Joseph Ferano
ce1426d1e9
plan.org says Flan is Clojure's brackets and a small slice of its API, and
(let [{:keys [x y]} p] ...) is one of the most-used parts of that surface.
A struct is Flan's map, so {:keys [x y]} and {inner :field} read fields off
one; [a b] and [a b & rest] read a fixed array.
It desugars in parse.ml into the Let bindings and Field accesses that already
exist — the same trade dotimes makes. Ast.binding carries a name and nothing
else, so nothing downstream learns that a pattern exists: not Load's renaming,
not Check, not a backend. That is not only taste. Load matches Ast.pattern
exhaustively and Shim builds Ast.binding literally, and neither file is
editable from here, so an AST variant was never on the table.
The value goes into a temporary first. A pattern over a call must call it
once, and (let [{:keys [p]} p] ...) must read the old p rather than the one
it is halfway through rebinding. The temporaries are named with a ~, which
the reader treats as a delimiter, so no source symbol can collide with one.
The arity is the one thing the parser cannot settle — it is a type — so the
pattern's shape travels to check.ml as destructure~nth, which knows how many
elements the value has and lowers to an ordinary at.
