(* Lexer specification for Ligo, to be processed by [ocamllex]. *)

{
(* START HEADER *)

type lexeme = string

(* STRING PROCESSING *)

(* The value of [mk_str len p] ("make string") is a string of length
   [len] containing the [len] characters in the list [p], in reverse
   order. For instance, [mk_str 3 ['c';'b';'a'] = "abc"]. *)

let mk_str (len: int) (p: char list) : string =
  let bytes = Bytes.make len ' ' in
  let rec fill i = function
         [] -> bytes
  | char::l -> Bytes.set bytes i char; fill (i-1) l
  in fill (len-1) p |> Bytes.to_string

(* The call [explode s a] is the list made by pushing the characters
   in the string [s] on top of [a], in reverse order. For example,
   [explode "ba" ['c';'d'] = ['a'; 'b'; 'c'; 'd']]. *)

let explode s acc =
  let rec push = function
    0 -> acc
  | i -> s.[i-1] :: push (i-1)
in push (String.length s)

(* LEXER ENGINE *)

(* Resetting file name and line number in the lexing buffer

   The call [reset ~file ~line buffer] modifies in-place the lexing
   buffer [buffer] so the lexing engine records that the file
   associated with [buffer] is named [file], and the current line is
   [line]. This function is useful when lexing a file that has been
   previously preprocessed by the C preprocessor, in which case the
   argument [file] is the name of the file that was preprocessed,
   _not_ the preprocessed file (of which the user is not normally
   aware). By default, the [line] argument is [1].
*)

let reset_file ~file buffer =
  let open Lexing in
  buffer.lex_curr_p <- {buffer.lex_curr_p with pos_fname = file}

let reset_line line_num buffer =
  let open Lexing in
  buffer.lex_curr_p <- {buffer.lex_curr_p with pos_lnum = line_num}

let reset ~file ?(line=1) buffer =
  (* Default value per the [Lexing] standard module convention *)
  reset_file ~file buffer; reset_line line buffer

(* Rolling back one lexeme _within the current semantic action_ *)

let rollback buffer =
  let open Lexing in
  let len = String.length (lexeme buffer) in
  let pos_cnum = buffer.lex_curr_p.pos_cnum - len in
  buffer.lex_curr_pos <- buffer.lex_curr_pos - len;
  buffer.lex_curr_p <- {buffer.lex_curr_p with pos_cnum}

(* ALIASES *)

let sprintf = Printf.sprintf

(* TOKENS *)

(* The signature [TOKEN] exports an abstract type [token], so a lexer
   can be a functor over tokens. Consequently, generic functions to
   construct tokens are provided. Note predicate [is_eof], which
   caracterises the virtual token for end-of-file, because it requires
   special handling. *)

module type TOKEN =
  sig
    type token

    (* Errors *)

    type   int_err = Non_canonical_zero
    type ident_err = Reserved_name

    (* Injections *)

    val mk_string : lexeme -> Region.t -> token
    val mk_bytes  : lexeme -> Region.t -> token
    val mk_int    : lexeme -> Region.t -> (token,   int_err) result
    val mk_ident  : lexeme -> Region.t -> (token, ident_err) result
    val mk_constr : lexeme -> Region.t -> token
    val mk_sym    : lexeme -> Region.t -> token
    val eof       : Region.t -> token

    (* Predicates *)

    val is_string : token -> bool
    val is_bytes  : token -> bool
    val is_int    : token -> bool
    val is_ident  : token -> bool
    val is_kwd    : token -> bool
    val is_constr : token -> bool
    val is_sym    : token -> bool
    val is_eof    : token -> bool

    (* Projections *)

    val to_lexeme : token -> lexeme
    val to_string : token -> ?offsets:bool -> [`Byte | `Point] -> string
    val to_region : token -> Region.t
  end

(* The module type for lexers is [S]. *)

module type S = sig
  module Token : TOKEN
  type token = Token.token

  type file_path = string
  type logger = Markup.t list -> token -> unit

  val output_token :
    ?offsets:bool -> [`Byte | `Point] ->
    EvalOpt.command -> out_channel -> logger

  type instance = {
    read     : ?log:logger -> Lexing.lexbuf -> token;
    buffer   : Lexing.lexbuf;
    get_pos  : unit -> Pos.t;
    get_last : unit -> Region.t;
    close    : unit -> unit
  }

  val open_token_stream : file_path option -> instance

  (* Error reporting *)

  exception Error of Error.t Region.reg

  val print_error :
    ?offsets:bool -> [`Byte | `Point] -> Error.t Region.reg -> unit

  (* Standalone tracer *)

  val trace :
    ?offsets:bool -> [`Byte | `Point] ->
    file_path option -> EvalOpt.command -> unit
end

(* The functorised interface

   Note that the module parameter [Token] is re-exported as a
   submodule in [S].
 *)

module Make (Token: TOKEN) : (S with module Token = Token) =
  struct
    module Token = Token
    type token = Token.token

    type file_path = string

    (* THREAD FOR STRUCTURED CONSTRUCTS (STRINGS, COMMENTS) *)

    (* When scanning structured constructs, like strings and comments,
       we need to keep the region of the opening symbol (like double
       quote, "#" or "(*") in order to report any error more
       precisely. Since ocamllex is byte-oriented, we need to store
       the parsed bytes as characters in an accumulator [acc] and
       also its length [len], so, we are done, it is easy to build the
       string making up the structured construct with [mk_str] (see
       above).

       The resulting data structure is called a _thread_.
       (Note for Emacs: "*)".)
    *)

    type thread = {
      opening : Region.t;
      len     : int;
      acc     : char list
    }

    let push_char char {opening; len; acc} = {opening; len=len+1; acc=char::acc}

    let push_string str {opening; len; acc} =
      {opening;
       len = len + String.length str;
       acc = explode str acc}

    (* STATE *)

    (* Beyond tokens, the result of lexing is a state (a so-called
       _state monad_). The type [state] represents the logical state
       of the lexing engine, that is, a value which is threaded during
       scanning and which denotes useful, high-level information
       beyond what the type [Lexing.lexbuf] in the standard library
       already provides for all generic lexers.

        Tokens are the smallest units used by the parser to build the
       abstract syntax tree. The state includes a queue of recognised
       tokens, with the markup at the left of its lexeme until either
       the start of the file or the end of the previously recognised
       token.

         The markup from the last recognised token or, if the first
       token has not been recognised yet, from the beginning of the
       file is stored in the field [markup] of the state because it is
       a side-effect, with respect to the output token list, and we
       use a record with a single field [units] because that record
       may be easily extended during the future maintenance of this
       lexer.

         The state also includes a field [pos] which holds the current
       position in the Ligo source file. The position is not always
       updated after a single character has been matched: that depends
       on the regular expression that matched the lexing buffer.

         The fields [decoder] and [supply] offer the support needed
       for the lexing of UTF-8 encoded characters in comments (the
       only place where they are allowed in Ligo). The former is the
       decoder proper and the latter is the effectful function
       [supply] that takes a byte, a start index and a length and feed
       it to [decoder]. See the documentation of the third-party
       library Uutf.
    *)

    type state = {
      units   : (Markup.t list * token) FQueue.t;
      markup  : Markup.t list;
      last    : Region.t;
      pos     : Pos.t;
      decoder : Uutf.decoder;
      supply  : Bytes.t -> int -> int -> unit
    }

    (* The call [enqueue (token, state)] updates functionally the
       state [state] by associating the token [token] with the stored
       markup and enqueuing the pair into the units queue. The field
       [markup] is then reset to the empty list. *)

    let enqueue (token, state) = {
      state with
        units  = FQueue.enq (state.markup, token) state.units;
        markup = []
    }

    (* The call [sync state buffer] updates the current position in
       accordance with the contents of the lexing buffer, more
       precisely, depending on the length of the string which has just
       been recognised by the scanner: that length is used as a
       positive offset to the current column. *)

    let sync state buffer =
      let lex   = Lexing.lexeme buffer in
      let len   = String.length lex in
      let start = state.pos in
      let stop  = start#shift_bytes len in
      let state = {state with pos = stop}
      in Region.make ~start ~stop, lex, state

    (* MARKUP *)

    (* Committing markup to the current logical state *)

    let push_newline state buffer =
      let value  = Lexing.lexeme buffer
      and ()     = Lexing.new_line buffer
      and start  = state.pos in
      let stop   = start#new_line value in
      let state  = {state with pos = stop}
      and region = Region.make ~start ~stop in
      let unit   = Markup.Newline Region.{region; value} in
      let markup = unit :: state.markup
      in {state with markup}

    let push_line (thread, state) =
      let start  = thread.opening#start in
      let region = Region.make ~start ~stop:state.pos
      and value  = mk_str thread.len thread.acc in
      let unit   = Markup.LineCom Region.{region; value} in
      let markup = unit :: state.markup
      in {state with markup}

    let push_block (thread, state) =
      let start  = thread.opening#start in
      let region = Region.make ~start ~stop:state.pos
      and value  = mk_str thread.len thread.acc in
      let unit   = Markup.BlockCom Region.{region; value} in
      let markup = unit :: state.markup
      in {state with markup}

    let push_space state buffer =
      let region, lex, state = sync state buffer in
      let value  = String.length lex in
      let unit   = Markup.Space Region.{region; value} in
      let markup = unit :: state.markup
      in {state with markup}

    let push_tabs state buffer =
      let region, lex, state = sync state buffer in
      let value  = String.length lex in
      let unit   = Markup.Tabs Region.{region; value} in
      let markup = unit :: state.markup
      in {state with markup}

    let push_bom state buffer =
      let region, value, state = sync state buffer in
      let unit   = Markup.BOM Region.{region; value} in
      let markup = unit :: state.markup
      in {state with markup}

    (* ERRORS *)

    type Error.t += Invalid_utf8_sequence
    type Error.t += Unexpected_character of char
    type Error.t += Undefined_escape_sequence
    type Error.t += Missing_break
    type Error.t += Unterminated_string
    type Error.t += Unterminated_integer
    type Error.t += Odd_lengthed_bytes
    type Error.t += Unterminated_comment
    type Error.t += Orphan_minus
    type Error.t += Non_canonical_zero
    type Error.t += Negative_byte_sequence
    type Error.t += Broken_string
    type Error.t += Invalid_character_in_string
    type Error.t += Reserved_name

    let error_to_string = function
      Invalid_utf8_sequence ->
        "Invalid UTF-8 sequence.\n"
    | Unexpected_character c ->
        sprintf "Unexpected character '%c'.\n" c
    | Undefined_escape_sequence ->
        "Undefined escape sequence.\n\
         Hint: Remove or replace the sequence.\n"
    | Missing_break ->
        "Missing break.\n\
         Hint: Insert some space.\n"
    | Unterminated_string ->
        "Unterminated string.\n\
         Hint: Close with double quotes.\n"
    | Unterminated_integer ->
        "Unterminated integer.\n\
         Hint: Remove the sign or proceed with a natural number.\n"
    | Odd_lengthed_bytes ->
        "The length of the byte sequence is an odd number.\n\
         Hint: Add or remove a digit.\n"
    | Unterminated_comment ->
        "Unterminated comment.\n\
         Hint: Close with \"*)\".\n"
    | Orphan_minus ->
        "Orphan minus sign.\n\
         Hint: Remove the trailing space.\n"
    | Non_canonical_zero ->
        "Non-canonical zero.\n\
         Hint: Use 0.\n"
    | Negative_byte_sequence ->
        "Negative byte sequence.\n\
         Hint: Remove the leading minus sign.\n"
    | Broken_string ->
        "The string starting here is interrupted by a line break.\n\
         Hint: Remove the break or close the string before.\n"
    | Invalid_character_in_string ->
        "Invalid character in string.\n\
         Hint: Remove or replace the character.\n"
    | Reserved_name ->
        "Reserved named.\n\
         Hint: Change the name.\n"
    | _ -> assert false

    exception Error of Error.t Region.reg

    let fail region value = raise (Error Region.{region; value})

    (* TOKENS *)

    (* Making tokens *)

    let mk_string (thread, state) =
      let start  = thread.opening#start in
      let stop   = state.pos in
      let region = Region.make ~start ~stop in
      let lexeme = mk_str thread.len thread.acc in
      let token  = Token.mk_string lexeme region
      in token, state

    let mk_bytes bytes state buffer =
      let region, _, state = sync state buffer in
      let token = Token.mk_bytes bytes region
      in token, state

    let mk_int state buffer =
      let region, lexeme, state = sync state buffer in
      match Token.mk_int lexeme region with
        Ok token -> token, state
      | Error Token.Non_canonical_zero ->
          fail region Non_canonical_zero

    let mk_ident state buffer =
      let region, lexeme, state = sync state buffer in
      match Token.mk_ident lexeme region with
        Ok token -> token, state
      | Error Token.Reserved_name -> fail region Reserved_name

    let mk_constr state buffer =
      let region, lexeme, state = sync state buffer
      in Token.mk_constr lexeme region, state

    let mk_sym state buffer =
      let region, lexeme, state = sync state buffer
      in Token.mk_sym lexeme region, state

    let mk_eof state buffer =
      let region, _, state = sync state buffer
      in Token.eof region, state

(* END HEADER *)
}

(* START LEXER DEFINITION *)

(* Named regular expressions *)

let utf8_bom   = "\xEF\xBB\xBF" (* Byte Order Mark for UTF-8 *)
let nl         = ['\n' '\r'] | "\r\n"
let digit      = ['0'-'9']
let natural    = digit | digit (digit | '_')* digit
let integer    = '-'? natural
let small      = ['a'-'z']
let capital    = ['A'-'Z']
let letter     = small | capital
let ident      = small (letter | '_' | digit)*
let constr     = capital (letter | '_' | digit)*
let hexa_digit = digit | ['A'-'F']
let byte       = hexa_digit hexa_digit
let byte_seq   = byte | byte (byte | '_')* byte
let bytes      = "0x" (byte_seq? as seq)
let esc        = "\\n" | "\\\"" | "\\\\" | "\\b"
               | "\\r" | "\\t" | "\\x" byte
let symbol     = ';' | ','
               | '(' | ')' | '{' | '}' | '[' | ']'
               | "<:" | '|' | "->" | ":=" | '=' | ':'
               | "||" | "&&" | '<' | "<=" | '>' | ">=" | "=/="
               | '+' | '-' | '*' | '.' | '_' | '^'

(* RULES *)

(* Except for the first rule [init], all rules bear a name starting
   with "scan".

   All have a parameter [state] that they thread through their
   recursive calls. The rules for the structured constructs (strings
   and comments) have an extra parameter of type [thread] (see above).
*)

rule init state = parse
  utf8_bom { scan (push_bom state lexbuf) lexbuf }
| _        { rollback lexbuf; scan state lexbuf  }

and scan state = parse
  nl      { scan (push_newline state lexbuf) lexbuf }
| ' '+    { scan (push_space   state lexbuf) lexbuf }
| '\t'+   { scan (push_tabs    state lexbuf) lexbuf }

| ident   { mk_ident       state lexbuf |> enqueue }
| constr  { mk_constr      state lexbuf |> enqueue }
| bytes   { (mk_bytes seq) state lexbuf |> enqueue }
| integer { mk_int         state lexbuf |> enqueue }
| symbol  { mk_sym         state lexbuf |> enqueue }
| eof     { mk_eof         state lexbuf |> enqueue }

| '"'  { let opening, _, state = sync state lexbuf in
         let thread = {opening; len=1; acc=['"']} in
         scan_string thread state lexbuf |> mk_string |> enqueue }

| "(*" { let opening, _, state = sync state lexbuf in
         let thread = {opening; len=2; acc=['*';'(']} in
         let state = scan_block thread state lexbuf |> push_block
         in scan state lexbuf }

| '#'  { let opening, _, state = sync state lexbuf in
         let thread = {opening; len=1; acc=['#']} in
         let state = scan_line thread state lexbuf |> push_line
         in scan state lexbuf }

  (* Some special errors

     Some special errors are recognised in the semantic actions of the
     following regular expressions. The first error is a minus sign
     separated from the integer it modifies by some markup (space or
     tabs). The second is a minus sign immediately followed by
     anything else than a natural number (matched above) or markup and
     a number (previous error). The third is the strange occurrence of
     an attempt at defining a negative byte sequence. Finally, the
     catch-all rule reports unexpected characters in the buffer (and
     is not so special, after all).
  *)

| '-' { let region, _, state = sync state lexbuf in
        let state = scan state lexbuf in
        let open Markup in
        match FQueue.peek state.units with
          None -> assert false
        | Some (_, ((Space _ | Tabs _)::_, token))
            when Token.is_int token ->
              fail region Orphan_minus
        | _ -> fail region Unterminated_integer }

| '-' "0x" byte_seq?
      { let region, _, _ = sync state lexbuf
        in fail region Negative_byte_sequence }

| _ as c { let region, _, _ = sync state lexbuf
           in fail region (Unexpected_character c) }

(* Finishing a string *)

and scan_string thread state = parse
  nl         { fail thread.opening Broken_string }
| eof        { fail thread.opening Unterminated_string }
| ['\t' '\r' '\b']
             { let region, _, _ = sync state lexbuf
               in fail region Invalid_character_in_string }
| '"'        { let _, _, state = sync state lexbuf
               in push_char '"' thread, state }
| esc        { let _, lexeme, state = sync state lexbuf
               in scan_string (push_string lexeme thread) state lexbuf }
| '\\' _     { let region, _, _ = sync state lexbuf
               in fail region Undefined_escape_sequence }
| _ as c     { let _, _, state = sync state lexbuf in
               scan_string (push_char c thread) state lexbuf }

(* Finishing a block comment

   (Note for Emacs: ("(*")
   The lexing of block comments must take care of embedded block
   comments that may occur within, as well as strings, so no substring
   "*)" may inadvertently close the block. This is the purpose
   of the first case of the scanner [scan_block].
*)

and scan_block thread state = parse
  '"' | "(*" { let opening = thread.opening in
               let opening', lexeme, state = sync state lexbuf in
               let thread = push_string lexeme thread in
               let thread = {thread with opening=opening'} in
               let next   = if lexeme = "\"" then scan_string
                            else scan_block in
               let thread, state = next thread state lexbuf in
               let thread = {thread with opening}
               in scan_block thread state lexbuf }
| "*)"       { let _, lexeme, state = sync state lexbuf
               in push_string lexeme thread, state }
| nl as nl   { let ()     = Lexing.new_line lexbuf
               and state  = {state with pos = state.pos#new_line nl}
               and thread = push_string nl thread
               in scan_block thread state lexbuf }
| eof        { fail thread.opening Unterminated_comment }
| _          { let     () = rollback lexbuf in
               let len    = thread.len in
               let thread,
                   status = scan_utf8 thread state lexbuf in
               let delta  = thread.len - len in
               let pos    = state.pos#shift_one_uchar delta in
               match status with
                 None -> scan_block thread {state with pos} lexbuf
               | Some error ->
                   let region = Region.make ~start:state.pos ~stop:pos
                   in fail region error }

(* Finishing a line comment *)

and scan_line thread state = parse
  nl as nl { let     () = Lexing.new_line lexbuf
             and thread = push_string nl thread
             and state  = {state with pos = state.pos#new_line nl}
             in thread, state }
| eof      { fail thread.opening Unterminated_comment }
| _        { let     () = rollback lexbuf in
             let len    = thread.len in
             let thread,
                 status = scan_utf8 thread state lexbuf in
             let delta  = thread.len - len in
             let pos    = state.pos#shift_one_uchar delta in
             match status with
               None -> scan_line thread {state with pos} lexbuf
             | Some error ->
                 let region = Region.make ~start:state.pos ~stop:pos
                 in fail region error }

and scan_utf8 thread state = parse
     eof { fail thread.opening Unterminated_comment }
| _ as c { let thread = push_char c thread in
           let lexeme = Lexing.lexeme lexbuf in
           let () = state.supply (Bytes.of_string lexeme) 0 1 in
           match Uutf.decode state.decoder with
             `Uchar _     -> thread, None
           | `Malformed _ -> thread, Some Invalid_utf8_sequence
           | `Await       -> scan_utf8 thread state lexbuf
           | `End         -> assert false }

(* END LEXER DEFINITION *)

{
(* START TRAILER *)

(* Scanning the lexing buffer for tokens (and markup, as a
   side-effect).

     Because we want the lexer to have access to the right lexical
   context of a recognised lexeme (to enforce stylistic constraints or
   report special error patterns), we need to keep a hidden reference
   to a queue of recognised lexical units (that is, tokens and markup)
   that acts as a mutable state between the calls to
   [read_token]. When [read_token] is called, that queue is consulted
   first and, if it contains at least one token, that token is
   returned; otherwise, the lexing buffer is scanned for at least one
   more new token. That is the general principle: we put a high-level
   buffer (our queue) on top of the low-level lexing buffer.

     One tricky and important detail is that we must make any parser
   generated by Menhir (and calling [read_token]) believe that the
   last region of the input source that was matched indeed corresponds
   to the returned token, despite that many tokens and markup may have
   been matched since it was actually read from the input. In other
   words, the parser requests a token that is taken from the
   high-level buffer, but the parser requests the source regions from
   the _low-level_ lexing buffer, and they may disagree if more than
   one token has actually been recognised.

     Consequently, in order to maintain a consistent view for the
   parser, we have to patch some fields of the lexing buffer, namely
   [lex_start_p] and [lex_curr_p], as these fields are read by parsers
   generated by Menhir when querying source positions (regions). This
   is the purpose of the function [patch_buffer]. After reading one
   ore more tokens and markup by the scanning rule [scan], we have to
   save in the hidden reference [buf_reg] the region of the source
   that was matched by [scan]. This atomic sequence of patching,
   scanning and saving is implemented by the _function_ [scan]
   (beware: it shadows the scanning rule [scan]). The function
   [patch_buffer] is, of course, also called just before returning the
   token, so the parser has a view of the lexing buffer consistent
   with the token.

     Note that an additional reference [first_call] is needed to
   distinguish the first call to the function [scan], as the first
   scanning rule is actually [init] (which can handle the BOM), not
   [scan].
*)

type logger = Markup.t list -> token -> unit

type instance = {
  read     : ?log:logger -> Lexing.lexbuf -> token;
  buffer   : Lexing.lexbuf;
  get_pos  : unit -> Pos.t;
  get_last : unit -> Region.t;
  close    : unit -> unit
}

let  file_path = match EvalOpt.input with
                   None | Some "-" -> ""
                 | Some file_path  -> file_path
let        pos = Pos.min#set_file file_path
let    buf_reg = ref (pos#byte, pos#byte)
and first_call = ref true
and    decoder = Uutf.decoder ~encoding:`UTF_8 `Manual
let     supply = Uutf.Manual.src decoder
let      state = ref {units = FQueue.empty;
                      last = Region.ghost;
                      pos;
                      markup = [];
                      decoder;
                      supply}

let get_pos () = !state.pos

let get_last () = !state.last

let patch_buffer (start, stop) buffer =
  let open Lexing in
  let file_path = buffer.lex_curr_p.pos_fname in
  buffer.lex_start_p <- {start with pos_fname = file_path};
  buffer.lex_curr_p  <- {stop  with pos_fname = file_path}

and save_region buffer =
  buf_reg := Lexing.(buffer.lex_start_p, buffer.lex_curr_p)

let scan buffer =
  patch_buffer !buf_reg buffer;
  (if   !first_call
   then (state := init !state buffer; first_call := false)
   else state := scan !state buffer);
  save_region buffer

let next_token buffer =
  scan buffer;
  match FQueue.peek !state.units with
                       None -> assert false
  | Some (units, ext_token) ->
      state := {!state with units}; Some ext_token

let check_right_context token buffer =
  let open Token in
  if is_int token || is_bytes token then
    match next_token buffer with
      Some ([], next) ->
        let pos    = (Token.to_region token)#stop in
        let region = Region.make ~start:pos ~stop:pos in
        if is_bytes token && is_int next then
          fail region Odd_lengthed_bytes
        else
        if is_ident next || is_string next
           || is_bytes next || is_int next then
          fail region Missing_break
    | _ -> ()
  else
  if Token.is_ident token || Token.is_string token then
    match next_token buffer with
      Some ([], next) ->
        if Token.is_ident next || Token.is_string next
        || Token.is_bytes next || Token.is_int next
        then
          let pos    = (Token.to_region token)#stop in
          let region = Region.make ~start:pos ~stop:pos
          in fail region Missing_break
    | _ -> ()

let rec read_token ?(log=fun _ _ -> ()) buffer =
  match FQueue.deq !state.units with
    None ->
      scan buffer;
      read_token ~log buffer
  | Some (units, (left_mark, token)) ->
      log left_mark token;
      state := {!state with units; last = Token.to_region token};
      check_right_context token buffer;
      patch_buffer (Token.to_region token)#byte_pos buffer;
      token

let open_token_stream file_path_opt =
  let      cin = match file_path_opt with
                   None | Some "-" -> stdin
                 | Some file_path  -> open_in file_path in
  let   buffer = Lexing.from_channel cin in
  let       () = match file_path_opt with
                   None | Some "-" -> ()
                 | Some file_path  -> reset ~file:file_path buffer
  and close () = close_in cin in
  {read = read_token; buffer; get_pos; get_last; close}

(* Standalone lexer for debugging purposes *)

(* Pretty-printing in a string the lexemes making up the markup
   between two tokens, concatenated with the last lexeme itself. *)

let output_token ?(offsets=true) mode command
                 channel left_mark token : unit =
  let output    str = Printf.fprintf channel "%s%!" str in
  let output_nl str = output (str ^ "\n") in
  match command with
    EvalOpt.Quiet -> ()
  | EvalOpt.Tokens -> Token.to_string token ~offsets mode |> output_nl
  | EvalOpt.Copy ->
      let lexeme = Token.to_lexeme token
      and apply acc markup = Markup.to_lexeme markup :: acc
      in List.fold_left apply [lexeme] left_mark
         |> String.concat "" |> output
  | EvalOpt.Units ->
      let abs_token = Token.to_string token ~offsets mode
      and apply acc markup =
        Markup.to_string markup ~offsets mode :: acc
      in List.fold_left apply [abs_token] left_mark
         |> String.concat "\n" |> output_nl

let print_error ?(offsets=true) mode Region.{region; value} =
  let  msg = error_to_string value in
  let file = match EvalOpt.input with
               None | Some "-" -> false
             |         Some _  -> true in
  let  reg = region#to_string ~file ~offsets mode in
  Utils.highlight (sprintf "Lexical error %s:\n%s%!" reg msg)

let trace ?(offsets=true) mode file_path_opt command : unit =
  try
    let {read; buffer; close; _} = open_token_stream file_path_opt
    and cout = stdout in
    let log = output_token ~offsets mode command cout
    and close_all () = close (); close_out cout in
    let rec iter () =
      match read ~log buffer with
        token ->
          if Token.is_eof token then close_all ()
          else iter ()
      | exception Error e -> print_error ~offsets mode e; close_all ()
    in iter ()
  with Sys_error msg -> Utils.highlight (sprintf "%s\n" msg)

end (* of functor [Make] in HEADER *)
(* END TRAILER *)
}