Support for if ... then ... else following #911. (#944)

executable_semantics/syntax/parser.ypp

@@ -134,6 +134,8 @@
 %type <Nonnull<Expression*>> and_expression
 %type <Nonnull<Expression*>> or_lhs
 %type <Nonnull<Expression*>> or_expression
+%type <Nonnull<Expression*>> statement_expression
+%type <Nonnull<Expression*>> if_expression
 %type <Nonnull<Expression*>> expression
 %type <Nonnull<GenericBinding*>> generic_binding
 %type <std::vector<Nonnull<AstNode*>>> deduced_params
@@ -216,6 +218,7 @@
   SEMICOLON
   SLASH
   STRING
+  THEN
   TRUE
   TYPE
   UNDERSCORE
@@ -477,12 +480,20 @@ or_expression:
           std::vector<Nonnull<Expression*>>({$1, $3}));
     }
 ;
-expression:
+statement_expression:
   ref_deref_expression
 | predicate_expression
 | and_expression
 | or_expression
 ;
+if_expression:
+  statement_expression
+| IF expression THEN if_expression ELSE if_expression
+    { $$ = arena->New<IfExpression>(context.source_loc(), $2, $4, $6); }
+;
+expression:
+  if_expression
+;
 designator: PERIOD identifier { $$ = $2; }
 ;
 paren_expression: paren_expression_base
@@ -645,11 +656,11 @@ clause_list:
     }
 ;
 statement:
-  expression EQUAL expression SEMICOLON
+  statement_expression EQUAL expression SEMICOLON
     { $$ = arena->New<Assign>(context.source_loc(), $1, $3); }
 | VAR pattern EQUAL expression SEMICOLON
     { $$ = arena->New<VariableDefinition>(context.source_loc(), $2, $4); }
-| expression SEMICOLON
+| statement_expression SEMICOLON
     { $$ = arena->New<ExpressionStatement>(context.source_loc(), $1); }
 | if_statement
     { $$ = $1; }