overview: |
  Lambdas are a special-cased data type for use in interpolations and
  sections.

  When used as the data value for an Interpolation tag, the lambda MUST be
  treatable as an arity 0 function, and invoked as such.  The returned value
  MUST be rendered against the default delimiters, then interpolated in place
  of the lambda.

  When used as the data value for a Section tag, the lambda MUST be treatable
  as an arity 1 function, and invoked as such (passing a String containing the
  unprocessed section contents).  The returned value MUST be rendered against
  the current delimiters, then interpolated in place of the section.
tests:
  - name: Interpolation
    desc: A lambda's return value should be interpolated.
    data:
      lambda: !code
        ruby:    'proc { "world" }'
        raku:    'sub { "world" }'
        perl:    'sub { "world" }'
        js:      'function() { return "world" }'
        php:     'return "world";'
        python:  'lambda: "world"'
        clojure: '(fn [] "world")'
        lisp:    '(lambda () "world")'
        pwsh:    '"world"'
        go:      'func() string { return "world" }'
    template: "Hello, {{lambda}}!"
    expected: "Hello, world!"

  - name: Interpolation - Expansion
    desc: A lambda's return value should be parsed.
    data:
      planet: "world"
      lambda: !code
        ruby:    'proc { "{{planet}}" }'
        raku:    'sub { q+{{planet}}+ }'
        perl:    'sub { "{{planet}}" }'
        js:      'function() { return "{{planet}}" }'
        php:     'return "{{planet}}";'
        python:  'lambda: "{{planet}}"'
        clojure: '(fn [] "{{planet}}")'
        lisp:    '(lambda () "{{planet}}")'
        pwsh:    '"{{planet}}"'
        go:      'func() string { return "{{planet}}" }'
    template: "Hello, {{lambda}}!"
    expected: "Hello, world!"

  - name: Interpolation - Alternate Delimiters
    desc: A lambda's return value should parse with the default delimiters.
    data:
      planet: "world"
      lambda: !code
        ruby:    'proc { "|planet| => {{planet}}" }'
        raku:    'sub { q+|planet| => {{planet}}+ }'
        perl:    'sub { "|planet| => {{planet}}" }'
        js:      'function() { return "|planet| => {{planet}}" }'
        php:     'return "|planet| => {{planet}}";'
        python:  'lambda: "|planet| => {{planet}}"'
        clojure: '(fn [] "|planet| => {{planet}}")'
        lisp:    '(lambda () "|planet| => {{planet}}")'
        pwsh:    '"|planet| => {{planet}}"'
        go:      'func() string { return "|planet| => {{planet}}" }'
    template: "{{= | | =}}\nHello, (|&lambda|)!"
    expected: "Hello, (|planet| => world)!"

  - name: Interpolation - Multiple Calls
    desc: Interpolated lambdas should not be cached.
    data:
      lambda: !code
        ruby:    'proc { $calls ||= 0; $calls += 1 }'
        raku:    'sub { state $calls += 1 }'
        perl:    'sub { no strict; $calls += 1 }'
        js:      'function() { return (g=(function(){return this})()).calls=(g.calls||0)+1 }'
        php:     'global $calls; return ++$calls;'
        python:  'lambda: globals().update(calls=globals().get("calls",0)+1) or calls'
        clojure: '(def g (atom 0)) (fn [] (swap! g inc))'
        lisp:    '(let ((g 0)) (lambda () (incf g)))'
        pwsh:    'if (($null -eq $script:calls) -or ($script:calls -ge 3)){$script:calls=0}; ++$script:calls; $script:calls'
        go:      'func() func() int { g := 0; return func() int { g++; return g } }()'
    template: '{{lambda}} == {{{lambda}}} == {{lambda}}'
    expected: '1 == 2 == 3'

  - name: Escaping
    desc: Lambda results should be appropriately escaped.
    data:
      lambda: !code
        ruby:    'proc { ">" }'
        raku:    'sub { ">" }'
        perl:    'sub { ">" }'
        js:      'function() { return ">" }'
        php:     'return ">";'
        python:  'lambda: ">"'
        clojure: '(fn [] ">")'
        lisp:    '(lambda () ">")'
        pwsh:    '">"'
        go:      'func() string { return ">" }'
    template: "<{{lambda}}{{{lambda}}}"
    expected: "<&gt;>"

  - name: Section
    desc: Lambdas used for sections should receive the raw section string.
    data:
      x: 'Error!'
      lambda: !code
        ruby:    'proc { |text| text == "{{x}}" ? "yes" : "no" }'
        raku:    'sub { $^section eq q+{{x}}+ ?? "yes" !! "no" }'
        perl:    'sub { $_[0] eq "{{x}}" ? "yes" : "no" }'
        js:      'function(txt) { return (txt == "{{x}}" ? "yes" : "no") }'
        php:     'return ($text == "{{x}}") ? "yes" : "no";'
        python:  'lambda text: text == "{{x}}" and "yes" or "no"'
        clojure: '(fn [text] (if (= text "{{x}}") "yes" "no"))'
        lisp:    '(lambda (text) (if (string= text "{{x}}") "yes" "no"))'
        pwsh:    'if ($args[0] -eq "{{x}}") {"yes"} else {"no"}'
        go:      'func(text string) string { if text == "{{x}}" { return "yes" } else { return "no" } }'
    template: "<{{#lambda}}{{x}}{{/lambda}}>"
    expected: "<yes>"

  - name: Section - Expansion
    desc: Lambdas used for sections should have their results parsed.
    data:
      planet: "Earth"
      lambda: !code
        ruby:    'proc { |text| "#{text}{{planet}}#{text}" }'
        raku:    'sub { $^section ~ q+{{planet}}+ ~ $^section }'
        perl:    'sub { $_[0] . "{{planet}}" . $_[0] }'
        js:      'function(txt) { return txt + "{{planet}}" + txt }'
        php:     'return $text . "{{planet}}" . $text;'
        python:  'lambda text: "%s{{planet}}%s" % (text, text)'
        clojure: '(fn [text] (str text "{{planet}}" text))'
        lisp:    '(lambda (text) (format nil "~a{{planet}}~a" text text))'
        pwsh:    '"$($args[0]){{planet}}$($args[0])"'
        go:      'func(text string) string { return text + "{{planet}}" + text }'
    template: "<{{#lambda}}-{{/lambda}}>"
    expected: "<-Earth->"

  - name: Section - Alternate Delimiters
    desc: Lambdas used for sections should parse with the current delimiters.
    data:
      planet: "Earth"
      lambda: !code
        ruby:    'proc { |text| "#{text}{{planet}} => |planet|#{text}" }'
        raku:    'sub { $^section ~ q+{{planet}} => |planet|+ ~ $^section }'
        perl:    'sub { $_[0] . "{{planet}} => |planet|" . $_[0] }'
        js:      'function(txt) { return txt + "{{planet}} => |planet|" + txt }'
        php:     'return $text . "{{planet}} => |planet|" . $text;'
        python:  'lambda text: "%s{{planet}} => |planet|%s" % (text, text)'
        clojure: '(fn [text] (str text "{{planet}} => |planet|" text))'
        lisp:    '(lambda (text) (format nil "~a{{planet}} => |planet|~a" text text))'
        pwsh:    '"$($args[0]){{planet}} => |planet|$($args[0])"'
        go:      'func(text string) string { return text + "{{planet}} => |planet|" + text }'
    template: "{{= | | =}}<|#lambda|-|/lambda|>"
    expected: "<-{{planet}} => Earth->"

  - name: Section - Multiple Calls
    desc: Lambdas used for sections should not be cached.
    data:
      lambda: !code
        ruby:    'proc { |text| "__#{text}__" }'
        raku:    'sub { "__" ~ $^section ~ "__" }'
        perl:    'sub { "__" . $_[0] . "__" }'
        js:      'function(txt) { return "__" + txt + "__" }'
        php:     'return "__" . $text . "__";'
        python:  'lambda text: "__%s__" % (text)'
        clojure: '(fn [text] (str "__" text "__"))'
        lisp:    '(lambda (text) (format nil "__~a__" text))'
        pwsh:    '"__$($args[0])__"'
        go:      'func(text string) string { return "__" + text + "__" }'
    template: '{{#lambda}}FILE{{/lambda}} != {{#lambda}}LINE{{/lambda}}'
    expected: '__FILE__ != __LINE__'

  - name: Inverted Section
    desc: Lambdas used for inverted sections should be considered truthy.
    data:
      static: 'static'
      lambda: !code
        ruby:    'proc { |text| false }'
        raku:    'sub { 0 }'
        perl:    'sub { 0 }'
        js:      'function(txt) { return false }'
        php:     'return false;'
        python:  'lambda text: 0'
        clojure: '(fn [text] false)'
        lisp:    '(lambda (text) (declare (ignore text)) nil)'
        pwsh:    '$false'
        go:      'func(text string) bool { return false }'
    template: "<{{^lambda}}{{static}}{{/lambda}}>"
    expected: "<>"