at: Python-style negative indexing#183
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Follow-up to PR #161 at builtin. Negative index wraps from end: at xs -1 returns last element, at xs -2 second-last, etc. Integer-only guard preserved. The at xs -(len xs) 1 workaround now becomes the much terser at xs -1.
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resolve_slice_bound, resolve_take_count, resolve_drop_count: small inline helpers that take (raw_i64, len_usize) and return the resolved prefix position. Negative inputs count from the end and clamp at -len, matching the semantics PR #183 already established for `at xs i`. Splitting these out of the engine call sites lets the tree-walker, VM, and Cranelift JIT helpers all dispatch through the same logic — no chance of one backend silently diverging on a `-len` edge case. 8 unit tests covering positive, -1, -len, beyond-len, len=0 boundaries.
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slc accepts negative start and end (each resolved against the input length), matching `at xs -1` from PR #183. take and drop accept negative counts: `take -k xs` keeps all but the last k, `drop -k xs` keeps only the last k. Both equivalent to Python's xs[:-k] and xs[-k:]. slc also picks up the same fract!=0 integer-validation check that take and drop already had — previously slc silently cast non-integer floats to usize. Brings the three slice builtins to parity with each other and with at. Routes through the resolve_* helpers in builtins.rs so the rule is defined once and reused by VM + JIT.
This was referenced May 16, 2026
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Follow-up to PR #161. Negative index wraps from end:
at xs -1returns last,at xs -2second-last, etc. Integer-only guard preserved (at xs -0.5still errors). The verboseat xs -(len xs) 1workaround becomesat xs -1. 17 new cross-engine tests; old 'negative errors' tests replaced per the design change.