adding comments and some newlines (#294)

* adding comments and some newlines

* Update executable_semantics/interpreter/typecheck.cpp

Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com>

* Update executable_semantics/interpreter/typecheck.cpp

Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com>

* Update executable_semantics/interpreter/typecheck.cpp

Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com>

* Update executable_semantics/interpreter/typecheck.cpp

Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com>

* Update executable_semantics/interpreter/typecheck.cpp

Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com>

* changed comment style

* Update executable_semantics/interpreter/typecheck.cpp

Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com>

* Update executable_semantics/interpreter/typecheck.cpp

Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com>

Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com>

executable_semantics/interpreter/typecheck.cpp

@@ -132,9 +132,26 @@ auto ReifyType(Value* t, int line_num) -> Expression* {
   }
 }
 
+// The TypeCheckExp function performs semantic analysis on an expression.
+// It returns a new version of the expression, its type, and an
+// updated environment which are bundled into a TCResult object.
+// The purpose of the updated environment is
+// to bring pattern variables into scope, for example, in a match case.
+// The new version of the expression may include more information,
+// for example, the type arguments deduced for the type parameters of a
+// generic.
+//
+// e is the expression to be analyzed.
+// env maps variable names to the type of their run-time value.
+// ct_env maps variable names to their compile-time values. It is not
+//    directly used in this function but is passed to InterExp.
+// expected is the type that this expression is expected to have.
+//    This parameter is non-null when the expression is in a pattern context
+//    and it is used to implement `auto`, otherwise it is null.
+// context says what kind of position this expression is nested in,
+//    whether it's a position that expects a value, a pattern, or a type.
 auto TypeCheckExp(Expression* e, TypeEnv* env, Env* ct_env, Value* expected,
-                  TCContext context)
-    -> TCResult {  //                   expected can be null
+                  TCContext context) -> TCResult {
   switch (e->tag) {
     case ExpressionKind::PatternVariable: {
       if (context != TCContext::PatternContext) {
@@ -389,6 +406,13 @@ auto TypecheckCase(Value* expected, Expression* pat, Statement* body,
   return std::make_pair(pat, res.stmt);
 }
 
+// The TypeCheckStmt function performs semantic analysis on a statement.
+// It returns a new version of the statement and a new type environment.
+//
+// The ret_type parameter is used for analyzing return statements.
+// It is the declared return type of the enclosing function definition.
+// If the return type is "auto", then the return type is inferred from
+// the first return statement.
 auto TypeCheckStmt(Statement* s, TypeEnv* env, Env* ct_env, Value* ret_type)
     -> TCStatement {
   if (!s) {