Support 'while when'

compile.c

@@ -623,7 +623,7 @@ CORD compile_statement(env_t *env, ast_t *ast)
         CORD body = compile_statement(scope, while_->body);
         if (loop_ctx.skip_label)
             body = CORD_all(body, "\n", loop_ctx.skip_label, ": continue;");
-        CORD loop = CORD_all("while (", compile(scope, while_->condition), ") {\n\t", body, "\n}");
+        CORD loop = CORD_all("while (", while_->condition ? compile(scope, while_->condition) : "yes", ") {\n\t", body, "\n}");
         if (loop_ctx.stop_label)
             loop = CORD_all(loop, "\n", loop_ctx.stop_label, ":;");
         return loop;

parse.c

@@ -983,10 +983,18 @@ PARSER(parse_while) {
     // while condition [do] [<indent>] body
     const char *start = pos;
     if (!match_word(&pos, "while")) return NULL;
+
+    const char *tmp = pos;
+    // Shorthand form: `while when ...`
+    if (match_word(&tmp, "when")) {
+        ast_t *when = expect(ctx, start, &pos, parse_when, "I expected a 'when' block after this");
+        if (!when->__data.When.else_body) when->__data.When.else_body = NewAST(ctx->file, pos, pos, Stop);
+        return NewAST(ctx->file, start, pos, While, .body=when);
+    }
     ast_t *condition = expect(ctx, start, &pos, parse_expr, "I don't see a viable condition for this 'while'");
     expect_str(ctx, start, &pos, ":", "I expected a ':' here");
     ast_t *body = expect(ctx, start, &pos, parse_opt_indented_block, "I expected a body for this 'while'"); 
-    const char *tmp = pos;
+    tmp = pos;
     whitespace(&tmp);
     return NewAST(ctx->file, start, pos, While, .condition=condition, .body=body);
 }

test/enums.tm

@@ -55,3 +55,9 @@ func main():
 	>> choose_text(Foo.Last("XX"))
 	= "else: Foo.Last(t=\"XX\")"
 
+	i := 1
+	cases := [Foo.One(1), Foo.One(2), Foo.Zero]
+	while when cases[i] is One(x):
+		>> x
+		i += 1
+

typecheck.c

@@ -948,7 +948,9 @@ type_t *get_type(env_t *env, ast_t *ast)
                     break;
                 }
             }
-            if (!any_unhandled)
+            // HACK: `while when ...` is handled by the parser adding an implicit
+            // `else: stop`, which has an empty source code span.
+            if (!any_unhandled && when->else_body->end > when->else_body->start)
                 code_err(when->else_body, "This 'else' block will never run because every tag is handled");
 
             type_t *else_t = get_type(env, when->else_body);