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From: tmoran@bix.com (Tom Moran)
Subject: Re: Ada type (yacc)YYSTYPE is ?
Date: 1999/01/18
Message-ID: <36a39cec.1456120@news.pacbell.net>#1/1
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References: <77qu4d$jt8@lotho.delphi.com> <77vmri$sc3$1@nnrp1.dejanews.com>
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 18 Jan 1999 12:46:37 PDT)
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NNTP-Posting-Date: Mon, 18 Jan 1999 12:46:37 PDT
Newsgroups: comp.lang.ada
Date: 1999-01-18T00:00:00+00:00
List-Id: <comp.lang.ada>

>What I did for my token analysis packages is I created an abstract tagged
>"token" type. That way the tag acts as the variant, and you can put whatever
A BNF production like
  name : simple_name
might have an action like 
  {$$ := $1;}
which would translate into something like
  yyval := yy.value_stack(yy.tos);
where yyval is a YYStype and yy.value_stack is an array of YYStype.
Would you define
  type YYStype is Root_Token'Class;   ?
I don't quite understand just how you are using the tagged type.

   What I think I'd like is compile time, rather than execution time,
type checking.  Instead of a single YYStype I'd rather have different
ones for different tokens.  Thus for intance
  subtype name_YYSType is -- a bounded string
  subtype param_YYStype is -- some appropriate record structure
and
  name : simple_name
would do an assignment where the types are both name_YYStype while
  param : formal_name COLON type_name
would have code
  {$$ := make_param($1,$3);}
where
  function make_param(formal_name, type_name : name_YYStype) return
param_YYStype;
   name :  simple_name TIC attribute_name
might be coded simply as
  {$$ := $1 & "'" $ $3;}
since the type would be bounded string and "&" is available.
Has anyone done that?  Does it work in practice?