Question

I’m starting to play with Fslex/Fsyacc. When trying to generate the parser using this input

Parser.fsy:

%{
open Ast
%}

// The start token becomes a parser function in the compiled code:
%start start

// These are the terminal tokens of the grammar along with the types of
// the data carried by each token:
%token <System.Int32> INT
%token <System.String> STRING
%token <System.String> ID
%token PLUS MINUS ASTER SLASH LT LT EQ GTE GT
%token LPAREN RPAREN LCURLY RCURLY LBRACKET RBRACKET COMMA
%token ARRAY IF THEN ELSE WHILE FOR TO DO LET IN END OF BREAK NIL FUNCTION VAR TYPE IMPORT PRIMITIVE
%token EOF

// This is the type of the data produced by a successful reduction of the 'start'
// symbol:
%type <Ast.Program> start

%%

// These are the rules of the grammar along with the F# code of the 
// actions executed as rules are reduced.  In this case the actions 
// produce data using F# data construction terms.
start: Prog { Program($1) }

Prog:
    | Expr EOF                  { $1 }

Expr: 
    // literals
    | NIL                   { Ast.Nil ($1) }
      | INT                       { Ast.Integer($1) }
    | STRING                    { Ast.Str($1) }

    // arrays and records
    | ID LBRACKET Expr RBRACKET OF Expr { Ast.Array ($1, $3, $6) } 
    | ID LCURLY AssignmentList RCURLY { Ast.Record ($1, $3) }

AssignmentList:
    | Assignment { [$1] }
    | Assignment COMMA AssignmentList {$1 :: $3 }

Assignment:
    | ID EQ Expr { Ast.Assignment ($1,$3) }

Ast.fs

namespace Ast
open System

type Integer = 
    | Integer of Int32

and Str = 
    | Str of string

and Nil = 
    | None

and Id = 
    | Id of string

and Array = 
    | Array of Id * Expr * Expr

and Record = 
    | Record of Id * (Assignment list)

and Assignment = 
    | Assignment of Id * Expr

and Expr =
    | Nil
    | Integer
    | Str
    | Array
    | Record

and Program =
    | Program of Expr

Fsyacc reports the following error: “FSYACC: error FSY000: An item with the same key has already been added.”

I believe the problem is in the production for AssignmentList, but can’t find a way around…

Any tips will be appreciated

Answer 1

Hate answering my own questions, but the problem was here (line 15 of parser input file)

%token PLUS MINUS ASTER SLASH LT LT EQ GTE GT

Note the double definition (should have been LTE)

My vote goes for finding a way to improve the output of the Fslex/Fsyacc executables/msbuild tasks