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special-form heads are not shadowable

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Found in a read/eval review: a local named like a special form wrongly took
over operator position. (let [if (fn ...)] (if true 1 2)) returned the fn, but
per spec section 3 (and the reference) special-form heads are not shadowable;
only macros are. Two fixes: drop the (not shadowed) guard on the special-form
branch of analyze-list (so an (if ...) head is always the special), and prefix
a local whose name is a Scheme keyword when emitting (so a value local legally
named if does not shadow the (if ...) the back end emits). Value-position
locals named if/or/case still work.
This commit is contained in:
Yogthos 2026-06-22 01:01:53 -04:00
parent f18ae3bd46
commit 212cd0399a
5 changed files with 106 additions and 81 deletions
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5
jolt-core/jolt/analyzer.clj
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@ -465,7 +465,10 @@
;; read -> macroexpand -> analyze. A local shadows both.
(and (form-sym? head) (not shadowed) (form-macro? ctx head))
(analyze ctx (form-expand-1 ctx form) env)
(and hname (not shadowed) (contains? handled hname))
;; special-form heads are NOT shadowable (unlike macros): a local named
;; `if` does not change the meaning of (if …) in operator position, per
;; spec §3 and the reference. No (not shadowed) guard here.
(and hname (contains? handled hname))
(analyze-special ctx hname items env)
(and hname (not shadowed) (method-head? hname))
(analyze-host-call ctx hname items env)

17
jolt-core/jolt/backend_scheme.clj
View file

@ -105,10 +105,23 @@
(def ^:private gensym-counter (atom 0))
(defn- fresh-label [prefix] (str prefix (swap! gensym-counter inc)))
;; Scheme syntactic keywords. A jolt local with one of these names would, when
;; emitted verbatim, shadow the Scheme form in operator position (a local named
;; `if` would turn the special form (if …) the back end emits into a call), so
;; such locals are prefixed. Matches the spec: special-form heads are not
;; shadowable, but a value local may legally be named `if`.
(def ^:private scheme-reserved
#{"if" "begin" "lambda" "let" "let*" "letrec" "letrec*" "quote" "quasiquote"
"unquote" "set!" "define" "define-syntax" "cond" "case" "when" "unless"
"and" "or" "do" "else" "guard" "parameterize" "delay" "values"})
;; Most jolt names are already valid Scheme identifiers. The one that isn't is
;; `#`, which jolt auto-gensyms use as a suffix (p1__0000X4# from #(...)) — `#`
;; starts a datum in Scheme, so replace it with `_`.
(defn- munge-name [s] (str/replace s "#" "_"))
;; starts a datum in Scheme, so replace it with `_`. A name that collides with a
;; Scheme keyword is prefixed with `_` so it can never shadow the emitted form.
(defn- munge-name [s]
(let [s (str/replace s "#" "_")]
(if (contains? scheme-reserved s) (str "_" s) s)))
(declare emit)