{:keys [x y]} and {inner :field} over a struct, [a b] and [a & rest] over a
fixed array, nesting through each other. All of it becomes Let plus Field plus
at plus slice in parse.ml, so nothing downstream learns a pattern exists - the
same shape dotimes already has.
The constraint turned out to be stronger than "do not add IR". load.ml matches
Ast.pattern exhaustively with no wildcard and shim.ml builds Ast.binding as a
full record literal, and both files belong to other agents this session, with
warning 8 an error - so no new frontend shape was available either. The
desugaring is what fits through that, and it is the better answer anyway.
The value goes into a temporary named destructure~N. The tilde is a reader
delimiter, so no source symbol can collide with one, and (let [{a :a} a] ...)
therefore reads the old a. The one thing the parser cannot settle is arity, so
that travels to check.ml as a call to destructure~nth, which knows the array's
length - a name in call position is an open namespace check.ml already owns and
dispatches, which is why that is not the same compromise as tagging a pattern.
Sequential patterns over a *slice* are refused rather than lowered to a
bounds-checked at. [a b] over [2 f32] is a claim the checker settles; over [T]
it is a claim about a number that does not exist until runtime, and lowering it
would turn a compile-time-checkable pattern into a program that type checks and
then traps.
match over enums is left unshipped on the same reasoning, and that restraint is
worth recording: it is fully desugarable and wanted, but a keyword needs a case
in Ast.pattern, and the alternative - tagging Pctor (":lo", []) - puts a second
meaning into a field another file destructures as a constructor name. One line
in load.ml unblocks it for whoever owns that file. The old refusal blamed
milestone 2, which was never the reason; both paths now name the enum and say
what actually stops it.
Joseph Ferano
50f5ea4db8