Support {...} interpolation in body

[josevalim]

The first commit has the code changes. The second commit runs mix format to migrate the codebase, so you can see if you prefer the new style or not.

cc @SteffenDE @chrismccord @connorlay @msaraiva

[wojtekmach]

For this template:

{if true do}

we currently receive this error:

** (TokenMissingError) token missing on iex:5:11:
    error: missing terminator: end
    │
  5 │ if true do
    │         │ └ missing closing delimiter (expected "end")
    │         └ unclosed delimiter
    │
    └─ iex:5:11

which is great but maybe there's still room to improve?

If we changed Code.string_to_quoted! to Code.string_to_quoted we could maybe improve the error message, telling people to use :if or <%= ... %>. The error tuple is quite expressive:

iex> Code.string_to_quoted("if true do")
{:error,
 {[
    opening_delimiter: :do,
    expected_delimiter: :end,
    line: 1,
    column: 9,
    end_line: 1,
    end_column: 11
  ], "missing terminator: end", ""}}

and I see it's like this since Elixir 1.16. Let me know if you'd like to pursue this and I'm happy to send a patch.

[bcardarella]

if the EEx syntax is going away it seem odd to keep it around just for comment tags. Why not a {% %} style?

[wojtekmach]

It is not going away though, it is not possible to do {if ... do}, {for ... do}, etc. (Not that people should reach for those given :if and :for exists.) We can't use {...} interpolation inside <script> and <style> either and for that <%= ... %> is the way to go.

[josevalim]

The EEx syntax is not going away per se because it is still needed in some situations (such as inside script, style, and template, as escaping { in there would be a pain) but I agree we should probably consider introducing something specific for comments later on.

[simonmcconnell]

The <%!-- --%> comment syntax was inspired by Surface's {!-- --}. If we're moving to curly braces in the HTML, I'd vote we use the curly comments too.

[bcardarella]

+1 for {!-- --} style