add ternary operator

Author: trillllian

docs/src/math.md

@@ -77,8 +77,9 @@ Asar supports the 6 usual comparison operators:
 | --- | --- |
 | <code>a \|\| b</code> | Returns 1 if at least one of `a` and `b` evaluates to true |
 | `a && b` | Returns 1 if both of `a` and `b` evaluate to true |
+| `?(a, b, c)` | Returns `b` if `a` is true, or `c` if `a` is false |
 
-These operators are lazy: they will not evaluate the right-hand argument if the result is already determined by the left-hand argument. (Specifically, `1 || anything` immediately returns `1` and doesn't evaluate `anything`, and similarly, `0 && anything` immediately returns `0`.)
+These operators are lazy: they will not evaluate the right-hand argument if the result is already determined by the left-hand argument. More specifically, `1 || anything` immediately returns `1` and doesn't evaluate `anything`; `0 && anything` immediately returns `0`; and `?(1, x, anything)` returns `x` without evaluating `anything`.
 
 ## Operator precedence
 

src/asar/math_ast.h

@@ -61,6 +61,17 @@ class math_ast_unop : public math_ast_node {
 	int get_len(bool could_be_bank_ex) const;
 };
 
+class math_ast_ternary_cond : public math_ast_node {
+	owned_node m_cond, m_true, m_false;
+public:
+	math_ast_ternary_cond(owned_node cond_in, owned_node true_in, owned_node false_in)
+	: m_cond(std::move(cond_in)), m_true(std::move(true_in)), m_false(std::move(false_in)) {}
+
+	math_val evaluate(const eval_context& ctx) const;
+	int has_label() const;
+	int get_len(bool could_be_bank_ex) const;
+};
+
 class math_ast_literal : public math_ast_node {
 	math_val m_value;
 	int m_len;

src/asar/math_eval.cpp

@@ -231,6 +231,22 @@ int math_ast_unop::get_len(bool could_be_bank_ex) const {
 	return m_arg->get_len(false);
 }
 
+math_val math_ast_ternary_cond::evaluate(const eval_context& ctx) const {
+	if(m_cond->evaluate(ctx).get_bool()) {
+		return m_true->evaluate(ctx);
+	} else {
+		return m_false->evaluate(ctx);
+	}
+}
+
+int math_ast_ternary_cond::has_label() const {
+	return m_cond->has_label() | m_true->has_label() | m_false->has_label();
+}
+
+int math_ast_ternary_cond::get_len(bool could_be_bank_ex) const {
+	return std::max(m_true->get_len(false), m_false->get_len(false));
+}
+
 math_val math_ast_label::evaluate(const eval_context &ctx) const {
 	if (m_cur_ns && labels.exists(m_cur_ns + m_labelname)) {
 		return math_val::make_identifier(m_cur_ns + m_labelname);

src/asar/math_parse.cpp

@@ -64,7 +64,7 @@ owned_node parse_context::parse_atom() {
 	}
 	if (is_ualpha(*str) || *str=='.' || *str=='?') {
 		const char * start=str;
-		while (is_ualnum(*str) || *str == '.') str++;
+		while (is_ualnum(*str) || *str == '.' || *str == '?') str++;
 		int len=(int)(str-start);
 		while (*str==' ') str++;
 		if (*str=='(') {
@@ -80,6 +80,16 @@ owned_node parse_context::parse_atom() {
 				if(*str == ',') str++;
 			}
 			str++;
+			// ternary is basically just a function called "?"
+			if(func_name == "?") {
+				if(arguments.size() != 3) {
+					throw_err_block(2, err_argument_count, 3, (int)arguments.size());
+				}
+				return std::make_unique<math_ast_ternary_cond>(
+					std::move(arguments[0]),
+					std::move(arguments[1]),
+					std::move(arguments[2]));
+			}
 			return std::make_unique<math_ast_function_call>(std::move(arguments), std::move(func_name));
 		} else {
 			string name_part(start, len);

tests/ternary.asm

@@ -0,0 +1,4 @@
+assert ?(1, "a", "b") == "a"
+assert ?(0, "a", "b") == "b"
+assert ?(1, 1, 1/0) == 1 ; short-circuiting
+assert ?(1, "a", 0) == "a" ; mixed types