Writing DSLs with Parslet - NYC.rb

A well-designed DSL improves programmer productivity and communication with domain experts. The Ruby community has produced a number of very popular external DSLs—Coffeescript, HAML, SASS, and Cucumber to name a few.

Parslet makes it easy to write these kinds of DSLs in pure Ruby. In this talk you’ll learn the basics, feel out the limitations of several approaches and find some common solutions. In no time, you’ll have the power to make a great new DSL, slurp in obscure file formats, modify and contribute to other people’s grammars (like Gherkin, TOML, or JSON), or even write your own programming language!

Presented by Jason Garber at Wicked Good Ruby Conf and NYC.rb. https://www.promptworks.com

Transcript

1. Writing DSLs
   with Parslet Jason Garber N Y C . r b A p r i l 8 , 2 0 1 4
3. TDD TATFT Agile! Scrum Pair Programming VIM Pomodoro Continuous Integration Continuous Delivery
5. Parsing
6. DSLS Domain-Specific Languages
7. <?xml version="1.0"?> <configuration><configSections><sectionGr
 oup name="userSettings" type="System.Configuration.UserSettings
 Group, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken
 =b77a5c561934e089"><section name="MSDNSampleSettings.My.MySetti
 ngs" type="System.Configuration.ClientSettingsSection, System, 
 Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e0
 89" allowExeDefinition="MachineToLocalUser" requirePermission="
 false"/></sectionGroup></configSections> ... <userSettings><MSD
 NSampleSettings.My.MySettings><setting name="Setting" serialize
 As="String"><value>SomeDefaultValue</value></setting></MSDNSamp
 leSettings.My.MySettings></userSettings> </configuration>
8. </xml>
9. I DSLs
10. RssReader::Application.routes.draw do devise_for :users ! resources :feeds, only: [:edit, :update, :show, :index] post '/feeds', to: 'feeds#create', as: 'create_feed' resources :users do get 'page/:page', action: :index, on: :collection end resources :posts do member do put :update_category end end get '/my_profile', to: 'users#my_profile', as: :my_profile root to: "home#home" end
11. describe Stack do context "stack with one item" do let(:stack) { a_stack_with_one_item } ! context "when popped" do before { stack.pop } it { should be_empty } end end end
12. click_on "Sign Up" fill_in "Email", with: account[:email] fill_in "Password", with: account[:password] fill_in "Confirmation", with: account[:password_confirmation] fill_in "Name", with: account[:name] select account[:birthyear].to_s, from: "Year born" check "Terms" click_on "I'm ready to join!" current_path.should eq "/accounts/#{account.id}/dashboard" page.should have_content "Dashboard"
13. desc 'Generate markup and stylesheets and open browser preview' task :preview => FileList['*.html'] + FileList['*.css'] do |t| sh 'open -g index.html' end rule '.html' => '.haml' do |t| puts "Rebuilding #{t.name}" sh "haml #{t.source} #{t.name}" end rule '.css' => lambda { |cssfile| source(cssfile) } do |t| puts "Rebuilding #{t.name}" sh "sass #{t.source} #{t.name}" end
14. get '/' do @posts = Post.all(:order => [:id.desc], :limit => 20) erb :index end ! get '/post/new' do erb :new end ! get '/post/:id' do @post = Post.get(params[:id]) erb :post end ! post '/post/create' do post = Post.new(params) status 201 redirect "/post/#{post.id}" end
15. rule(:table) do (str("table") >> attributes?.as(:attributes) >> str(".\n")).maybe >> table_row.repeat(1).as(:content) >> block_end end rule(:table_row) do table_row_attributes >> table_row_content >> end_table_row end rule(:table_row_attributes) { (attributes?.as(:attributes) >> str(". ")).maybe } rule(:table_row_content) { (table_header | table_data).repeat(1).as(:content) } rule(:end_table_row) { str("|") >> (block_end.present? | (str("\n"))) } rule(:table_header) do str("|_. ") >> table_content.as(:content) end rule(:table_data) do str("|") >> str("\n").absent? >> td_attributes? >> table_content.as(:content) end
16. Internal DSLs Fluent Interfaces
17. EXTERNAL
    DSLS
18. upstream puma { server unix:///tmp/sockets/puma.sock fail_timeout=0; } server { listen 80 default deferred; server_name promptworks.com www.promptworks.com; root /srv/promptworks/public; charset utf-8; if (-f $document_root/system/maintenance.html) { return 503; } error_page 503 @maintenance; location @maintenance { rewrite ^(.*)$ /system/maintenance.html last; break; } }
19. <([A-Z][A-Z0-9]*)\b[^>]*>(.*?)<\/\1> ! ! \b(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3} (?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\b
20. SELECT DISTINCT sc1.id FROM ( SELECT DATE_ADD(entry_point.start_date, INTERVAL (digits_1.digit * 10 + d (1 << (DAYOFWEEK(DATE_ADD(entry_point.start_date, INTERVAL (digits_1.digi FROM (SELECT CAST('2012-03-01' AS date) AS start_date) AS entry_point INNER JOIN (SELECT 0 AS digit UNION SELECT 1 UNION SELECT 2 UNION SELECT ON (digits_1.digit * 10 + digits_2.digit) <= (DATEDIFF(DATE_ADD(entry_poi ) AS md INNER JOIN schedules AS sc1 INNER JOIN negative_link_influences AS neg on (sc1.call_type_id = neg.call_type_id or neg.call_type_id = -1) WHERE sc1.schedule_on = md.date AND neg.affected_shift = 0;
21. BEGIN { TOTAL=0 } { TOTAL = TOTAL + $1 } END { print TOTAL/NR } {print "<li><a href=\"" $1 "\">" $1 "</a></li>"}
22. ! doctype html html head title Test body - unless items.empty? ol - items.each do |item| li = item - else p No items
23. Feature: Searching music As a User I want to be able to search music So I can play it Background: Given I am logged in Scenario: Search music Given I am on the search screen When I search for "mix" Then I should see the mixtape
24. class erlang { file { "/etc/apt/sources.list.d/esl-erlang.list": ensure => present, owner => root, content => 'deb http://example.com/debian precise contrib'; } exec { "apt-update": command => "/usr/bin/apt-get update", refreshonly => true; } package { "esl-erlang": ensure => installed, require => Exec['apt-update', 'import-key']; } } include erlang
25. Heroku buildpack: Ruby ====================== This is a [Heroku buildpack](http://devcenter.heroku.com/article It uses [Bundler](http://gembundler.com) for dependency manageme Usage ----- ### Ruby Example Usage: $ heroku create --stack cedar --buildpack https://github.com $ git push heroku master
26. title = "T??? Example" ! [owner] name = "Tom Preston-Werner" organization = "GitHub" bio = "GitHub Cofounder & CEO\nLikes tater tots and beer." dob = 1979-05-27T07:32:00Z # First class dates? Why not? ! [database] server = "192.168.1.1" ports = [ 8001, 8001, 8002 ] connection_max = 5000 enabled = true
27. global= { \time 4/4 \key c \major \tempo "Allegro" 4 = 132 \set Score.skipBars = ##t } ! Violinone = \new Voice { \relative c''{ \set Staff.midiInstrument = #"violin" \set tupletSpannerDuration = #(ly:make-moment 1 4) ! % Jason solo R1 * 9 r2 r4 \times 2/3 { c4( b8)
28. gsave 1 0.5 scale 70 100 48 0 360 arc fill grestore /Helvetica-Bold 14 selectfont 1.0 setgray 29 45 moveto (Hello, world!) show showpage
29. .accordion { li { border-top: 1px solid #e2e4e6; &:first-child { border-top-color: transparent; } } a { @include rem(padding, 5px 10px 6px); &.icon { padding-left: 40px; position: relative; } &.icon img { @include rem(left, 10px); margin-top: -2px; position: absolute; } } }
32. h1. Give Textile a try! ! A *simple* paragraph with a line break, some _emphasis_ and a "link":http://redcloth.org ! * an item * and another ! # one # two # three
33. h1. Give Textile a try! ! A *simple* paragraph with a line break, some _emphasis_ and a "link":http://redcloth.org ! * an item * and another ! # one # two # three
35. A_HLGN = /(?:\<(?!>)|\<\>|\=|[()]+)/ A_VLGN = /[\-^~]/ C_CLAS = '(?:\([^)]+\))' C_LNGE = '(?:\[[^\]]+\])' C_STYL = '(?:\{[^}]+\})' S_CSPN = '(?:\\\\\d+)' S_RSPN = '(?:/\d+)' A = "(?:#{A_HLGN}?#{A_VLGN}?|#{A_VLGN}?#{A_HLGN}?)" S = "(?:#{S_CSPN}?#{S_RSPN}|#{S_RSPN}?#{S_CSPN}?)" C = "(?:#{C_CLAS}?#{C_STYL}?#{C_LNGE}?|#{C_STYL}? #{C_LNGE}?#{C_CLAS}?|#{C_LNGE}?#{C_STYL}?#{C_CLAS}?)" ! PUNCT = Regexp::quote( '!"#$%&\'*+,-./:;=?@\\^_`|~' ) HYPERLINK = '(\S+?)([^\w\s/;=\?]*?)(\s|$)'
36. def links( text ) text.gsub!( / ([\s\[{(]|[#{PUNCT}])? # $pre " # start (#{C}) # $atts ([^"]+?) # $text \s? (?:\(([^)]+?)\)(?="))? # $title ": (\S+?) # $url (\/)? # $slash ([^\w\/;]*?) # $post (?=\s|\.[#{PUNCT}]+|$) /x ) do |m| pre,atts,text,title,url,slash,post = $~[1..7] ! url = check_refs( url ) ! atts = pba( atts ) atts << " title=\"#{ title }\"" if title atts = shelve( atts ) if atts ! "#{ pre }<a href=\"#{ url }#{ slash }\"#{ atts }>" + "#{ text }</a>#{ post }" end end
37. “Some people, when confronted with a problem, think ‘I know, I'll use regular expressions.’
 Now they have two problems.” — Jamie Zawinski
39. %%{ machine superredcloth_scan; include superredcloth_common "ext/superredcloth_scan/superredcloth_common.rl"; action extend { extend = rb_hash_aref(regs, ID2SYM(rb_intern("type"))); } # blocks notextile_tag_start = "<notextile>" ; notextile_tag_end = "</notextile>" CRLF? ; notextile_block_start = ( "notextile" >A %{ STORE(type) } A C :> "." ( "." %extend | "" ) " "+ ) ; pre_tag_start = "<pre" [^>]* ">" (space* "<code>")? ; pre_tag_end = ("</code>" space*)? "</pre>" CRLF? ; pre_block_start = ( "pre" >A %{ STORE(type) } A C :> "." ( "." %extend | "" ) " "+ ) ; bc_start = ( "bc" >A %{ STORE(type) } A C :> "." ( "." %extend | "" ) " "+ ) ; btype = ( "p" | "h1"... block_start = ( btype >A %{ STORE(type) } A C :> "." ( "." %extend | "" ) " "+ ) ; next_block_start = ( btype A C :> "."+ " " ) >A @{ p = reg - 1; } ; pre_tag := |* pre_tag_end { CAT(block); DONE(block); fgoto main; }; default => esc_pre; *|; notextile_tag := |* notextile_tag_end { DONE(block); fgoto main; }; default => cat; *|; }%%
40. void rb_str_cat_escaped(VALUE str, char *ts, char *te, unsigned int opts); void rb_str_cat_escaped_for_preformatted(VALUE str, char *ts, char *te, unsigned int opts); VALUE superredcloth_inline(VALUE, char *, char *, VALUE); VALUE superredcloth_inline2(VALUE, VALUE, VALUE); ! #define CAT(H) rb_str_cat(H, ts, te-ts) #define CLEAR(H) H = rb_str_new2("") #define INLINE(H, T) rb_str_append(H, rb_funcall(rb_formatter, rb_intern(#T), 1, regs)) ! VALUE superredcloth_transform(rb_formatter, p, pe, refs) VALUE rb_formatter; char *p, *pe; VALUE refs; { if (RSTRING(block)->len > 0) { ADD_BLOCK(); } ! if ( NIL_P(refs) && rb_funcall(refs_found, rb_intern("empty?"), 0) == Qfalse ) { return superredcloth_transform(rb_formatter, orig_p, orig_pe, refs_found); } else { rb_funcall(rb_formatter, rb_intern("after_transform"), 1, html); return html; } }
43. %%{ machine superredcloth_scan; include superredcloth_common "ext/superredcloth_scan/superredcloth_common.rl"; action extend { extend = rb_hash_aref(regs, ID2SYM(rb_intern("type"))); } # blocks notextile_tag_start = "<notextile>" ; notextile_tag_end = "</notextile>" CRLF? ; notextile_block_start = ( "notextile" >A %{ STORE(type) } A C :> "." ( "." %extend | "" ) " "+ ) ; pre_tag_start = "<pre" [^>]* ">" (space* "<code>")? ; pre_tag_end = ("</code>" space*)? "</pre>" CRLF? ; pre_block_start = ( "pre" >A %{ STORE(type) } A C :> "." ( "." %extend | "" ) " "+ ) ; bc_start = ( "bc" >A %{ STORE(type) } A C :> "." ( "." %extend | "" ) " "+ ) ; btype = ( "p" | "h1"... block_start = ( btype >A %{ STORE(type) } A C :> "." ( "." %extend | "" ) " "+ ) ; next_block_start = ( btype A C :> "."+ " " ) >A @{ p = reg - 1; } ; pre_tag := |* pre_tag_end { CAT(block); DONE(block); fgoto main; }; default => esc_pre; *|; notextile_tag := |* notextile_tag_end { DONE(block); fgoto main; }; default => cat; *|; }%%
44. %%{ machine superredcloth_scan; include superredcloth_common "ext/superredcloth_scan/superredcloth_common.rl"; action extend { extend = rb_hash_aref(regs, ID2SYM(rb_intern("type"))); } # blocks notextile_tag_start = "<notextile>" ; notextile_tag_end = "</notextile>" CRLF? ; notextile_block_start = ( "notextile" >A %{ STORE(type) } A C :> "." ( "." %extend | "" ) " "+ ) ; pre_tag_start = "<pre" [^>]* ">" (space* "<code>")? ; pre_tag_end = ("</code>" space*)? "</pre>" CRLF? ; pre_block_start = ( "pre" >A %{ STORE(type) } A C :> "." ( "." %extend | "" ) " "+ ) ; bc_start = ( "bc" >A %{ STORE(type) } A C :> "." ( "." %extend | "" ) " "+ ) ; btype = ( "p" | "h1"... block_start = ( btype >A %{ STORE(type) } A C :> "." ( "." %extend | "" ) " "+ ) ; next_block_start = ( btype A C :> "."+ " " ) >A @{ p = reg - 1; } ; pre_tag := |* pre_tag_end { CAT(block); DONE(block); fgoto main; }; default => esc_pre; *|; notextile_tag := |* notextile_tag_end { DONE(block); fgoto main; }; default => cat; *|; }%% /* * redcloth_scan.c.rl * * Copyright (C) 2009 Jason Garber */ #define redcloth_scan_c ! #define RSTRING_NOT_MODIFIED #include <ruby.h> #include "redcloth.h" ! VALUE mRedCloth, super_ParseError, super_RedCloth, super_HTML, super_LATEX; VALUE SYM_escape_preformatted, SYM_escape_attributes; ! #line 23 "ext/redcloth_scan/redcloth_scan.c" static const unsigned char _redcloth_scan_actions[] = { 0, 1, 0, 1, 2, 1, 3, 1, 4, 1, 5, 1, 6, 1, 7, 1, 9, 1, 10, 1, 11, 1, 12, 1, 13, 1, 14, 1, 15, 1, 16, 1, 17, 1, 18, 1, 19, 1, 20, 1, 21, 1, 22, 1, 23, 1, 24, 1, 25, 1, 26, 1, 27, 1, 28, 1, 29, 1, 30, 1, 34, 1, 35, 1, 36, 1, 38, 1, 40, 1, 42, 1, 43, 1, 44, 1, 45, 1, 48, 1, 57, 1, 58, 1, 59, 1, 60, 1, 61, 1, 62, 1, 63, 1, 64, 1, 65, 1, 66, 1, 70, 1, 73, 1,
47. grammar Arithmetic rule additive multitive ( '+' multitive )* end ! rule multitive primary ( [*/%] primary )* end ! rule primary '(' additive ')' / number end ! rule number '-'? [1-9] [0-9]* end end
48. require 'parslet' ! class MiniParser < Parslet::Parser rule(:integer) { match('[0-9]').repeat(1) } root(:integer) end ! MiniParser.new.parse("1324321") # => "1324321"@0
49. Parslet::Atoms str('NYC.rb')
50. Parslet::Atoms str('NYC.rb').parse('NYC.rb')
51. Parslet::Atoms str('NYC.rb').parse('NYC.rb') # => "NYC.rb"@0
52. Parslet::Atoms str('NYC.rb').parse('NYC.rb') # => "NYC.rb"@0 ! match('[0-9a-f]')
53. Parslet::Atoms str('NYC.rb').parse('NYC.rb') # => "NYC.rb"@0 ! match('[0-9a-f]') ! any
54. Operators str('jim') >> str('weirich') ! str('Ruby') | str('Elixir') ! match('[Nn]') >> str('ew') | match('[Yy]') >> str('ork City')
55. Repetition str('foo').repeat str('foo').repeat(1) str('foo').repeat(1,3) str('foo').repeat(0, nil) str('foo').maybe
56. Presence str('Java') >> str('Script').present? ! ! str('0').repeat(1).absent? >> match('[\d]').repeat(1)
57. 1. Create a grammar
 What should be legal syntax? 2. Annotate the grammar:
 What is important data? 3. Create a transformation:
 How do I want to work with that data?
58. 1. Create a grammar
 What should be legal syntax? 2. Annotate the grammar:
 What is important data? 3. Create a transformation:
 How do I want to work with that data?
59. Capture str('Madison Sq').parse('Madison Sq') # => "Madison Sq"@0 ! str('Madison Sq').as(:park).parse('Madison Sq') # => {:park=>"Madison Sq"@0}
60. Capture str('a').repeat.as(:b) # => {:b=>"aaa"@0} str('a').as(:b).repeat # => [{:b=>"a"@0}, {:b=>"a"@1}, {:b=>"a"@2}] str('a').as(:a) >> str('b').as(:b) >> str('c') # => {:a=>"a"@0, :b=>"b"@1}
61. rule(:table) do (str("table") >> attributes?.as(:attributes) >> str(".\n")).maybe >> table_row.repeat(1).as(:content) >> block_end end rule(:table_row) do table_row_attributes >> table_row_content >> end_table_row end rule(:table_row_attributes) { (attributes?.as(:attributes) >> str(". ")).maybe } rule(:table_row_content) { (table_header | table_data).repeat(1).as(:content) } rule(:end_table_row) { str("|") >> (block_end.present? | (str("\n"))) } rule(:table_header) do str("|_. ") >> table_content.as(:content) end rule(:table_data) do str("|") >> str("\n").absent? >> td_attributes? >> table_content.as(:content) end
62. table(#prices).
 | Adults | $5 |
 | Children | $2 | <table id="prices"> <tr> <td>Adults</td> <td>$5</td> </tr> <tr> <td>Children</td> <td>$2</td> </tr> </table>
63. rule(:table) do (str("table") >> attributes?.as(:attributes) >> str(".\n")).maybe >> table_row.repeat(1).as(:content) >> block_end end rule(:table_row) do table_row_attributes >> table_row_content >> end_table_row end rule(:table_row_attributes) { (attributes?.as(:attributes) >> str(". ")).maybe } rule(:table_row_content) { (table_header | table_data).repeat(1).as(:content) } rule(:end_table_row) { str("|") >> (block_end.present? | (str("\n"))) } rule(:table_header) do str("|_. ") >> table_content.as(:content) end rule(:table_data) do str("|") >> str("\n").absent? >> td_attributes? >> table_content.as(:content) end
64. def parenthesized(atom) str('(') >> atom >> str(')') end ! parenthesized(match['\d']).parse("(5)")
65. class HtmlTag < Parslet::Parser root(:tag) rule(:tag) { open_tag | close_tag | self_closing_tag | comment_tag } rule(:open_tag) { str("<") >> tag_name >> attributes? >> str(">") } rule(:close_tag) { str("</") >> tag_name >> str(">") } rule(:tag_name) { match("[A-Za-z_:]") >> name_char.repeat } ... end ! class BlockHtmlTag < HtmlTag rule(:tag_name) do inline_tag_name.absent? >> any_tag_name end ! rule(:inline_tag_name) do INLINE_TAGS.map {|name| str(name) }.reduce(:|) end end
66. > HtmlTag.new.open_tag.methods => [:name, :block, :try, :parslet, :to_s_inner, :parse, :apply, :cached?, ...] ! > HtmlTag.new.open_tag.parse("<blockquote>") => "<blockquote>"@0 ! > HtmlTag.new.open_tag.parse("</blockquote>") ! Parslet::ParseFailed: Failed to match sequence ('<' TAG_NAME ATTRIBUTES? '>') at line 1 char 2. from /Users/jasongarber/.rvm/gems/ruby-2.0.0-p247/gems/parslet-1.5.0/lib/parslet/ cause.rb:63:in `raise' from /Users/jasongarber/.rvm/gems/ruby-2.0.0-p247/gems/parslet-1.5.0/lib/parslet/ atoms/base.rb:46:in `parse' from (irb):8 from /Users/jasongarber/.rvm/rubies/ruby-2.0.0-p247/bin/irb:16:in `<main>'
67. begin RedClothParslet::Parser::BlockHtmlTag.new.tag.parse("<img>") rescue Parslet::ParseFailed => failure puts failure.cause.ascii_tree end ! Expected one of [OPEN_TAG, CLOSE_TAG, SELF_CLOSING_TAG, COMMENT_TAG] at line 1 char 1. |- Failed to match sequence ('<' TAG_NAME ATTRIBUTES? '>') at line 1 char 2. | `- Failed to match sequence ((!INLINE_TAG_NAME / &(INLINE_TAG_NAME NAME_CHAR{1, })) ANY_TAG_NAME) at line 1 char 2. | `- Expected one of [!INLINE_TAG_NAME, &(INLINE_TAG_NAME NAME_CHAR{1, })] at line 1 char 2. | |- Input should not start with INLINE_TAG_NAME at line 1 char 2. | `- Input should start with INLINE_TAG_NAME NAME_CHAR{1, } at line 1 char 2. |- Failed to match sequence ('</' TAG_NAME '>') at line 1 char 1. | `- Expected "</", but got "<i" at line 1 char 1. |- Failed to match sequence ('<' TAG_NAME ATTRIBUTES? SPACES? '/' '>') at line 1 char 2. | `- Failed to match sequence ((!INLINE_TAG_NAME / &(INLINE_TAG_NAME NAME_CHAR{1, })) ANY_TAG_NAME) at line 1 char 2. | `- Expected one of [!INLINE_TAG_NAME, &(INLINE_TAG_NAME NAME_CHAR{1, })] at line 1 char 2. | |- Input should not start with INLINE_TAG_NAME at line 1 char 2. | `- Input should start with INLINE_TAG_NAME NAME_CHAR{1, } at line 1 char 2. `- Failed to match sequence ('<!--' (!COMMENT_TAG_END .){0, } COMMENT_TAG_END) at line 1 char 1. `- Expected "<!--", but got "<img" at line 1 char 1.
68. I Objects
69. describe RedClothParslet::Parser::HtmlTag do it { should parse("<div>") } it { should parse("<hr />") } it { should parse("</div>") } it { should parse("<!-- an HTML comment -->") } ! describe "attribute" do subject { described_class.new.attribute } it { should parse(" class='awesome'") } it { should_not parse(' 9kittens="cute"') } end end
70. $ rspec spec/parser/html_tag_spec.rb ! 1) RedClothParslet::Parser::HtmlTag tag Failure/Error: it { should parse("</div>") } expected TAG to be able to parse "</div>" Expected one of [OPEN_TAG, CLOSE_TAG, SELF_CLOSING_TAG, COMMENT_TAG] at line 1 char 1. |- Failed to match sequence ('<' TAG_NAME ATTRIBUTES? '>') at line 1 char 2. | `- Failed to match sequence ([A-Za-z_:] NAME_CHAR{0, }) at line 1 char 2. | `- Failed to match [A-Za-z_:] at line 1 char 2. |- Failed to match sequence ('<' TAG_NAME '>') at line 1 char 2. | `- Failed to match sequence ([A-Za-z_:] NAME_CHAR{0, }) at line 1 char 2. | `- Failed to match [A-Za-z_:] at line 1 char 2. |- Failed to match sequence ('<' TAG_NAME ATTRIBUTES? SPACES? '/' '>') at line 1 char 2. | `- Failed to match sequence ([A-Za-z_:] NAME_CHAR{0, }) at line 1 char 2. | `- Failed to match [A-Za-z_:] at line 1 char 2. `- Failed to match sequence ('<!--' (!COMMENT_TAG_END .){0, } COMMENT_TAG_END) at line 1 char 1. `- Expected "<!--", but got "</di" at line 1 char 1. # ./spec/parser/html_tag_spec.rb:9:in `block (3 levels) in <top (required)>' ! 13/13: 100% |==========================================| Time: 00:00:00 ! Finished in 0.01879 seconds 13 examples, 1 failure
71. 1. Create a grammar
 What should be legal syntax? 2. Annotate the grammar:
 What is important data? 3. Create a transformation:
 How do I want to work with that data?
72. Transformations tree = {left: {int: '1'}, op: '+', right: {int: '2'}} class T < Parslet::Transform rule(int: simple(:x)) { Integer(x) } end ! T.new.apply(tree) # => {:left=>1, :op=>"+", :right=>2}
73. Transformations tree = {left: {int: '1'}, op: '+', right: {int: '2'}} ! class T < Parslet::Transform rule(int: simple(:x)) { Integer(x) } rule(op: '+', left: simple(:l), right: simple(:r)) { l + r } end T.new.apply(tree) # => 3
74. Transformations rule(:content => subtree(:c), :attributes => subtree(:a)) do |dict| {:content => dict[:c], :opts => RedCloth::Ast::Attributes.new(dict[:a])} end ! rule(:table => subtree(:a)) do RedCloth::Ast::Table.new(a[:content], a[:opts]) end ! rule(:bq => subtree(:a)) do RedCloth::Ast::Blockquote.new(a[:content], a[:opts]) end
75. Who you callin’ slow?
76. class ErbParser < Parslet::Parser rule(:ruby) { (str('%>').absent? >> any).repeat.as(:ruby) } ! rule(:expression) { (str('=') >> ruby).as(:expression) } rule(:comment) { (str('#') >> ruby).as(:comment) } rule(:code) { ruby.as(:code) } rule(:erb) { expression | comment | code } ! rule(:erb_with_tags) { str('<%') >> erb >> str('%>') } rule(:text) { (str('<%').absent? >> any).repeat(1) } ! rule(:text_with_ruby) { (text.as(:text) | erb_with_tags).repeat.as(:t root(:text_with_ruby) end
77. class ErbParser < Parslet::Parser rule(:ruby) { (str('%>').absent? >> any).repeat.as(:ruby) } ! rule(:expression) { (str('=') >> ruby).as(:expression) } rule(:comment) { (str('#') >> ruby).as(:comment) } rule(:code) { ruby.as(:code) } rule(:erb) { expression | comment | code } ! rule(:erb_with_tags) { str('<%') >> erb >> str('%>') } rule(:text) { (str('<%').absent? >> any).repeat(1) } ! rule(:text_with_ruby) { (text.as(:text) | erb_with_tags).repeat.as(:t root(:text_with_ruby) end class rule( ! rule( rule( rule( rule( ! rule( rule( ! rule( root( end
78. class ErbParser < Parslet::Parser rule(:ruby) { (str('%>').absent? >> any).repeat.as(:ruby) } ! rule(:expression) { (str('=') >> ruby).as(:expression) } rule(:comment) { (str('#') >> ruby).as(:comment) } rule(:code) { ruby.as(:code) } rule(:erb) { expression | comment | code } ! rule(:erb_with_tags) { str('<%') >> erb >> str('%>') } rule(:text) { (str('<%').absent? >> any).repeat(1) } ! rule(:text_with_ruby) { (text.as(:text) | erb_with_tags).repeat.as(:t root(:text_with_ruby) end class rule( ! rule( rule( rule( rule( ! rule( rule( ! rule( root( end include Parslet::Accelerator optimized = apply( parser, rule( (str(:x).absent? >> any).repeat ) { GobbleUp.new(x, 0) } )
79. Custom Atoms • Parse a limited set of natural-language queries • “Who makes the best cheesesteaks in NYC?” • EngTagger: a corpus-trained, probabilistic tagger • Custom Parslet::Atom
80. rule(:question) do interrogative.maybe.as(:int) >> verb_phrase.maybe.as(:verb) >> superlative_phrase.maybe.as(:sup) >> subject.as(:subj) >> prepositional_phrase.repeat.as(:preps) >> sentence_end.repeat.as(:punct) end ! rule(:verb_phrase) { (verb_present | verb_past | verb_future).as(:vp) } rule(:verb_present) { word(:VBZ, :VB).as(:present) } rule(:verb_past) { word(:VBD).as(:past) } rule(:verb_future) { word(:MD).as(:future) >> word(:VB).maybe.as(:infinitive) }
81. Other Crazy Uses •User-supplied formulas / logic •Logs •Streaming text •The Right Reverend and Right Honourable the Lord Bishop of London Richard John Carew Chartres
82. Who will write the next awesome DSL?
83. Who will write the next awesome DSL!. You
84. Conclusions • DSLs make life better • internal_dsl > external_dsl if 
 internal_dsl.practicable? • Keep your parser clean, unit test • “Situational awareness!”
85. The State of RedCloth •Needs lots of love + less neglectful maintainer •Finish the rewrite •Merge + release RedCloth 5.0
86. @jasongarber @promptworks jason@promptworks.com Rate Me: http://spkr8.com/t/30651
87. SpeakerRate Next time, post a link to my page