Module | Parslet |
In: |
lib/parslet.rb
lib/parslet/error_reporter/tree.rb lib/parslet/error_reporter/deepest.rb lib/parslet/source.rb lib/parslet/cause.rb |
A simple parser generator library. Typical usage would look like this:
require 'parslet' class MyParser < Parslet::Parser rule(:a) { str('a').repeat } root(:a) end pp MyParser.new.parse('aaaa') # => 'aaaa'@0 pp MyParser.new.parse('bbbb') # => Parslet::Atoms::ParseFailed: # Don't know what to do with bbbb at line 1 char 1.
The simple DSL allows you to define grammars in PEG-style. This kind of grammar construction does away with the ambiguities that usually comes with parsers; instead, it allows you to construct grammars that are easier to debug, since less magic is involved.
Parslet is typically used in stages:
The first stage is traditionally intermingled with the second stage; output from the second stage is usually called the ‘Abstract Syntax Tree’ or AST.
The stages are completely decoupled; You can change your grammar around and use the second stage to isolate the rest of your code from the changes you‘ve effected.
All parslet atoms are subclasses of {Parslet::Atoms::Base}. You might want to look at all of those: {Parslet::Atoms::Re}, {Parslet::Atoms::Str}, {Parslet::Atoms::Repetition}, {Parslet::Atoms::Sequence}, {Parslet::Atoms::Alternative}.
A parse that fails will raise {Parslet::ParseFailed}. This exception contains all the details of what went wrong, including a detailed error trace that can be printed out as an ascii tree. ({Parslet::Cause})
Returns an atom matching any character. It acts like the ’.’ (dot) character in regular expressions.
any.parse('a') # => 'a'
@return [Parslet::Atoms::Re] a parslet atom
Designates a piece of the parser as being dynamic. Dynamic parsers can either return a parser at runtime, which will be applied on the input, or return a result from a parse.
Dynamic parse pieces are never cached and can introduce performance abnormalitites - use sparingly where other constructs fail.
Example:
# Parses either 'a' or 'b', depending on the weather dynamic { rand() < 0.5 ? str('a') : str('b') }
A special kind of atom that allows embedding whole treetop expressions into parslet construction.
# the same as str('a') >> str('b').maybe exp(%Q("a" "b"?))
@param str [String] a treetop expression @return [Parslet::Atoms::Base] the corresponding parslet parser
Returns an atom matching a character class. All regular expressions can be used, as long as they match only a single character at a time.
match('[ab]') # will match either 'a' or 'b' match('[\n\s]') # will match newlines and spaces
There is also another (convenience) form of this method:
match['a-z'] # synonymous to match('[a-z]') match['\n'] # synonymous to match('[\n]')
@param str [String] character class to match (regexp syntax) @return [Parslet::Atoms::Re] a parslet atom
Introduces a new capture scope. This means that all old captures stay accessible, but new values stored will only be available during the block given and the old values will be restored after the block.
Example:
# :a will be available until the end of the block. Afterwards, # :a from the outer scope will be available again, if such a thing # exists. scope { str('a').capture(:a) }
Returns a placeholder for a tree transformation that will only match a sequence of elements. The symbol you specify will be the key for the matched sequence in the returned dictionary.
# This would match a body element that contains several declarations. { :body => sequence(:declarations) }
The above example would match :body => [‘a’, ‘b’], but not :body => ‘a‘.
see {Parslet::Transform}
Returns a placeholder for a tree transformation that will only match simple elements. This matches everything that sequence doesn‘t match.
# Matches a single header. { :header => simple(:header) }
see {Parslet::Transform}
Returns an atom matching the str given:
str('class') # will match 'class'
@param str [String] string to match verbatim @return [Parslet::Atoms::Str] a parslet atom