Hello, Aaron Ecay <aarone...@gmail.com> writes:
> If this means “can it ever work?” then I think the answer is “yes it > can”. But I think the current implementation is broken and likely to > remain so for the foreseeable future. The issues are: > > 1. :cache only works for code which is a pure function of its header args > 2. When combined with :session, the environment that the code is evaluated > in is not created anew each time it is run. This makes it much easier > to leak references to (e.g.) variables defined in other blocks > 3. The proper notion of purity is not easily defined when the code does > things like modifying the emacs environment, touching the filesystem, > or accessing the language’s RNG. > 4. We (org devs) don’t actually understand how the semantics of cache > interacts with other babel features. See: > <http://mid.gmane.org/86fvqqc8jb....@somewhere.org>. > > 1-3 are likely to be extremely confusing for users, especially less > technically sophisticated ones (what’s a “pure function” anyway)? The > inability to give a clear introductory explanation of the feature in > combination with 4 indicating we don’t actually understand it ourselves > makes me feel like we should not be advertising, let alone recommending, > it. > > The only other literate programming environment that I know of that > implements such a feature is knitr (for R). They address these issues > by providing (optional) free-variable analysis to construct a dependency > graph between code blocks. There is also some handling of RNG seed > values. The documentation <http://yihui.name/knitr/demo/cache/> is much > more comprehensive, including a prominent statement about the dangers of > side effect-ful code and a nuanced discussion of several issues, > including the RNG. Thank you for the explanations. Assuming the user knows what s?he is doing (so I'm not bothered by issues 2 and 3), :cache is still a somewhat useful feature. I don't mind advertising it. Though, I agree we could include big fat warnings about it in the manual. Also, :cache might be a bit misleading as it implies more than what this feature offer, i.e., a dumb "don't update results if contents didn't change". I cannot think of a better name, tho. Regards, -- Nicolas Goaziou
