I have written a function in SML that is giving me this compilation error: Can you help me fix this code so i will not get compilation errors when loading into the compiler.

datatype token = DI | EQ | ID of string | MI | PL | TI val isLetter = fn : char -> bool val isOp = fn : char -> bool val isWhiteSpace = fn : char -> bool val getOpToken = fn : char -> token p4.sml:35.10-35.15 Error: syntax error: replacing WHILE with EQUALOP [unexpected exception: Compile]

uncaught exception Compile [Compile: "syntax error"] raised at: ../compiler/Parse/main/smlfile.sml:19.24-19.46 ../compiler/TopLevel/interact/evalloop.sml:45.54 ../compiler/TopLevel/interact/evalloop.sml:306.20-306.23 ../compiler/TopLevel/interact/interact.sml:65.39-65.42

This is my SML code:

datatype token = ID of string | EQ | PL | MI | TI | DI;

(* Helper function to check if a character is a valid letter *)

fun isLetter(c: char) = c >= #"a" andalso c <= #"z" orelse c >= #"A" andalso c <= #"Z";

(* Helper function to check if a character is a valid operation character *)

fun isOp(c: char) = c = #"+" orelse c = #"-" orelse c = #"*" orelse c = #"/" orelse c = #"=";

(* Helper function to check if a character is whitespace *)

fun isWhiteSpace(c: char) = c = #" " orelse c = #"\t" orelse c = #" ";

(* Helper function to get the token of an operation character *)

fun getOpToken(c: char) : token =

if c = #"+" then PL

else if c = #"-" then MI

else if c = #"*" then TI

else if c = #"/" then DI

else EQ;

(* Main parse function *)

fun parse (fileName: string) : token list =

let

(* Read the file into a string *)

val file = TextIO.openIn fileName

val fileStr = TextIO.inputAll file

val len = String.size fileStr

(* Helper function to parse the string recursively *)

fun parseHelper (i: int, acc: token list) : token list =

if i = len then acc

else if isWhiteSpace(String.sub(fileStr, i)) then parseHelper(i + 1, acc)

else if isLetter(String.sub(fileStr, i)) then

(* Build the string of letters *)

let

val j = ref (i + 1)

while !j < len andalso isLetter(String.sub(fileStr, !j)) orelse j := !j + 1

val id = String.substring(fileStr, i, !j - i)

in parseHelper(!j, acc @ [ID id]) end

else if isOp(String.sub(fileStr, i)) then

parseHelper(i + 1, acc @ [getOpToken(String.sub(fileStr, i))])

else (* Invalid character *)

(print "Compilation error"; [])

in

TextIO.closeIn file;

parseHelper(0, [])

end;