Added 'else if' support.

language/interpreter.cc

@@ -56,12 +56,19 @@ private:
     }
 
     static Value eval_if(const Ast& ast, Env& env) {
-        auto cond = eval(*ast.nodes[0], env);
-        if (cond.to_bool()) {
-            return eval(*ast.nodes[1], env);
-        } else if (ast.nodes.size() > 2) {
-            return eval(*ast.nodes[2], env);
+        const auto& nodes = ast.nodes;
+
+        for (auto i = 0u; i < nodes.size(); i += 2) {
+            if (i + 1 == nodes.size()) {
+                return eval(*nodes[i], env);
+            } else {
+                auto cond = eval(*nodes[i], env);
+                if (cond.to_bool()) {
+                    return eval(*nodes[i + 1], env);
+                }
+            }
         }
+
         return Value();
     }
 

language/parser.cc

@@ -11,7 +11,7 @@ static auto g_grammar = R"(
     EXPRESSION          <-  ASSIGNMENT / PRIMARY
     ASSIGNMENT          <-  IDENTIFIER '=' _ EXPRESSION
     WHILE               <- 'while' _ EXPRESSION BLOCK
-    IF                  <- 'if' _ EXPRESSION BLOCK ('else' _ BLOCK)?
+    IF                  <- 'if' _ EXPRESSION BLOCK ('else' _ 'if' _ EXPRESSION BLOCK)* ('else' _ BLOCK)?
     FUNCTION            <-  'fn' _ PARAMETERS BLOCK
     PARAMETERS          <-  '(' _ IDENTIFIER* ')' _ 
     FUNCTION_CALL       <-  IDENTIFIER ARGUMENTS