parsing - Creating Simple Parser in F# -

i attempting create extremely simple parser in f# using fslex , fsyacc. @ first, functionality trying achieve allowing program take in string represents addition of integers , output result. example, want parser able take in "5 + 2" , output string "7". interested in string arguments , outputs because import parser excel using excel dna once extend functionality support more operations. however, struggling simple integer addition 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, have program.fs file acts (extremely small) ast:

module program  type value =      | int of int  type op = plus 

finally, have file main.fs supposed test out functionality of interpreter (as import function 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 run function, parser doesn't work @ all. when build project, parser.fs , lexer.fs files created correctly. feel there simple missing, have no idea how make function correctly.