noob wants make a parser for a small language - parsing

Question

I want make a parser in happy for the let-in-expression language. For example, i want parse the following string:
let x = 4 in x*x
At the university we study attribute grammars, and i want use this tricks to calculate directly the value of the parsed let-in-expression. So in the happy file, i set the data type of the parsing function to Int, and i created a new attribute called env. This attribute is a function from String to Int that associates variable name to value. Referring to my example:
env "x" = 4
Now i put here below the happy file, where there is my grammar:
{
module Parser where
import Token
import Lexer
}
%tokentype { Token }
%token
let { TLet }
in { TIn }
int { TInt $$ }
var { TVar $$ }
'=' { TEq }
'+' { TPlus }
'-' { TMinus }
'*' { TMul }
'/' { TDiv }
'(' { TOB }
')' { TCB }
%name parse
%attributetype { Int }
%attribute env { String -> Int }
%error { parseError }
%%
Exp : let var '=' Exp in Exp
{
$4.env = $$.env;
$2.env = (\_ -> 0);
$6.env = (\str -> if str == $2 then $4 else 0);
$$ = $6;
}
| Exp1
{
$1.env = $$.env;
$$ = $1;
}
Exp1 : Exp1 '+' Term
{
$1.env = $$.env;
$2.env = $$.env;
$$ = $1 + $3;
}
| Exp1 '-' Term
{
$1.env = $$.env;
$2.env = $$.env;
$$ = $1 - $3;
}
| Term
{
$1.env = $$.env;
$$ = $1;
}
Term : Term '*' Factor
{
$1.env = $$.env;
$2.env = $$.env;
$$ = $1 * $3;
}
| Term '/' Factor
{
$1.env = $$.env;
$2.env = $$.env;
$$ = div $1 $3;
}
| Factor
{
$1.env = $$.env;
$$ = $1;
}
Factor
: int
{
$$ = $1;
}
| var
{
$$ = $$.env $1;
}
| '(' Exp ')'
{
$1.env = $$.env;
$$ = $1;
}
{
parseError :: [Token] -> a
parseError _ = error "Parse error"
}
When i load the haskell file generated from the happy file above, i get the following error:
Ambiguous occurrence `Int'
It could refer to either `Parser.Int', defined at parser.hs:271:6
or `Prelude.Int',
imported from `Prelude' at parser.hs:2:8-13
(and originally defined in `GHC.Types')
I don't know why i get this, because i don't define the type Parser.Int in my happy file. I tried to replace Int with Prelude.Int, but i get other errors.
How can i resolve? Can i have also some general tips if I'm doing something not optimal?

Answer 1

See the happy explaination of attributetype: http://www.haskell.org/happy/doc/html/sec-AtrributeGrammarsInHappy.html
Your line:
%attributetype { Int }
Is declaring a type named Int. This is what causes the ambiguity.