RFE: Add shorthand syntax for default macro values

[pmatilai]

A common use-case with macros is wanting to fallback to a default value if the macro in question doesn't exist, eg "use value of %foo if defined, or 5 otherwise". This seems unnecessarily clumsy in the current macro implementation: %{?foo}%{!?foo:5} and the new expression stuff doesn't seem to help here.

[mlschroe]

Just thinking out loud:

%{!foo:bar}
%{?foo-bar}

[pmatilai]

More thinking out loud, %{foo|bar} as a shorthand for the actual expression (%["%{?foo}" || "bar"])

[mlschroe]

Ohh, non-emptyness instead of plain definedness? Like in most shells: ${foo-bar} versus ${foo:-bar}...