The problems people have with XML is not the syntax. Also, how would you represent (optional) attributes in s-expression? I could think of a couple ideas but none that is nice.

Basic S-expressions also don't distinguish between lists and maps, which is something that turns out to be very convenient in practice. Sure, a map is just a list of pairs - but the deserializer needs to be aware of its meaning to parse it into the appropriate data structure. So you either need a schema even for the most trivial cases, or you need a distinct syntax.

EDN is a better Lisp-flavored candidate, IMO.

Basic S-exp syntax can easily be extended to denote dictionaries. Just like #(...) gives us vectors and #S structures, some #H can provide hash tables.

Of course, but then we're not talking about raw S-exps, but something built on top of them. And then you get something like EDN anyway.

> Of course, but then we're not talking about raw S-exps, but something built on top of them.

I don't understand what this means.

If you really needed, you could just define that the second list element is an attribute list/map, turning <foo bar="baz"><quux /></foo> into (foo ((bar "baz")) (quux)).

The problem with XML itself is being unnecessarily verbose (and thus difficult for both human and machine to read) for what's just a way to encode trees. Attributes are arguably XML's self-inflicted gunshot wound in the foot; you mainly need them because of visual noise caused by regular nodes.

I believe that more than being verbose (closing tags for example) it is that the whole spec is enormous, with entities and namespaces making it even more complex. Still we see that some of that is actually needed as various json path/schema projects show.

mapping a 1-1 s-expression translation on HTML/XML/JSON/YAML etc. solves nothing. YAML has (had) code execution security problem and parser incompatibility issues. HTML/XML actually need namespaces in a few cases. JSON is abused as "compile target".

There can be no one format that works for everything. This is why I like TOML, it is really good at what it tries to do and stops there.

There is no need for optional attributes in the first place, because you can always just do this:

  <thing>
     <optattr>42</optattr>
     ...
  </thing>

insteadof

  <thing optattr="42">
    ...
  </thing>

Attributes exist in order to create a more compact syntax within XML/HTML tags, in ironic recognition of these notations being horribly verbose.