lang: 배타 대여를 호출에 넘기는 것은 이동이 아니라 재대여다

&mut T 를 &mut T 파라미터에 넘기면 호출이 끝날 때 돌려받는다. 호출이 도는 동안 호출자는 그 값에 손댈 수 없으므로 별칭이 생기지 않는다. 이것이 없으면 배타 파라미터를 다시 넘기는 일이 함수당 한 번만 가능해서 &mut 가 사실상 쓸 수 없었다 -- 재귀 하강 파서를 쓰다가 걸렸다. 페이로드 없는 enum 은 이름 붙은 수라서 수로 읽을 수 있다. 반대 방향은 안 된다: 임의의 수는 변이가 아니다. calc 프로그램: 재귀 하강 수식 계산기. 우선순위, 괄호, 오류 전파. 1+2*3 = 7 (1+2)*3 = 9 2*(3+4)-5 = 9 10/3 = 3 1+ = error (1+2 = error
2026-08-17 06:45:19 +09:00
parent 206799d1cb
commit 432d073104
3 changed files with 146 additions and 1 deletions
@@ -156,9 +156,27 @@ static int compatible(FeType *want, FeType *got, FeNode *value)
        value->text[0] != '\'' && value->text[0] != '"';
 }

+/* Does passing `arg` to a parameter of type `param` lend it rather than give
+   it away? An exclusive borrow handed to a call comes back when the call
+   returns, so it is not a move. */
+static int call_reborrows(const FeType *param, const FeType *arg)
+{
+    if (!param || !arg) return 0;
+    if (param->kind==FE_TYPE_REF && arg->kind==FE_TYPE_REF &&
+        param->ref_mut && arg->ref_mut) return 1;
+    if (param->kind==FE_TYPE_SLICE && arg->kind==FE_TYPE_SLICE &&
+        param->ref_mut && arg->ref_mut) return 1;
+    return 0;
+}
+
 static int explicit_castable(FeType *a, FeType *b)
 {
    if (!a || !b) return 0;
+    /* An enum without a payload is a number with names on it, so reading it
+       as one is a widening or narrowing and nothing more. The other direction
+       is not allowed: an arbitrary number is not a variant. */
+    if (a->kind == FE_TYPE_ENUM && !a->fields &&
+        (fe_type_is_integer(b) || b->kind == FE_TYPE_CHAR)) return 1;
    return (fe_type_is_integer(a) || a->kind == FE_TYPE_CHAR) &&
           (fe_type_is_integer(b) || b->kind == FE_TYPE_CHAR);
 }
@@ -2820,6 +2838,11 @@ static FeType *check_call_args(FeCheckerState *s, FeNode *n, FeSym *sym,
        } else if (b && a && b->kind==FE_TYPE_SLICE && !b->ref_mut &&
                   a->kind==FE_TYPE_SLICE && a->ref_mut) {
            /* Call-only []mut -> [] weakening is a temporary view. */
+        } else if (call_reborrows(b, a)) {
+            /* Handing an exclusive borrow to a call lends it for the length of
+               that call and takes it back after: the caller cannot touch it
+               meanwhile, so nothing is aliased. Without this an exclusive
+               parameter could be passed onwards exactly once. */
        } else mark_moved(s,arg,a);
        if (!compatible(b, a, arg) &&
            !(b && a && b->kind==FE_TYPE_SLICE && a->kind==FE_TYPE_SLICE &&
@@ -2916,7 +2939,8 @@ static FeType *check_call(FeCheckerState *s, FeNode *n)
                if (root && root->borrow_root) root=root->borrow_root;
                if (root) fe_own_call_shared_view(c->diags,&root->own,arg->loc);
            } else if (!(b && a && b->kind==FE_TYPE_SLICE &&
-                         a->kind==FE_TYPE_SLICE && !b->ref_mut && a->ref_mut))
+                         a->kind==FE_TYPE_SLICE && !b->ref_mut && a->ref_mut) &&
+                       !call_reborrows(b, a))
                mark_moved(s,arg,arg->sem_type ? arg->sem_type : a);
            if (!fe_type_equal(b,a) && !m7_actual_compatible(b,a,arg) &&
                !(b && a && b->kind==FE_TYPE_SLICE && a->kind==FE_TYPE_SLICE &&
@@ -0,0 +1,120 @@
+// EXIT:0
+// OUTPUT:1+2*3 = 7
+// OUTPUT:(1+2)*3 = 9
+// OUTPUT:2*(3+4)-5 = 9
+// OUTPUT:10/3 = 3
+// OUTPUT:1+ = error
+// OUTPUT:(1+2 = error
+unit calc;
+import std.io;
+import std.fmt;
+
+// A recursive-descent evaluator over a byte slice.
+//
+// The position travels in a `&mut usize` rather than in a struct beside the
+// text: a struct cannot hold a slice, because a slice is a borrowed view and
+// R4 keeps borrows out of aggregate storage. Passing both is the honest way
+// to say "this text, and how far we have read".
+
+fn done(src: []u8, at: usize) -> bool { return at >= src.n; }
+
+fn peek(src: []u8, at: usize) -> u8 {
+    if done(src, at) { return 0; }
+    return src[at];
+}
+
+fn skip_spaces(src: []u8, at: &mut usize) -> void {
+    while not done(src, at.^) {
+        if src[at.^] != 32 { break; }
+        at.^ = at.^ + 1;
+    }
+}
+
+fn number(src: []u8, at: &mut usize) -> !i32 {
+    var value: i32 = 0;
+    var digits: usize = 0;
+    while not done(src, at.^) {
+        let c: u8 = src[at.^];
+        if c < 48 or c > 57 { break; }
+        value = value * 10 + ((c - 48) as i32);
+        digits = digits + 1;
+        at.^ = at.^ + 1;
+    }
+    if digits == 0 { return error.BadNumber; }
+    return value;
+}
+
+fn factor(src: []u8, at: &mut usize) -> !i32 {
+    skip_spaces(src, at);
+    if peek(src, at.^) == 40 {
+        at.^ = at.^ + 1;
+        let inner: i32 = try expr(src, at);
+        skip_spaces(src, at);
+        if peek(src, at.^) != 41 { return error.Unbalanced; }
+        at.^ = at.^ + 1;
+        return inner;
+    }
+    return number(src, at);
+}
+
+fn term(src: []u8, at: &mut usize) -> !i32 {
+    var left: i32 = try factor(src, at);
+    while true {
+        skip_spaces(src, at);
+        let op: u8 = peek(src, at.^);
+        if op != 42 and op != 47 { break; }
+        at.^ = at.^ + 1;
+        let right: i32 = try factor(src, at);
+        if op == 42 { left = left * right; }
+        else {
+            if right == 0 { return error.DivideByZero; }
+            left = left / right;
+        }
+    }
+    return left;
+}
+
+fn expr(src: []u8, at: &mut usize) -> !i32 {
+    var left: i32 = try term(src, at);
+    while true {
+        skip_spaces(src, at);
+        let op: u8 = peek(src, at.^);
+        if op != 43 and op != 45 { break; }
+        at.^ = at.^ + 1;
+        let right: i32 = try term(src, at);
+        if op == 43 { left = left + right; }
+        else { left = left - right; }
+    }
+    return left;
+}
+
+fn evaluate(text: []u8) -> !i32 {
+    var at: usize = 0;
+    let value: i32 = try expr(text, &mut at);
+    skip_spaces(text, &mut at);
+    if not done(text, at) { return error.Trailing; }
+    return value;
+}
+
+fn show(text: []u8) -> void {
+    var buf: [16]u8 = [0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0];
+    io.print(text);
+    io.print(" = ");
+    let value: i32 = evaluate(text) catch |e| {
+        io.print("error\n");
+        return;
+    };
+    let n: usize = fmt.fmt_i32(buf[..], value);
+    io.print(buf[0..n]);
+    io.print("\n");
+}
+
+fn main() -> i32 {
+    show("1+2*3");
+    show("(1+2)*3");
+    show("2*(3+4)-5");
+    show("10/3");
+    show("1+");
+    show("(1+2");
+    return 0;
+}