# Breaking Changes From Market Commons to Market Commons ⅠⅠ

The following is a list of known incompatibilities between documents
written in the original Market Commons format and a Market Commons ⅠⅠ
processor.

- Market Commons ⅠⅠ occasionally throws errors! Most of these are
  related to Declaration of Jargon processing, but you should be aware
  of the fact that not every call to the parse function will
  necessarily succeed (and compensate for this fact accordingly). In
  comparison with the original Market Commons, one noticable source of
  errors is literals with invalid X·M·L content, like
  `` `| <tag> |` content `| </tag> |` ``, which is no longer permitted
  (see below).

  **Rationale:** Sometimes this is better!

- Spaces are now allowed between any two line‐initial sigils, including
  in section and heading chunks.

  **Rationale:** It is simpler to keep this behaviour consistent, and
  the old behaviour allowed for the easy specification of empty
  headings (which is not something which should be easy). Also and
  importantly, this allows for the use of sigils which repeat a single
  character multiple times.

- You can no longer specify attributes for a “default” block.

  **Rationale:** It wasn’t really obvious that a line beginning with a
  `{` was actually setting the attributes for the paragraph.

- Literals are now processed differently :—

  - Literal contents must now be welformed X·M·L `content`.

  - Newlines are not allowed in inline literals.

    **Rationale:** This makes literal behaviour more predictable and
    better conform to expectations.

- Support for media elements has changed and been reduced; for
  simplicity, Market Commons ⅠⅠ only supports `<img>` and not `<video>`
  or multiple sources. Use inline or block literals if you need this
  functionality.

  **Rationale:** The way media was handled was pretty magic and
  difficult to generalize in an extensible manner.

- Ruby support has been modified to work like other inline elements.
  Instead of `.| base | text |.`, write `.| base }| text |} |.` (or
  `.| {| base |{}| text |} |.`).

  **Rationale:** The original ruby implementation was always finicky
  and may have had bugs.

- Market Commons ⅠⅠ preserves references (like `&this;`) instead of
  escaping the ampersand. (Note that H·T·M·L entities like `&aelig;`
  are _not_ defined by default in Market Commons ⅠⅠ.) This can be
  combined with Market Commons ⅠⅠ Declarations of Jargon to define
  reusable components for documents. Backslash‐escapes like `\U+2764\`
  are no longer supported; use character references like `&#x2764;`
  instead.

  **Rationale:** Entity references are a powerful capability of X·M·L
  and form a better solution for re·usable content than anything Market
  Commons could offer. There’s no point in maintaining two ways of
  escaping characters; if X·M·L references are supported, then they
  should be used. This frees up `\` as a potential sigil and makes
  inserting literal whitespace into documents simpler.

- Empty inline elements can now be specified using a `#` surrounded by
  sigils.

  **Rationale:** Having a distinct empty element syntax makes it
  clearer when a lack of content is intentional. In particular, this
  allows distinguishing between TEXTTO inlines with empty contents, and
  TEXTTO inlines with no content.

- `||` is no longer supported as a way of specifying linebreaks.
  Instead, use `\#\`.

  **Rationale:** Having a special syntax for just one kind of empty
  element is silly. `||` was always kind of a weird special case in
  processing anyway.

- Sections are now closed by some number of `|` pipe characters,
  instead of `/` solidi. For _some_ backwards‐compatibility, `/` has
  been defined as a `<div>` whose title will be sent to the
  `data-title` property in the default Declaration of Jargon.

  **Rationale:** This frees up `/` as an ordinary sigil instead of
  making it a special case. Importantly, `/| |/` was already being used
  as an inline tag.

- Whitespace behaviour has been made more sensible, which means that
  whitespace output may differ in some cases.

  **Rationale:** The original behaviour was just “what is easy to
  program in Racket”.

- Various changes in the formatting of output (which should not affect
  the rendered result). In general, less whitespace has been
  introduced, to enable Market Commons ⅠⅠ to be used more readily in
  contexts where whitespace is significant.