static bool Fson::save( Document const& document, std::string const& filename );

Pretty-prints document to filename using FsonWriter's default Fson dialect — 2-space indent, one member/element per line, key: value. Returns false on I/O failure, true otherwise.

Writes back everything the parser captured: comments in all positions (header, leading, trailing, dangling, footer; nested blocks), member and element order, quoted-vs-bare key form, -- disabled markers, |-block string form, and number lexemes exactly as written (1.50 stays 1.50).

if( !Fson::save( *result.document, "config.fson" ) )
  std::cerr << "cannot write config.fson\n";

For a stream, a string, a non-default dialect, or preferBareKeys, use FsonWriter directly.

%include and save

save writes only the host file. %include lines are reproduced verbatim; values merged in from included files are not written into the host. A programmatic setPath on a merged value promotes it to a host member — see Object — dotted paths.