[types] Add type annotation to aid translation.

MyPy and mycpp are working!

Fewer compile errors in examples/lexer_main.

Update type checking manifest too.

frontend/reader.py

@@ -32,7 +32,8 @@ def GetLine(self):
     # type: () -> Tuple[int, Optional[str], int]
     line = self._GetLine()
     if line is None:
-      return -1, None, 0
+      eof_line = None  # type: Optional[str]
+      return -1, eof_line, 0
 
     line_id = self.arena.AddLine(line, self.line_num)
     self.line_num += 1
@@ -113,7 +114,9 @@ def __init__(self, lines, arena):
   def GetLine(self):
     # type: () -> Tuple[int, Optional[str], int]
     if self.pos == self.num_lines:
-      return -1, None, 0
+      eof_line = None  # type: Optional[str]
+      return -1, eof_line, 0
+
     line_id, line, start_offset = self.lines[self.pos]
 
     self.pos += 1

mycpp/mylib.h

@@ -355,13 +355,15 @@ namespace mylib {  // MyPy artifact
 class File {
  public:
   virtual void write(Str* s) = 0;
+  virtual Str* readline() = 0;
 };
 
 class Buf : public File {
  public:
   Buf() : data_(nullptr), len_(0) {
   };
   virtual void write(Str* s);
+  virtual Str* readline();
   Str* getvalue() { return new Str(data_, len_); }
 
  private:
@@ -376,6 +378,7 @@ class CFile : public File {
   CFile(FILE* f) : f_(f) {
   };
   virtual void write(Str* s);
+  virtual Str* readline();
 
  private:
   FILE* f_;

types/osh-parse-manifest.txt

@@ -1,5 +1,6 @@
 ./asdl/format.py
 ./asdl/pretty.py
+./asdl/pybase.py
 ./asdl/runtime.py
 ./bin/osh_parse.py
 ./core/alloc.py