From my cold, dead hands

Back at the PLT Scheme blog, Matthias Felleisen has just posted an entry on the rationale behind the limitations imposed to DrScheme’s REPL, namely, the inability to evaluate individual expressions. If you’ve read this blog before, you won’t be surprised to know that, as much as i respect Matthias, i totally disagree with his rationale. Well, not totally: let me explain. Matthias’ point is that allowing on the go redefinitions often leads to inconsistent states of the running image, which in turn leads to confusion and much head scratching, even when that head belongs to, say, Dan Friedman (let alone Matthias’ students’ heads, or mine, for that matter). Therefore, we’d better ban the offending feature from the development environment, right?

I’m willing to concede that evaluation of individual expressions is as dangerous as Matthias’ experience suggests, or even demonstrates. But i strongly take issue with his conclusion. Incremental development might be dangerous, but it’s also a powerful tool, and incredibly fun (all three come together most of the time, don’t they?) as any Common Lisp, Factor, Smaltalk, Scheme48 or Guile (to mention a few) hacker will tell you. As yours truly has been telling you since this blog started. Or PLT hackers, for that matter: Matthias’ post originates in this interesting email exchange, with people giving good reasons on behalf of an incremental REPL (think, for instance, of manipulating large chunks of intermediate data). I want this power. I want the fun. I’m willing to take the risk.

In my opinion, this is a good illustration of one of the points that Gerry Sussman was making in his talk on robust systems at ILC. For instance, injudicious use of generic functions will quickly paint you into a corner; but, at the same time, this sharp knife will cut for you beauties such as the scmutils library used in SICM, or help you in taming evolving requirements. Should we be deprived of generics for our own good?

Now, don’t take me wrong. I immensely appreciate the warnings and advice of people who, as Matthias, have much more experience and knowledge under their belts than i’ll ever have. I’m willing (eager, even) to learn from them. But i don’t understand why warnings should become prohibitions. I’m not dissing the work of Robby implementing DrScheme’s ‘restart-the-world’ functionality: it’s great to have it there (in fact, having it there as a safety net is yet another reason for allowing the incremental REPL variety). And i would have no objection if not having an incremental REPL were due to technical implementation details. What i fail to see is why undercutting the functionality of my tools in the name of ‘my own good’ is the right thing to do. I hated it when my parents did that to me ;-).

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