[Haskell-beginners] computing multiple attributes in Happy

3 Apr 2012

      Hello,
I am trying out the happy parser.
Take the second example Binary to Decimal given on the happy site:
http://www.haskell.org/happy/doc/html/sec-AttributeGrammarExample.html

This example computes the decimal value equivalent to the given binary
string.

I want to modify it so that along with the decimal value, it should compute
a list of data values, like MyLeft, MyRight.
I have modified the grammar given on the site as shown below (in the end).

What I want to do is generate a list of MyLeft, MyRight such that MyLeft
denotes 0 and MyRight denotes 1.
So, if my input is "1011\n" then I expect the list attribute to be:
[MyRight, Myleft, MyRight, MyRight]

The happy compiles this thing without any problem, ghci compiles and loads
it correctly. But when I run the following command I always get only the
decimal value and not the list:

So my questions are:

   1. *How shall one compute, access multiple attributes in happy first? **(use
   in Haskell code, write to file etc)*
   2. *How shall one access any one or multiple of the computed attributes
   in GHCi then?*
   3. *Am I computing the list correctly?
   *

Thanks in advance.
     kak

Here is my happy grammar code:
-----------------------------
{
module BitsParser (parse) where
test = parse "1011\n"

-- how to write the list attribute to a file here?

data Dirs = MyLeft | MyRight deriving Show
fun a b = a^b
}

%tokentype { Char }

%token minus { '-' }
%token plus  { '+' }
%token one   { '1' }
%token zero  { '0' }
%token newline { '\n' }

%attributetype { Attrs }
%attribute value { Integer }
%attribute pos   { Int }
%attribute list   { [Dirs] }

%name parse start

%%

start
   : num newline { $$ = $1 }

num
   : bits        { $$ = $1       ; $1.pos = 0 ; $1.list = [] }
   | plus bits   { $$ = $2       ; $2.pos = 0 ; $2.list = [] }
   | minus bits  { $$ = negate $2; $2.pos = 0 ; $2.list = [] }

bits
   : bit         { $$ = $1
                 ; $1.pos = $$.pos ; $1.list =  $$.list
                 }

   | bits bit    { $$ = $1 + $2 ; $$.list = $1.list ++ $2.list
                 ; $1.pos = $$.pos + 1
                 ; $2.pos = $$.pos
                 }

bit
   : zero        { $$ = 0 ; $$.list = [MyLeft] }
   | one         { $$ = fun 2 ($$.pos) ; $$.list = [MyRight] }

{
happyError = error "parse error"
}

[Haskell-beginners] computing multiple attributes in Happy

kak dod