Re: [Gnu-arch-users] "semantic" diff/merge tools

[Ludovic Courtès]

Thomas Lord <address@hidden> writes:

> Sexps denote abstract trees -- which is a semantics of a sort
> but a very weak one.   From the diff perspective, a bytestream
> denotes a list of lines, each a list of characters -- a semantics
> of comparable complexity.

Right, sexp are a simple semantics.  Yet, to the Scheme/Lisp programmer,
it might make sense to be able to visualize an sexp-diff (a tree diff in
fact) rather than that classic line-diff which is completely
disconnected from the actual data structure it is representing.

> In contrast, a semantically oriented application-specific diff/merge
> tool for a word processor document isn't performing generic operations
> on trees.   It would know, for example, not to try to recursively
> compare the contents of a tree node representing a text paragraph 
> to a tree node representing, say, a diagram.   It's the difficulty
> and expense of writing such rules, multiplied by the number of 
> present and future applications for which they are desirable, that
> I object too and that lead me to propose a family of data file formats
> that can make good use of generic diff/merge tools.

You rightfully note that document are actually trees.  I might add that
XML is really sexps with a different (weird) syntax.  This means that if
you have a tree differencer (which could be based on sexps), you're on
your way to having a document-aware diff tool.  Of course, more needs to
be done so that it knows more about what it is diffing.

> A "best of both worlds" approach is to use data file formats that
> can make good use of generic merge tools, but then permit (rather
> than "require") applications to provide non-generic alternatives.

That's one possibility.  For sexps/XML, for instance, I think the
tree diff approach may provide better results than per-line diff.

Thanks,
Ludovic.