Support else in if then conditions

source/parser.yy

@@ -59,7 +59,7 @@
 %token <std::string> IDENTIFIER "identifier"
 %token <std::int32_t> NUMBER "number"
 %token <bool> BOOLEAN
-%token IF THEN WHILE DO
+%token IF WHILE DO
 %token CONST VAR PROCEDURE
 %token BEGIN_BLOCK END_BLOCK
 %token LEFT_PAREN RIGHT_PAREN SEMICOLON DOT COMMA
@@ -67,6 +67,9 @@
 %token PLUS MINUS MULTIPLICATION DIVISION
 %token ASSIGNMENT COLON HAT AT
 
+%precedence THEN
+%precedence ELSE
+
 %type <std::unique_ptr<elna::source::integer_literal>> integer_literal;
 %type <std::unique_ptr<elna::source::boolean_literal>> boolean_literal;
 %type <std::unique_ptr<elna::source::constant_definition>> constant_definition;
@@ -169,6 +172,11 @@ if_statement:
             $$ = std::make_unique<elna::source::if_statement>(elna::source::make_position(@1),
                 std::move($2), std::move($4));
         }
+    | IF expression THEN statement ELSE statement
+        {
+            $$ = std::make_unique<elna::source::if_statement>(elna::source::make_position(@1),
+                std::move($2), std::move($4), std::move($6));
+        }
 pointer:
     integer_literal { $$ = std::move($1); }
     | boolean_literal { $$ = std::move($1); }