vm: prescriptive error for '+' on lists (#680)#682
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`[1,2] + [3,4]` reported `cannot apply '+' to list and list` — correct but not actionable. A Python/JS prior lands here (observed live: an AI given only a syntax primer wrote `xs + [v]` and crashed) and has to guess the idiom. The message now names the fix, mirroring the existing mixed-type f-string hint: cannot apply '+' to list and list (use 'append of [xs, v]' to add an element, or 'concat of [a, b]' to join two lists) Cold error path (rt_error), no hot-path cost. dict+num and other non-list cases keep the generic message; mixed str+X keeps the f-string hint. First of the prescriptive-error pass (#680): at points where the language diverges from mainstream priors, teach the fix — helps humans and AI alike, and a context-rich AI stumbling there is a lower bound on human friction. Regression: EE20b. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
… our own Contributors work from forks we can't push to, so if our merge stales their PR the only fixes are asking them to rebase or the fetch-reland dance (#657). We rebase our own for free. So: land ready contributor PRs first, then rebase ours. Folded here rather than a standalone docs PR to avoid a wasted CI cycle. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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First concrete output of the prescriptive-error pass (#680).
What
[1,2] + [3,4]reportedcannot apply '+' to list and list— correct, but not actionable. A Python/JS prior lands here and has to guess the idiom. The message now names the fix, mirroring the existing mixed-type f-string hint:Scope / safety
rt_error) — no hot-path cost.dict + numand other non-list mismatches keep the generic message; mixedstr + Xkeeps the f-string hint (verified).appendandconcatare both real builtins, so the suggestions are valid.Why
At points where the language diverges from mainstream priors, errors should teach the fix — for humans and AI alike. A context-rich AI stumbling here is a lower bound on human friction (it was observed live: an AI given only a syntax primer wrote
xs + [v]and crashed). This is the first site; the catalog of others is in #680.Tests
test_error_extra.eigsEE20b pins the new prescriptive text (asserts the message names bothappend ofandconcat of). Full suite 3113/3113 release, 3134/3134 ASan+leaks.Closes #680 is premature — this is one site of several; references #680.
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