avoid duplication of side effects in `upf`?

[gasche]

The README says:

Note that ppx_update is simple, so if you update with a long expression, that expression will be computed twice.

Maybe you could fix this in the syntax extension so that users don't have to worry about it?

Currently you generate code that looks like if <lexpr>.f == <rexpr> then () else <lexpr>.f <- <rexpr>, but I suppose you could just as easily generate let field = <rexpr> and obj = <lexpr> in if obj.f == field then () else obj.f <- field, which does not suffer from this issue.