# fold-1: fold becomes an imperative construct (first syntax-reform constuct)

[vpisarev]

fold is now a thin imperative sugar over for:

fold <acc> = <init> for <clauses> { body }
  ==>  { var <acc> = init; for <clauses> { body }; <acc> }

The accumulator is a real mutable var, ASSIGNED in the body (acc = f(acc,x) / acc += x), not yielded as the tail value; the whole-fold value is the accumulator after the loop. Multiple accumulators are multiple vars (fold a=0, b=1 for ..). break/continue/return are legal in the body (it is a plain for). A closure in the body captures the accumulator VAR. The named reduction sugars (all/exists/count/find/find_opt/filter/vector, spelled name(for ...)) are a separate construct, unchanged.

Highlights (full history in docs/fold1_report.md):

• Simultaneous tuple assignment (a,b) = (b,a+b): a parse-time desugar (materialize RHS into a temp before any store) — the Fibonacci/swap idiom.
• FB-023 fixed: a latent C-gen soundness hole (single-use temp reading mutable memory inlined past an aliasing store), surfaced by the swap; fixed with a recursive movement_unsafe_read guard in find_single_use_vals.
• Token-precise spans: the reform-prep-1 batch spans now give each token its own span; a side win is 12 negative goldens with tighter, correct carets.
• The whole 68K-line corpus AND the self-hosting compiler migrated to the new form (census docs/fold_census.md: 261 sites, zero closure-captures-accumulator sites, so the var-capture semantics change is behavior-neutral). Compiler migration handled a silent-bug class where an accumulator name shadows its own iteration collection / a body val — every such site renames the accumulator.
• An old-style value body warns "'fold' accumulator 's' is never assigned … the body must UPDATE the accumulator ('s = ' / 's += ...')".

Verification: full ladder green (unit 168, negative 90, T4 IR 63, cfold, corpus O0/O3); bootstrap fixpoint holds (the compiler self-builds deterministically after migrating its own folds); -pr-resolve census unaffected.

CLAUDE.md fold gotcha rewritten; language_changes_brief.md §2 -> IMPLEMENTED.