Getting along with YAML comments with Psych

by Masaki Hara

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© 2024 Wantedly, Inc. Psych-comments Psych-comments s = Psych::Comments.parse_stream(input) env = s.children[0].children[0].children[1] env.children << Psych::Nodes::Scalar.new("NEW_TOKEN") output = Psych::Comments.emit_yaml(s) bundle add psych-comments require "psych/comments"

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© 2024 Wantedly, Inc. YAML family tree XML Perl Marshaller YAML 1.0 YAML 1.2 YAML as “simple XML” YAML as “Portable Marshaller” YAML as “JSON upper-compat” JSON 1998 Data::Denter, Around 2001 2004 2001 (Launch of json.org) 2009

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© 2024 Wantedly, Inc. Three-fold YAML processing Ruby Object Node Graph Event Tree YAML "&a [1, true, *a]" [...] "1" "true" *a !!seq [...] !!int "1" !!str "true" * [1, true].tap { |a| a << a } Representation Serialization Presentation Native

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© 2024 Wantedly, Inc. Three-fold YAML processing Ruby Object Node Graph Event Tree YAML Process aliases and anchors (Cycles etc.) Process Tags (Custom Objects) Representation Serialization Presentation Native

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© 2024 Wantedly, Inc. YAML Kind and Tags Sequence Mapping Scalar !!seq !!omap !!pairs !!map !!set !ruby/object !!str !!binary !!null !!bool !!int !!float !!timestamp !!yaml !ruby/symbol !!merge !!value

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© 2024 Wantedly, Inc. YAML is… ● YAML is (in a way) a portable Marshal ● Abstraction layers to sort out complexity ○ Parsing: remove syntax details ○ Deserializing: connect anchors and aliases ○ Interpreting: resolve and realize tags

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© 2024 Wantedly, Inc. Psych’s API levels Ruby Object Node Graph Event Tree YAML Psych.load Psych.dump Psych.parse #to_yaml Psych::Parser Psych::Emitter High-level API Mid-level API Low-level API Representation Serialization Presentation Native

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© 2024 Wantedly, Inc. Psych’s API levels and psych-comments Ruby Object Node Graph Event Tree YAML Psych.parse #to_yaml Mid-level API psych-comments’ API Representation Serialization Presentation Native

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© 2024 Wantedly, Inc. Psych (YAML) mid-level API Psych (a.k.a. YAML): mid-level API s = Psych.parse_stream(input) env = s.children[0].children[0].children[1] env.children << Psych::Nodes::Scalar.new("NEW_TOKEN") output = s.to_yaml

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© 2024 Wantedly, Inc. Recap: Psych-comments Recap: Psych-comments s = Psych::Comments.parse_stream(input) env = s.children[0].children[0].children[1] env.children << Psych::Nodes::Scalar.new("NEW_TOKEN") output = Psych::Comments.emit_yaml(s)

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© 2024 Wantedly, Inc. Extending the parser 2-pass parser Psych::Nodes::Node libyaml YAML text without comments Source location Psych::Nodes::Node with comments psych- comments Parse nodes Parse comments

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© 2024 Wantedly, Inc. Comment scanning edge case 3 Edge case 3: comments on a key-value pair # foo foo: 1 bar: # bar 2 NOTE: Psych lacks a node type for key-value pairs, instead hanging keys and values alternatingly in a flat array

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© 2024 Wantedly, Inc. Comment scanning edge case 4 Edge case 4 solution: attach to the whole element root: # foo - foo: 1 - # bar bar: 2 NOTE: “foo: 1” implicitly generates a Mapping node, which the comment attaches to.

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© 2024 Wantedly, Inc. Internal commands for YAML formatting Prepared utilities for formatting print "foo" space! newline! indented do … end Print after generating reserved spaces and indentation Reserve spaces or indentation Bump indent level NOTE: there is one more “virtual indentation” util for bullets

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© 2024 Wantedly, Inc. Generating bullet comments Hoisting map comments above bullets → ● Lookahead the tree and generate comments ● Then avoid duplication via a queue ○ Note that we should not mutate the input

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© 2024 Wantedly, Inc. config/locales: translation mistakes Solution 1: check for correspondence Synchronizing locales... Generating en.our_new_service.try_it_out Generating ja.our_new_service.try_if_out

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© 2024 Wantedly, Inc. config/locales: intentional absense Solution 2: (ab)use YAML tags en: # No data due to # translation # cost ja: japan_only: start: !only:ja "開始" cf. https://github.com/creasty/i18n_flow

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© 2024 Wantedly, Inc. Recap: Three-fold YAML processing Ruby Object Node Graph Event Tree YAML "&a [1, true, *a]" [...] "1" "true" *a !!seq [...] !!int "1" !!str "true" * [1, true].tap { |a| a << a } Representation Serialization Presentation Native

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© 2024 Wantedly, Inc. Levels of abstraction Presentation Serialization Representation Anchors & aliases Non-specific tags Scalar content formatting Directives Node style Comments Spacing Key ordering Tag style Escapes Tags Node links Scalar content

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© 2024 Wantedly, Inc. Levels of abstraction – Psych Presentation Serialization Representation Anchors & aliases Non-specific tags Scalar content formatting Directives Node style Comments Spacing Key ordering Tag style Escapes Tags Node links Scalar content Psych Mid-level API

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© 2024 Wantedly, Inc. Recap: Psych-comments layer Presentation Serialization Representation Anchors & aliases Non-specific tags Scalar content formatting Directives Node style Comments Spacing Key ordering Tag style Escapes Tags Node links Scalar content Psych-comments

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© 2024 Wantedly, Inc. Wrap up ● I made psych-comments gem. ● It processes YAML comments. ● It neatly solves your problem, partly reusing Psych’s own algorithms. ● Thankfully people are interested in expanding it, but as a responsible maintainer, I’m going to limit its scope.