Creating Simple Parser in F#

Question

I am currently attempting to create an extremely simple parser in F# using FsLex and FsYacc. At first, the only functionality I am trying to achieve is allowing the program to take in a string that represents addition of integers and output a result. For example, I would want the parser to be able to take in "5 + 2" and output the string "7". I am only interested in string arguments and outputs because I would like to import the parser into Excel using Excel DNA once I extend the functionality to support more operations. However, I am currently struggling to get even this simple integer addition to work correctly.

My lexer.fsl file looks like:

{
module lexer
open System
open Microsoft.FSharp.Text.Lexing
open Parser

let lexeme = LexBuffer<_>.LexemeString

let ops = ["+", PLUS;] |> Map.ofList
}

let digit = ['0'-'9']
let operator = "+"
let integ = digit+

rule lang = parse
    | integ 
    {INT(Int32.Parse(lexeme lexbuf))}
    | operator 
    {ops.[lexeme lexbuf]}

My parser.fsy file looks like:

%{
open Program
%}

%token <int>INT
%token PLUS

%start input
%type <int> input

%%

input: 
    exp {$1}
;

exp: 
    | INT { $1 }
    | exp exp PLUS { $1 + $2 }
;

Additionally, I have a Program.fs file that acts like an (extremely small) AST:

module Program

type value = 
    | Int of int

type op = Plus

Finally, I have the file Main.fs that is supposed to test out the functionality of the interpreter (as well as import the function into Excel).

module Main

open ExcelDna.Integration
open System
open Program
open Parser

[<ExcelFunction(Description = "")>]
    let main () =
    let x = "5 + 2"
    let lexbuf = Microsoft.FSharp.Text.Lexing.LexBuffer<_>.FromString x
    let y = input lexer.lang lexbuf
    y

However, when I run this function, the parser doesn't work at all. When I build the project, the parser.fs and lexer.fs files are created correctly. I feel that there is something simple I am missing, but I have no idea how to make this function correctly.

Creating Simple Parser in F#

0 Answers0