Re: new parser: error recovery needs work

[dewar at gnat dot com (Robert Dewar)]

> (I think i heard something about carat errors being on
> the way,

A little note here. I assume carat error means errors where there is a pointer
to the column of the error.

What we found very useful in GNAT was to define the following routines:

   procedure Error_Msg_AP (Msg : String);
   --  Output a message just after the previous token. This routine can be
   --  called only from the parser, since it references Prev_Token_Ptr.

   procedure Error_Msg_BC (Msg : String);
   --  Output a message just before the current token. Note that the important
   --  difference between this and the previous routine is that the BC case
   --  posts a flag on the current line, whereas AP can post a flag at the
   --  end of the preceding line. This routine can be called only from the
   --  parser, since it references Token_Ptr.

   procedure Error_Msg_SC (Msg : String);
   --  Output a message at the start of the current token, unless we are at
   --  the end of file, in which case we always output the message after the
   --  last real token in the file. This routine can be called only from the
   --  parser, since it references Token_Ptr.

   procedure Error_Msg_SP (Msg : String);
   --  Output a message at the start of the previous token. This routine can
   --  be called only from the parser, since it references Prev_Token_Ptr.

This allows very precise placement of the message pointers. One danger of
message pointers is that if they are not exactly right, they can obfuscate
rather than clarify the error.