ cat > test.lua local epnf = require( "epnf" ) local g = epnf.define( function(_ENV) local _ = WS^0 local number = C( P"-"^-1 * R"09"^1 ) local err = E"variable, number, or '(' expected" START "expr" expr = _ * (V"aexpr"+err) * EOF"operator (+-*/) expected" aexpr = V"mexpr" * (S"+-" * _ * (V"mexpr"+err))^0 mexpr = V"term" * (S"*/" * _ * (V"term"+err))^0 term = (ID + number + (P"(" * _ * (V"aexpr"+err) * (P")"+E") expected"))) * _ end ) local s = "12 + 9" --local s = "12 + 9 * (c+11)" --local s = "1 1" --local s = "*" print( '"' .. s .. '"' ) local ast = epnf.parsestring( g, s ) epnf.dumpast( ast ) ^D The output is the abstract syntax tree (or an error message): $ lua test.lua "12 + 9" { id = expr, pos = 1 1 = { id = aexpr, pos = 1 1 = { id = mexpr, pos = 1 1 = { id = term, pos = 1 1 = 12 } } 2 = { id = mexpr, pos = 6 1 = { id = term, pos = 6 1 = 9 } } } } [1]: http://www.inf.puc-rio.br/~roberto/lpeg/lpeg.html (LPeg) ## Reference ## The `luaepnf` module provides the following public functions: * `epnf.define( func [, g [, e]] ) -> g` This function provides a suitable environment for the given function `func()` which must take a single parameter called `_ENV` for Lua 5.2 and up, and calls it to execute the rule definitions contained in `func()`. It returns an LPeg grammar table containing the patterns for the rule definitions. If a table is given as a second argument, the patterns are stored there. This can be used to change/complete an existing grammar. The parameter `e` can be used if `func` expects a function environment different from `_G`. * `epnf.parse( g, name, input [, ...] ) -> ast, name, input` This function calls LPeg's `match()` function with the given grammar `g` and `input` string. The input's `name` is passed via `Carg( 1 )`, and is used in error messages. All remaining arguments are free for use and can be accessed in the grammar using `Carg( 2 )` and up. Return value is a tree structure representing the AST of the input under the given grammar. This function can return nil or raise an error depending on the error handling strategy in the grammar if the matching fails. The last two return values are only useful if you intend to use the `epnf.raise()` function. * `epnf.parsefile( g, filename [, ...] ) -> ast, name, input` Calls `epnf.parse()` with the contents of the given file. * `epnf.parsestring( g, string [, ...] ) -> ast, name, input` Calls `epnf.parse()` with a name derived from the input string. * `epnf.dumpast( ast )` This function prints a human readable representation of the `ast` to `stderr`. It is useful during development or debugging. * `epnf.raise( name, msg, source, position )` This function raises an error similar to the LPeg pattern `E( msg )` (see below) with input name, line number and a visual marker for `position`. It can be used during validation of the AST, which has suitable positions in the `pos` field of its nodes. `name` and `source` are typically kept from the `epnf.parse*()` function calls. ### Custom Environment for Grammar Definition ### The `epnf.define()` function calls its argument with a custom environment, which contains all LPeg patterns and the extra patterns and functions listed below. The environment also has a `__newindex` metamethod which creates rules on each global assignment, that later can be referenced using LPeg's `V( name )` operator. The pattern on the right handside of an assignment is enhanced to construct an AST node (a table) when matched. Each AST node contains an `id` field with the name of the matching rule, a `pos` field with the byte position of the match in the input string, and all captures and AST nodes for matching rule references in the pattern in the array part of the table. * `START( name )` This function sets the name of the starting rule of the resulting LPeg grammar. * `SUPPRESS( ... )` Prevents the rules named as arguments from producing an AST node. * `E( [msg] )` Creates an LPeg pattern that, when matched by LPeg, raises a parse error using the custom error message with input name, line number, and error location. If `msg` is absent or nil, a generic parse error is raised. * `EOF( [msg] )` Raises a parse error using the optional error message if there are any unprocessed input characters left. Never use this pattern in recursive rules! * `ID` An LPeg pattern that matches and captures an identifier as used in programming languages, i.e. letters and `_` followed by letters, digits, or `_`. * `W( word )` Creates an LPeg pattern similar to `P( word )`, but makes sure, that word is not followed by letters, numbers, or `_` (so it matches complete words only). Does not capture anything. * `WS` Matches a single whitespace character. Captures nothing. Every predefined pattern or function in the custom environment starts with a capital letter, so using lower-case rule names is advisable to avoid naming conflicts. ## Download ## The source code (with documentation and test scripts) is available on [github][2]. [2]: https://github.com/siffiejoe/lua-luaepnf/ ## Installation ## There are two ways to install this module, either using luarocks (if this module already ended up in the [main luarocks repository][3]) or manually. Using luarocks, simply type: luarocks install luaepnf To install the module manually just drop `epnf.lua` somewhere into your Lua `package.path`. [3]: http://luarocks.org/repositories/rocks/ (Main Repository) ## Contact ## Philipp Janda, siffiejoe(a)gmx.net Comments and feedback are always welcome. ## License ## luaepnf is *copyrighted free software* distributed under the MIT license (the same license as Lua 5.1). The full license text follows: luaepnf (c) 2013-2015 Philipp Janda Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions: The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software. THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHOR OR COPYRIGHT HOLDER BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.