source-academy / py-slang / 36323013500

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Commit Message

fix(py2js): pair/llist/head/tail preserve sharing (#476)

In py2js, the linked-list builtins went through the generic stdlib
bridge, which converts every argument to a CSE Value and the result back
into new PyLists. So head(p) returned a fresh copy of p's head on every
call, and pair(x, x) stored two copies of x.

That broke everything depending on identity:
- draw_data drew shared substructure as separate copies, e.g.
  pair(head(xs), pair(head(xs), ...)) drew head(xs) twice
- set_head/set_tail on a value obtained via head/tail mutated a
  throwaway copy: after y = pair(x, x); set_head(head(y), 99),
  head(x) was still 1

pair, llist, head and tail now handle well-formed calls natively on the
caller's own PyValues. Any other call (wrong argument count, non-pair to
head/tail) falls through to the generic bridge, so every error message
is unchanged.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

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