<html><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>4.4. Example Attribute Grammars</title><link rel="stylesheet" href="fptools.css" type="text/css"><meta name="generator" content="DocBook XSL Stylesheets V1.73.2"><link rel="start" href="index.html" title="Happy User Guide"><link rel="up" href="sec-AttributeGrammar.html" title="Chapter 4. Attribute Grammars"><link rel="prev" href="sec-AttrGrammarLimits.html" title="4.3. Limits of Happy Attribute Grammars"><link rel="next" href="sec-invoking.html" title="Chapter 5. Invoking Happy"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">4.4. Example Attribute Grammars</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="sec-AttrGrammarLimits.html">Prev</a> </td><th width="60%" align="center">Chapter 4. Attribute Grammars</th><td width="20%" align="right"> <a accesskey="n" href="sec-invoking.html">Next</a></td></tr></table><hr></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="sec-AttributeGrammarExample"></a>4.4. Example Attribute Grammars</h2></div></div></div><p>
The following two toy attribute grammars may prove instructive. The first is
an attribute grammar for the classic context-sensitive grammar
{ a^n b^n c^n | n >= 0 }. It demonstrates the use of conditionals,
inherited and synthesized attributes.
</p><pre class="programlisting">
{
module ABCParser (parse) where
}
%tokentype { Char }
%token a { 'a' }
%token b { 'b' }
%token c { 'c' }
%token newline { '\n' }
%attributetype { Attrs a }
%attribute value { a }
%attribute len { Int }
%name parse abcstring
%%
abcstring
: alist blist clist newline
{ $$ = $1 ++ $2 ++ $3
; $2.len = $1.len
; $3.len = $1.len
}
alist
: a alist
{ $$ = $1 : $2
; $$.len = $2.len + 1
}
| { $$ = []; $$.len = 0 }
blist
: b blist
{ $$ = $1 : $2
; $2.len = $$.len - 1
}
| { $$ = []
; where failUnless ($$.len == 0) "blist wrong length"
}
clist
: c clist
{ $$ = $1 : $2
; $2.len = $$.len - 1
}
| { $$ = []
; where failUnless ($$.len == 0) "clist wrong length"
}
{
happyError = error "parse error"
failUnless b msg = if b then () else error msg
}
</pre><p>
This grammar parses binary numbers and
calculates their value. It demonstrates
the use of inherited and synthesized attributes.
</p><pre class="programlisting">
{
module BitsParser (parse) where
}
%tokentype { Char }
%token minus { '-' }
%token plus { '+' }
%token one { '1' }
%token zero { '0' }
%token newline { '\n' }
%attributetype { Attrs }
%attribute value { Integer }
%attribute pos { Int }
%name parse start
%%
start
: num newline { $$ = $1 }
num
: bits { $$ = $1 ; $1.pos = 0 }
| plus bits { $$ = $2 ; $2.pos = 0 }
| minus bits { $$ = negate $2; $2.pos = 0 }
bits
: bit { $$ = $1
; $1.pos = $$.pos
}
| bits bit { $$ = $1 + $2
; $1.pos = $$.pos + 1
; $2.pos = $$.pos
}
bit
: zero { $$ = 0 }
| one { $$ = 2^($$.pos) }
{
happyError = error "parse error"
}
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