Add foreach for S-expression lists

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>

src/sexpr/sexpr_funcs.h

@@ -10,6 +10,17 @@ Author: Leonardo de Moura
 
 namespace lean {
 
+template<typename F>
+void foreach(sexpr const & l, F f) {
+    lean_assert(is_list(l));
+    sexpr const * h = &l;
+    while (!is_nil(*h)) {
+        lean_assert(is_cons(*h));
+        f(head(*h));
+        h = &tail(*h);
+    }
+}
+
 template<typename F>
 sexpr map(sexpr const & l, F f) {
     lean_assert(is_list(l));

src/tests/sexpr/sexpr.cpp

@@ -71,7 +71,7 @@ static void tst1() {
     lean_assert(!contains(sexpr{10,20,-2,0,10}, [](sexpr e) { return to_int(e) < -10; }));
 }
 
-void tst2() {
+static void tst2() {
     sexpr a;
     a = 2;
     lean_assert(a == sexpr(2));
@@ -108,9 +108,17 @@ void tst2() {
     lean_assert(sexpr(1, 2) == sexpr(1, sexpr(2)));
 }
 
+static void tst3() {
+    int sum = 0;
+    foreach(sexpr{0, 1, 2, 3, 4},
+            [&](sexpr const & e) { sum += to_int(e); });
+    lean_assert(sum == 10);
+}
+
 int main() {
     continue_on_violation(true);
     tst1();
     tst2();
+    tst3();
     return has_violations() ? 1 : 0;
 }