= Oedipus Lex - This is not your father's lexer

home :: http://github.com/seattlerb/oedipus_lex
rdoc :: http://docs.seattlerb.org/oedipus_lex

== DESCRIPTION

Oedipus Lex is a lexer generator in the same family as Rexical and
Rex. Oedipus Lex is my independent lexer fork of Rexical. Rexical was
in turn a fork of Rex. We've been unable to contact the author of rex
in order to take it over, fix it up, extend it, and relicense it to
MIT. So, Oedipus was written clean-room in order to bypass licensing
constraints (and because bootstrapping is fun).

Oedipus brings a lot of extras to the table and at this point is only
historically related to rexical. The syntax has changed enough that
any rexical lexer will have to be tweaked to work inside of oedipus.
At the very least, you need to add slashes to all your regexps.

Oedipus, like rexical, is based primarily on generating code much like
you would a hand-written lexer. It is _not_ a table or hash driven
lexer. It uses StrScanner within a multi-level case statement. As such,
Oedipus matches on the _first_ match, not the longest (like lex and
its ilk).

This documentation is not meant to bypass any prerequisite knowledge
on lexing or parsing. If you'd like to study the subject in further
detail, please try [TIN321] or the [LLVM Tutorial] or some other good
resource for CS learning. Books... books are good. I like books.

== Syntax:

  lexer           = (misc_line)*
                    /class/ class_id
                    (option_section)?
                    (inner_section)?
                    (start_section)?
                    (macro_section)?
                    (rule_section)?
                    /end/
                    (misc_line)*

  misc_line       = /.*/

  class_id        = /\w+.*/

  option_section  = /options?/ NL (option)*
  option          = /stub/i
                  | /debug/i
                  | /do_parse/i
                  | /lineno/i
                  | /column/i

  inner_section   = /inner/ NL (misc_line)*

  start_section   = /start/ NL (misc_line)*

  macro_section   = /macros?/ NL (macro)*
  macro           = name regexp
  name            = /\w+/
  regexp          = /(\/(?:\\.|[^\/])+\/[io]?)/

  rule_section    = /rules?/ NL (rule|group)*
  rule            = (state)? regexp (action)?
  group           = /:/ regexp NL (rule)+
  state           = label
                  | predicate
  label           = /:\w+/
  predicate       = /\w+\?/
  action          = name
                  | /\{.*\}.*/

=== Basic Example

    class Calculator
    macros
      NUMBER /\d+/
    rules
           /rpn/       :RPN # sets @state to :RPN
           /#{NUMBER}/ { [:number, text.to_i] }
           /\s+/
           /[+-]/      { [:op, text] }

      :RPN /\s+/
      :RPN /[+-]/      { [:op2, text] }
      :RPN /#{NUMBER}/ { [:number2, text.to_i] }
      :RPN /alg/       nil # clears state
    end

==== Header

Anything before the class line is considered the "header" and will be
added to the top of your file. This includes extra lines like module
namespacing.

==== Class Line

The class line, like a regular ruby class declaration, specifies what
class all of the lexer code belongs to. You may simply specify a class
name like:

    class MyLexer

or it may specify a superclass as well:

    class MyLexer < MyParser

You might do this latter case to mix your lexer and your racc parser
together.

Personally, I recommend keeping them apart for cleanliness and
testability.

==== Options

All options are opt-in and can be specified either in the grammar or
via an options hash in `OedipusLex#initialize`.

Specify `debug` to turn on basic tracing output.

Specify `stub` to create a generic handler that processes all files
specified on the commandline with a rather generic handler. This makes
it easy to get up and running before you have the rest of your system
in place.

Specify `do_parse` to generate a generic do_parse method that
automatically dispatches off to `lex_<token_type>` methods.

Specify `lineno` to generate automatic line number handling at the
beginning of `next_token`. This was the default in 1.0.0 and you must
now activate it.

Specify `column` to generate automatic column number handling.

==== Inner

The inner section is just code, like header or footer, but inner gets
put _inside_ the class body. You can put extra methods here.

Personally, I recommend you don't use inner and you put all of your
extra methods and class code in a separate file. This makes lexer
generation faster and keeps things separate and small.

==== Macros

Macros define named regexps that you can use via interpolation inside
other subsequent macros or within rule matchers.

==== Start

The lexer runs in a loop until it finds a match or has to bail. Use
the `start` section to place extra code at the top of your
`next_token` method, before the loop. Eg:

    start
      space_seen = false

This code will get expanded into the very top of the lexer method. Do
note that this code gets run before _every token_, not just on lexer
initialization.

==== Rules

The rule section is the meat of the lexer. It contains one or more
rule lines where each line consists of:

* a required state (as a `:symbol`), a predicate method, or nothing.
* a regular expression.
* an action method, an action block, or nothing.

More often than not, a rule should not specify a required state. Only
use them when you're convinced you need them.

So a rule can very simple, including _just_ a regexp:

  rules
    /#.*/  # ignore comments

or can contain any combination of state checks or action types:

  rules
    :state     /token/    action_method
    predicate? /another/  { do_something }

===== States and Predicates

In order for the tokenizer to determine if the rule's regexp should
even be considered, a rule may specify a required state, a predicate
method to call, or leave it blank.

If the rule does not specify a state, it can be used whenever `@state`
is nil or a symbol that starts lowercase (an inclusive rule). If the
rule specifies a symbol that starts uppercase (an exclusive rule), it
will _only_ use those rules when `@state` matches.

Alternatively, a rule may specify a predicate method to check. If that
method returns a truthy value, the rule is currently valid. This is
equivalent to setting the required state to nil, as it will be used
with inclusive and nil states, and ignored for exclusive states.

==== End & Footer

Like the header, anything after the end line is considered the
"footer" and will be added to the bottom of your file.

== Suggested Structure

Here's how I suggest you structure things:

=== Rakefile

You only need a minimum of dependencies to wire stuff up if you use
the supplied rake rule.

  Rake.application.rake_require "oedipus_lex"

  task :lexer  => "lib/mylexer.rex.rb"
  task :parser => :lexer # plus appropriate parser rules/deps
  task :test   => :parser

=== lib/mylexer.rex

Put your lexer definition here. It will generate into
`"lib/mylexer.rex.rb"`.

  class MyLexer
  macros
    # ...
  rules
    # ...
  end

=== lib/mylexer.rb

  require "new_ruby_lexer.rex"

  class MyLexer
    # ... predicate methods and stuff
  end

=== lib/myparser.rb

Assuming you're using a racc based parser, you'll need to define a
`next_token` method that bridges over to your lexer:

  class MyParser
    def next_token
      lexer.next_token # plus any sanity checking / error handling...
    end
  end

== Differences with Rexical

If you're already familiar with rexical, this might help you get up
and running faster. If not, it could provide an overview of the
value-added.

=== Additions or Changes

==== A generic rake rule is defined for rex files.

Oedipus defines a rake rule that allows you simply define a file-based
dependency and rake will take care of the rest. Eg:

  file "lib/mylexer.rex.rb" => "lib/mylexer.rex"

  task :generated => %w[lib/mylexer.rex.rb]

  task :test => :generated

==== All regular expressions must be slash delimited.

Basically, regexps are now plain slashed ruby regexps. This allows for
regexp flags to be provided individually, rather than specifying an
entire grammar is case-insensitive, you can have a single rule be case
insensitive.

Right now only `/i` and `/o` are properly handled.

==== Regular expressions now use ruby interpolation.

Instead of `aaa{{macro}}ccc` it is `/aaa#{macro}ccc/`.

==== Macros define class constants.

Macros simply become class constants inside the lexer class. This
makes them immediately available to other macros and to the regexps in
the rules section.

This also implies that they must start uppercase, since that is
required by ruby.

==== Rules can be activated by predicate methods.

Instead of just switching on state, rules can now check predicate
methods to see if they should trigger. Eg:

    rules
      sad?   /\w+/    { [:sad,   text] }
      happy? /\w+/    { [:happy, text] }
    end

    # elsewhere:
    def sad?
      # ...
    end

    def happy?
      not sad?
    end

==== Rule actions are only a single-line.

In order to push complexity down, `{ rule actions }` may only be a
single line.

==== Rules can invoke methods.

For more complex actions, use a method by specifying its name:

    rules
      /\w+/    process_word
    end

And then define the handler method to return a result pair:

    def process_word text
      # do lots of normalization...
      [:word, token]
    end

This strikes a good balance between readability and maintainability.
It also makes it much easier to write unit tests for the complex
actions.

==== Rules can define state.

There are shortcuts built in to define or clear state:

  rules
         /rpn/  :RPN  # sets @state to :RPN
    # ...
    :RPN /alg/   nil  # clears @state

==== Use a `start` section to define pre-lex code.

The lexer runs in a loop until it finds a match or has to bail.
Sometimes more complex lexers need to set some local state. You can
now do this in a `start` section. Eg:

  start
    space_seen = false

This code will get expanded into the very top of the lexer method. Do
note that this code gets run before _every token_, not just on
initialization.

==== Rule state can be inclusive or exclusive.

This actually isn't new from rexical... It just wasn't really well
documented.

Exclusive states start with an uppercase letter (and are generally all
uppercase). Inclusive states start with a lowercase letter. Exclusive
states will _only_ try their own matchers. Inclusive states will also
try any matcher w/o a state.

In both cases, the order of generated matchers is strictly defined by
the source file. Nothing is re-ordered, ever. Eg:

  rules
          /\d+/
          /\s+/  # used in both nil-state and :rpn state
          /[+-]/

    :rpn  /\d+/  # won't hit, because of nil-state matcher above

    :OP   /\s+/  # must define its own because no-nil-state matchers are used
    :OP   /\d+/
  end

==== Default `do_parse` will dispatch to lex_xxx automatically.

The method `do_parse` is generated for you and automatically
dispatches off to user-defined methods named `lex_<token-type>` where
token-type is the first value returned from any matching action. Eg:

  rules
    /\s*(\#.*)/  { [:comment, text] }

  # elsewhere:

  def lex_comment line
    # do nothing
  end

==== `text` is passed in, or use `match[n]` or `matches`

You can use the `text` variable for the entire match inside an action,
or you can use `match[n]` to access a specific match group, or
`matches` to get an array of all match groups. Eg:

  /class ([\w:]+)(.*)/ { [:class, *matches] }

In this case, the action will return something like: `[:class,
"ClassName" "< Superclass"]`.

==== You can override the scanner class by defining `scanner_class`.

Oedipus will define the method `scanner_class` to return
`StringScanner` unless you define one yourself. Because it uses
reflection to figure out whether you've defined it or not, you may
need to require the generated lexer AFTER you've defined
`scanner_class`. Eg:

  class MyLexer
    # ...

    def scanner_class
      CustomStringScanner
    end

    # ...
  end

  require "my_lexer.rex"

**NOTE:** I'm _totally_ open to better ways of doing this. I simply
needed to get stuff done and this presented itself as _viable-enough_.

=== Removals

==== There is no command-line tool.

There is no command-line tool. Instead, use the rake rule described
above.

==== There are only two options: debug and stub.

All other options from rexical have been removed because they don't
make sense in Oedipus.

==== Probably others...

It's hard to think about what I took out. What I added is plain as
day. :P

[TIN321]: http://www.cse.chalmers.se/edu/year/2011/course/TIN321/lectures/proglang-04.html
[LLVM Tutorial]: http://llvm.org/docs/tutorial/LangImpl1.html#language

== Requirements:

* ruby version 1.8.x or later.

== Install

* sudo gem install oedipus_lex

== License

(The MIT License)

Copyright (c) Ryan Davis, seattle.rb

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 AUTHORS OR COPYRIGHT HOLDERS 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.
