Custom writer: default to single quotes for strings

Makes the code more consistent and makes it easier to use double quotes
in strings, which is the usual quoting style used for HTML attributes.

Closes: #7487

data/sample.lua

@@ -14,7 +14,7 @@
 -- syntax errors.
 
 local pipe = pandoc.pipe
-local stringify = (require "pandoc.utils").stringify
+local stringify = (require 'pandoc.utils').stringify
 
 -- The global variable PANDOC_DOCUMENT contains the full AST of
 -- the document which is going to be written. It can be used to
@@ -25,19 +25,19 @@ local meta = PANDOC_DOCUMENT.meta
 -- `image_format` meta value.
 local image_format = meta.image_format
   and stringify(meta.image_format)
-  or "png"
+  or 'png'
 local image_mime_type = ({
-    jpeg = "image/jpeg",
+    jpeg = 'image/jpeg',
-    jpg = "image/jpeg",
+    jpg = 'image/jpeg',
-    gif = "image/gif",
+    gif = 'image/gif',
-    png = "image/png",
+    png = 'image/png',
-    svg = "image/svg+xml",
+    svg = 'image/svg+xml',
   })[image_format]
-  or error("unsupported image format `" .. image_format .. "`")
+  or error('unsupported image format `' .. image_format .. '`')
 
 -- Character escaping
 local function escape(s, in_attribute)
-  return s:gsub("[<>&\"']",
+  return s:gsub('[<>&"\']',
     function(x)
       if x == '<' then
         return '&lt;'
@@ -60,7 +60,7 @@ end
 local function attributes(attr)
   local attr_table = {}
   for x,y in pairs(attr) do
-    if y and y ~= "" then
+    if y and y ~= '' then
       table.insert(attr_table, ' ' .. x .. '="' .. escape(y,true) .. '"')
     end
   end
@@ -72,7 +72,7 @@ local notes = {}
 
 -- Blocksep is used to separate block elements.
 function Blocksep()
-  return "\n\n"
+  return '\n\n'
 end
 
 -- This function is called once for the whole document. Parameters:
@@ -106,31 +106,31 @@ function Str(s)
 end
 
 function Space()
-  return " "
+  return ' '
 end
 
 function SoftBreak()
-  return "\n"
+  return '\n'
 end
 
 function LineBreak()
-  return "<br/>"
+  return '<br/>'
 end
 
 function Emph(s)
-  return "<em>" .. s .. "</em>"
+  return '<em>' .. s .. '</em>'
 end
 
 function Strong(s)
-  return "<strong>" .. s .. "</strong>"
+  return '<strong>' .. s .. '</strong>'
 end
 
 function Subscript(s)
-  return "<sub>" .. s .. "</sub>"
+  return '<sub>' .. s .. '</sub>'
 end
 
 function Superscript(s)
-  return "<sup>" .. s .. "</sup>"
+  return '<sup>' .. s .. '</sup>'
 end
 
 function SmallCaps(s)
@@ -142,33 +142,33 @@ function Strikeout(s)
 end
 
 function Link(s, tgt, tit, attr)
-  return "<a href='" .. escape(tgt,true) .. "' title='" ..
+  return '<a href="' .. escape(tgt,true) .. '" title="' ..
-         escape(tit,true) .. "'" .. attributes(attr) .. ">" .. s .. "</a>"
+         escape(tit,true) .. '"' .. attributes(attr) .. '>' .. s .. '</a>'
 end
 
 function Image(s, src, tit, attr)
-  return "<img src='" .. escape(src,true) .. "' title='" ..
+  return '<img src="' .. escape(src,true) .. '" title="' ..
-         escape(tit,true) .. "'/>"
+         escape(tit,true) .. '"/>'
 end
 
 function Code(s, attr)
-  return "<code" .. attributes(attr) .. ">" .. escape(s) .. "</code>"
+  return '<code' .. attributes(attr) .. '>' .. escape(s) .. '</code>'
 end
 
 function InlineMath(s)
-  return "\\(" .. escape(s) .. "\\)"
+  return '\\(' .. escape(s) .. '\\)'
 end
 
 function DisplayMath(s)
-  return "\\[" .. escape(s) .. "\\]"
+  return '\\[' .. escape(s) .. '\\]'
 end
 
 function SingleQuoted(s)
-  return "&lsquo;" .. s .. "&rsquo;"
+  return '&lsquo;' .. s .. '&rsquo;'
 end
 
 function DoubleQuoted(s)
-  return "&ldquo;" .. s .. "&rdquo;"
+  return '&ldquo;' .. s .. '&rdquo;'
 end
 
 function Note(s)
@@ -184,11 +184,11 @@ function Note(s)
 end
 
 function Span(s, attr)
-  return "<span" .. attributes(attr) .. ">" .. s .. "</span>"
+  return '<span' .. attributes(attr) .. '>' .. s .. '</span>'
 end
 
 function RawInline(format, str)
-  if format == "html" then
+  if format == 'html' then
     return str
   else
     return ''
@@ -200,8 +200,8 @@ function Cite(s, cs)
   for _,cit in ipairs(cs) do
     table.insert(ids, cit.citationId)
   end
-  return "<span class=\"cite\" data-citation-ids=\"" .. table.concat(ids, ",") ..
+  return '<span class="cite" data-citation-ids="' .. table.concat(ids, ',') ..
-    "\">" .. s .. "</span>"
+    '">' .. s .. '</span>'
 end
 
 function Plain(s)
@@ -209,16 +209,16 @@ function Plain(s)
 end
 
 function Para(s)
-  return "<p>" .. s .. "</p>"
+  return '<p>' .. s .. '</p>'
 end
 
 -- lev is an integer, the header level.
 function Header(lev, s, attr)
-  return "<h" .. lev .. attributes(attr) ..  ">" .. s .. "</h" .. lev .. ">"
+  return '<h' .. lev .. attributes(attr) ..  '>' .. s .. '</h' .. lev .. '>'
 end
 
 function BlockQuote(s)
-  return "<blockquote>\n" .. s .. "\n</blockquote>"
+  return '<blockquote>\n' .. s .. '\n</blockquote>'
 end
 
 function HorizontalRule()
@@ -234,39 +234,39 @@ function CodeBlock(s, attr)
   -- If code block has class 'dot', pipe the contents through dot
   -- and base64, and include the base64-encoded png as a data: URL.
   if attr.class and string.match(' ' .. attr.class .. ' ',' dot ') then
-    local img = pipe("base64", {}, pipe("dot", {"-T" .. image_format}, s))
+    local img = pipe('base64', {}, pipe('dot', {'-T' .. image_format}, s))
     return '<img src="data:' .. image_mime_type .. ';base64,' .. img .. '"/>'
   -- otherwise treat as code (one could pipe through a highlighter)
   else
-    return "<pre><code" .. attributes(attr) .. ">" .. escape(s) ..
+    return '<pre><code' .. attributes(attr) .. '>' .. escape(s) ..
-           "</code></pre>"
+           '</code></pre>'
   end
 end
 
 function BulletList(items)
   local buffer = {}
   for _, item in pairs(items) do
-    table.insert(buffer, "<li>" .. item .. "</li>")
+    table.insert(buffer, '<li>' .. item .. '</li>')
   end
-  return "<ul>\n" .. table.concat(buffer, "\n") .. "\n</ul>"
+  return '<ul>\n' .. table.concat(buffer, '\n') .. '\n</ul>'
 end
 
 function OrderedList(items)
   local buffer = {}
   for _, item in pairs(items) do
-    table.insert(buffer, "<li>" .. item .. "</li>")
+    table.insert(buffer, '<li>' .. item .. '</li>')
   end
-  return "<ol>\n" .. table.concat(buffer, "\n") .. "\n</ol>"
+  return '<ol>\n' .. table.concat(buffer, '\n') .. '\n</ol>'
 end
 
 function DefinitionList(items)
   local buffer = {}
   for _,item in pairs(items) do
     local k, v = next(item)
-    table.insert(buffer, "<dt>" .. k .. "</dt>\n<dd>" ..
+    table.insert(buffer, '<dt>' .. k .. '</dt>\n<dd>' ..
-                   table.concat(v, "</dd>\n<dd>") .. "</dd>")
+                   table.concat(v, '</dd>\n<dd>') .. '</dd>')
   end
-  return "<dl>\n" .. table.concat(buffer, "\n") .. "\n</dl>"
+  return '<dl>\n' .. table.concat(buffer, '\n') .. '\n</dl>'
 end
 
 -- Convert pandoc alignment to something HTML can use.
@@ -303,13 +303,13 @@ function Table(caption, aligns, widths, headers, rows)
   local function add(s)
     table.insert(buffer, s)
   end
-  add("<table>")
+  add('<table>')
-  if caption ~= "" then
+  if caption ~= '' then
-    add("<caption>" .. escape(caption) .. "</caption>")
+    add('<caption>' .. escape(caption) .. '</caption>')
   end
   if widths and widths[1] ~= 0 then
     for _, w in pairs(widths) do
-      add('<col width="' .. string.format("%.0f%%", w * 100) .. '" />')
+      add('<col width="' .. string.format('%.0f%%', w * 100) .. '" />')
     end
   end
   local header_row = {}
@@ -317,7 +317,7 @@ function Table(caption, aligns, widths, headers, rows)
   for i, h in pairs(headers) do
     local align = html_align(aligns[i])
     table.insert(header_row,'<th align="' .. align .. '">' .. h .. '</th>')
-    empty_header = empty_header and h == ""
+    empty_header = empty_header and h == ''
   end
   if not empty_header then
     add('<tr class="header">')
@@ -326,9 +326,9 @@ function Table(caption, aligns, widths, headers, rows)
     end
     add('</tr>')
   end
-  local class = "even"
+  local class = 'even'
   for _, row in pairs(rows) do
-    class = (class == "even" and "odd") or "even"
+    class = (class == 'even' and 'odd') or 'even'
     add('<tr class="' .. class .. '">')
     for i,c in pairs(row) do
       add('<td align="' .. html_align(aligns[i]) .. '">' .. c .. '</td>')
@@ -340,7 +340,7 @@ function Table(caption, aligns, widths, headers, rows)
 end
 
 function RawBlock(format, str)
-  if format == "html" then
+  if format == 'html' then
     return str
   else
     return ''
@@ -348,7 +348,7 @@ function RawBlock(format, str)
 end
 
 function Div(s, attr)
-  return "<div" .. attributes(attr) .. ">\n" .. s .. "</div>"
+  return '<div' .. attributes(attr) .. '>\n' .. s .. '</div>'
 end
 
 -- The following code will produce runtime warnings when you haven't defined
@@ -358,6 +358,6 @@ local meta = {}
 meta.__index =
   function(_, key)
     io.stderr:write(string.format("WARNING: Undefined function '%s'\n",key))
-    return function() return "" end
+    return function() return '' end
   end
 setmetatable(_G, meta)

test/writer.custom

@@ -5,7 +5,7 @@ John Gruber’s markdown test suite.</p>
 
 <h1 id="headers">Headers</h1>
 
-<h2 id="level-2-with-an-embedded-link">Level 2 with an <a href='/url' title=''>embedded link</a></h2>
+<h2 id="level-2-with-an-embedded-link">Level 2 with an <a href="/url" title="">embedded link</a></h2>
 
 <h3 id="level-3-with-emphasis">Level 3 with <em>emphasis</em></h3>
 
@@ -525,7 +525,7 @@ Blah
 
 <p>This is <strong>strong</strong>, and so <strong>is this</strong>.</p>
 
-<p>An <em><a href='/url' title=''>emphasized link</a></em>.</p>
+<p>An <em><a href="/url" title="">emphasized link</a></em>.</p>
 
 <p><strong><em>This is strong and em.</em></strong></p>
 
@@ -560,7 +560,7 @@ So is &lsquo;pine.&rsquo;</p>
 <p>&lsquo;He said, &ldquo;I want to go.&rdquo;&rsquo; Were you alive in the
 70’s?</p>
 
-<p>Here is some quoted &lsquo;<code>code</code>&rsquo; and a &ldquo;<a href='http://example.com/?foo=1&amp;bar=2' title=''>quoted link</a>&rdquo;.</p>
+<p>Here is some quoted &lsquo;<code>code</code>&rsquo; and a &ldquo;<a href="http://example.com/?foo=1&amp;bar=2" title="">quoted link</a>&rdquo;.</p>
 
 <p>Some dashes: one—two — three—four — five.</p>
 
@@ -660,70 +660,70 @@ So is &lsquo;pine.&rsquo;</p>
 
 <h2 id="explicit">Explicit</h2>
 
-<p>Just a <a href='/url/' title=''>URL</a>.</p>
+<p>Just a <a href="/url/" title="">URL</a>.</p>
 
-<p><a href='/url/' title='title'>URL and title</a>.</p>
+<p><a href="/url/" title="title">URL and title</a>.</p>
 
-<p><a href='/url/' title='title preceded by two spaces'>URL and title</a>.</p>
+<p><a href="/url/" title="title preceded by two spaces">URL and title</a>.</p>
 
-<p><a href='/url/' title='title preceded by a tab'>URL and title</a>.</p>
+<p><a href="/url/" title="title preceded by a tab">URL and title</a>.</p>
 
-<p><a href='/url/' title='title with &quot;quotes&quot; in it'>URL and title</a></p>
+<p><a href="/url/" title="title with &quot;quotes&quot; in it">URL and title</a></p>
 
-<p><a href='/url/' title='title with single quotes'>URL and title</a></p>
+<p><a href="/url/" title="title with single quotes">URL and title</a></p>
 
-<p><a href='/url/with_underscore' title=''>with_underscore</a></p>
+<p><a href="/url/with_underscore" title="">with_underscore</a></p>
 
-<p><a href='mailto:nobody@nowhere.net' title=''>Email link</a></p>
+<p><a href="mailto:nobody@nowhere.net" title="">Email link</a></p>
 
-<p><a href='' title=''>Empty</a>.</p>
+<p><a href="" title="">Empty</a>.</p>
 
 <h2 id="reference">Reference</h2>
 
-<p>Foo <a href='/url/' title=''>bar</a>.</p>
+<p>Foo <a href="/url/" title="">bar</a>.</p>
 
-<p>With <a href='/url/' title=''>embedded [brackets]</a>.</p>
+<p>With <a href="/url/" title="">embedded [brackets]</a>.</p>
 
-<p><a href='/url/' title=''>b</a> by itself should be a link.</p>
+<p><a href="/url/" title="">b</a> by itself should be a link.</p>
 
-<p>Indented <a href='/url' title=''>once</a>.</p>
+<p>Indented <a href="/url" title="">once</a>.</p>
 
-<p>Indented <a href='/url' title=''>twice</a>.</p>
+<p>Indented <a href="/url" title="">twice</a>.</p>
 
-<p>Indented <a href='/url' title=''>thrice</a>.</p>
+<p>Indented <a href="/url" title="">thrice</a>.</p>
 
 <p>This should [not][] be a link.</p>
 
 <pre><code>[not]: /url</code></pre>
 
-<p>Foo <a href='/url/' title='Title with &quot;quotes&quot; inside'>bar</a>.</p>
+<p>Foo <a href="/url/" title="Title with &quot;quotes&quot; inside">bar</a>.</p>
 
-<p>Foo <a href='/url/' title='Title with &quot;quote&quot; inside'>biz</a>.</p>
+<p>Foo <a href="/url/" title="Title with &quot;quote&quot; inside">biz</a>.</p>
 
 <h2 id="with-ampersands">With ampersands</h2>
 
-<p>Here’s a <a href='http://example.com/?foo=1&amp;bar=2' title=''>link with an ampersand in the URL</a>.</p>
+<p>Here’s a <a href="http://example.com/?foo=1&amp;bar=2" title="">link with an ampersand in the URL</a>.</p>
 
-<p>Here’s a link with an amersand in the link text: <a href='http://att.com/' title='AT&amp;T'>AT&amp;T</a>.</p>
+<p>Here’s a link with an amersand in the link text: <a href="http://att.com/" title="AT&amp;T">AT&amp;T</a>.</p>
 
-<p>Here’s an <a href='/script?foo=1&amp;bar=2' title=''>inline link</a>.</p>
+<p>Here’s an <a href="/script?foo=1&amp;bar=2" title="">inline link</a>.</p>
 
-<p>Here’s an <a href='/script?foo=1&amp;bar=2' title=''>inline link in pointy braces</a>.</p>
+<p>Here’s an <a href="/script?foo=1&amp;bar=2" title="">inline link in pointy braces</a>.</p>
 
 <h2 id="autolinks">Autolinks</h2>
 
-<p>With an ampersand: <a href='http://example.com/?foo=1&amp;bar=2' title='' class="uri">http://example.com/?foo=1&amp;bar=2</a></p>
+<p>With an ampersand: <a href="http://example.com/?foo=1&amp;bar=2" title="" class="uri">http://example.com/?foo=1&amp;bar=2</a></p>
 
 <ul>
 <li>In a list?</li>
-<li><a href='http://example.com/' title='' class="uri">http://example.com/</a></li>
+<li><a href="http://example.com/" title="" class="uri">http://example.com/</a></li>
 <li>It should.</li>
 </ul>
 
-<p>An e-mail address: <a href='mailto:nobody@nowhere.net' title='' class="email">nobody@nowhere.net</a></p>
+<p>An e-mail address: <a href="mailto:nobody@nowhere.net" title="" class="email">nobody@nowhere.net</a></p>
 
 <blockquote>
-<p>Blockquoted: <a href='http://example.com/' title='' class="uri">http://example.com/</a></p>
+<p>Blockquoted: <a href="http://example.com/" title="" class="uri">http://example.com/</a></p>
 </blockquote>
 
 <p>Auto-links should not occur here: <code>&lt;http://example.com/&gt;</code></p>
@@ -740,7 +740,7 @@ So is &lsquo;pine.&rsquo;</p>
 <img src="lalune.jpg" id="" alt="lalune"/><figcaption>lalune</figcaption>
 </figure>
 
-<p>Here is a movie <img src='movie.jpg' title=''/> icon.</p>
+<p>Here is a movie <img src="movie.jpg" title=""/> icon.</p>
 
 <hr/>
 
@@ -774,7 +774,7 @@ footnote (as with list items).</p>
 lazy and just indent the first line of each block. <a href="#fnref2">&#8617;</a></p></li>
 <li id="fn3"><p>This
 is <em>easier</em> to type. Inline notes may contain
-<a href='http://google.com' title=''>links</a> and <code>]</code> verbatim characters,
+<a href="http://google.com" title="">links</a> and <code>]</code> verbatim characters,
 as well as [bracketed text]. <a href="#fnref3">&#8617;</a></p></li>
 <li id="fn4"><p>In quote. <a href="#fnref4">&#8617;</a></p></li>
 <li id="fn5"><p>In list. <a href="#fnref5">&#8617;</a></p></li>