parser.fs

//
// Parser for simple C programs.  This component checks 
// the input program to see if it meets the syntax rules
// of simple C.  The parser returns a string denoting
// success or failure. 
//
// Returns: the string "success" if the input program is
// legal, otherwise the string "syntax_error: ..." is
// returned denoting an invalid simple C program.
//
// <<YOUR NAME>>
//
// Original author:
//   Prof. Joe Hummel
//   U. of Illinois, Chicago
//   CS 341, Spring 2022
//

namespace compiler

module parser =
  //
  // NOTE: all functions in the module must be indented.
  //

  //
  // matchToken
  //
  let private matchToken expected_token tokens =
    //
    // if the next token matches the expected token,  
    // keep parsing by returning the rest of the tokens.
    // Otherwise throw an exception because there's a 
    // syntax error, effectively stopping compilation
    // at the first error.
    //
    let next_token = List.head tokens

    if expected_token = next_token then  
      List.tail tokens
    else
      failwith ("expecting " + expected_token + ", but found " + next_token)


  //
  // simpleC
  //
  let private simpleC tokens = 
    matchToken "$" tokens


  //
  // parse tokens
  //
  // Given a list of tokens, parses the list and determines
  // if the list represents a valid simple C program.  Returns
  // the string "success" if valid, otherwise returns a 
  // string of the form "syntax_error:...".
  //
  let parse tokens = 
    try
      let result = simpleC tokens
      "success"
    with 
      | ex -> "syntax_error: " + ex.Message