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.

{
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
}

This grammar parses binary numbers and calculates their value. It demonstrates the use of inherited and synthesized attributes.

{
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"
}