How do I read in lines from a text file in OCaml?

Question

This is what I have so far. Isn't this all that you need? I keep getting the error "Error: Unbound module Std"

let r file =
    let chan = open_in file in
    Std.input_list (chan)

Answer 1

Editor's note: this answer was good for previous versions of OCaml; with version 4.14.0 and above, please consider these answers instead: https://stackoverflow.com/a/73019499/ and https://stackoverflow.com/a/77625257/

An imperative solution using just the standard library:

let read_file filename = 
let lines = ref [] in
let chan = open_in filename in
try
  while true; do
    lines := input_line chan :: !lines
  done; !lines
with End_of_file ->
  close_in chan;
  List.rev !lines ;;

If you have the Batteries-included library you could read a file into an Enum.t and iterate over it as follows:

let filelines = File.lines_of filename in
Enum.iter ( fun line -> (*Do something with line here*) ) filelines

Answer 2

Fortunately the OCaml standard library is continuously improving. Since OCaml 5.1, one can do the following:

let read_lines (file_name : string) : string list =
  In_channel.with_open_text file_name In_channel.input_lines

The In_channel module contains several functions that are useful for file content reading.

Answer 3

This reads the file's lines and prints each of them:

open Core.Std

let handle_line line =
  printf "Your line: %s \n" line

let () =
  let file_to_read = "./file_to_read.txt" in
  let lines = In_channel.read_lines file_to_read in
  List.iter ~f: handle_line lines

Answer 4

What happens with very large files? Maybe you don't want to read all lines, so why should you? A lazy list or sequence would be ideal. Fortunately as of OCaml 4.07 and later, we have the Seq module which permits us to create a sequence of lines read from a file.

let rec read_lines_seq file () =
  match In_channel.input_line file with
  | None -> Seq.Nil
  | Some line -> Seq.Cons (line, read_lines_seq file)

Where we might get all of the lines from a file now with:

let read_lines file_name =
  In_channel.with_open_text 
    file_name 
    (fun file -> file |> read_lines_seq |> List.of_seq)

But we could use this just as easily to print all of the lines in a file without having to first generate a complete list and then iterate that.

OCaml 5.1 also introduces In_channel.fold_lines which could do much the same.

let read_lines file_name =
  In_channel.(
    with_open_text
      file_name
      (fun file -> file |> fold_lines (Fun.flip List.cons) [] 
                        |> List.rev)
  )

Answer 5

To build upon @alwaysday1's solution you can make a simple function that will read all of the lines of a file using In_channel from OCaml 4.14.0, and Str like so:

let read_lines file =
  In_channel.with_open_text file In_channel.input_all
  |> Str.(split (regexp "\n"))

This will produce a list of all lines in the file.

Answer 6

There are new modules In_channel and Out_channel in the standard library of OCaml version 4.14.0, which was released on 2022-03-28. refer

So now, we could easily read the content from a file as follows:

# let conx = In_channel.with_open_text "example.dat" In_channel.input_all;;
val conx : string = "hey!\nworld"

Say, the content of the example.dat:

hey!
world

Answer 7

I know this is an old question, but there is another recursive variation that is interesting (at least in my opinion), without libraries or reversal:

let read_lines filename =
  let f = open_in filename in
  let rec loop () =
    try
      let next = input_line f in
      next :: loop ()
    with End_of_file ->
      close_in f;
      []
  in loop ()

Note that the let next = ... line is important, because it ensures that the next line is read before recursion.

Answer 8

This just loads the whole file into one big string, but you could always split it into a list later.

let read_file path =
  let channel = open_in path in
  let buffer = Buffer.create 1024 in
  let rec go () =
    try
      Buffer.add_channel buffer channel 1024; go ()
    with End_of_file ->
      Buffer.contents buffer in
  go ();;

Answer 9

Here is a simple recursive solution that does not accumulate the lines or use external libraries, but lets you read a line, process it using a function, read the next recursively until done, then exit cleanly. The exit function closes the open filehandle and signals success to the calling program.

let read_lines file process =
  let in_ch = open_in file in
  let rec read_line () =
    let line = try input_line in_ch with End_of_file -> exit 0
    in (* process line in this block, then read the next line *)
       process line;
       read_line ();
in read_line ();;

read_lines some_file print_endline;;

Answer 10

If you have the OCaml Core library installed, then it is as simple as:

open Core.Std
let r file = In_channel.read_lines file

If you have corebuild installed, then you can just compile your code with it:

corebuild filename.byte

if your code resides in a file named filename.ml.

If you don't have the OCaml Core, or do not want to install it, or some other standard library implementation, then, of course, you can implement it using a vanilla OCaml's standard library. There is a function input_line, defined in the Pervasives module, that is automatically opened in all OCaml programs (i.e. all its definitions are accessible without further clarification with a module name). This function accepts a value of type in_channel and returns a line, that was read from the channel. Using this function you can implement the required function:

let read_lines name : string list =
  let ic = open_in name in
  let try_read () =
    try Some (input_line ic) with End_of_file -> None in
  let rec loop acc = match try_read () with
    | Some s -> loop (s :: acc)
    | None -> close_in ic; List.rev acc in
  loop []

This implementation uses recursion, and is much more natural to OCaml programming.

Answer 11

Another style to read lines from a file using Scanf "string indiciation" and zero-width character. It is like traditional imperative style.

open Scanf 
open Printf

(* little helper functions *)
let id x = x 
let const x = fun _ -> x
let read_line file = fscanf file "%s@\n" id 
let is_eof file = try fscanf file "%0c" (const false) with End_of_file -> true

let _ = 
  let file = open_in "/path/to/file" in 

  while not (is_eof file) do 
    let s = read_line file in
    (* do something with s *) 
    printf "%s\n" s 
  done;

  close_in file

NOTE:

1. read_line ignore one trailing \n, so if the last character of your file is \n, it may seems like you have missed the last empty line.
2. when using of Scanf, due to bufferization, do not mix other low level reading on the same channel, otherwise it will result in strange behaviour.

Answer 12

Here's a recursive solution using Scanf:

let read_all_lines file_name =
  let in_channel = open_in file_name in
  let rec read_recursive lines =
    try
      Scanf.fscanf in_channel "%[^\r\n]\n" (fun x -> read_recursive (x :: lines))
    with
      End_of_file ->
        lines in
  let lines = read_recursive [] in
  let _ = close_in_noerr in_channel in
  List.rev (lines);;

Usage:

let all_lines = read_all_lines "input.txt";;

However, I'd prefer to stream line-by-line:

let make_reader file_name =
  let in_channel = open_in file_name in
  let closed = ref false in
  let read_next_line = fun () ->
    if !closed then
      None
    else
      try
        Some (Scanf.fscanf in_channel "%[^\r\n]\n" (fun x -> x))
      with
        End_of_file ->
          let _ = close_in_noerr in_channel in
          let _ = closed := true in
          None in
  read_next_line;;

Usage:

let read_next = make_reader "input.txt";;
let next_line = read_next ();;

And may be a bit of icing:

type reader = {read_next : unit -> string option};;

let make_reader file_name =
  let in_channel = open_in file_name in
  let closed = ref false in
  let read_next_line = fun () ->
    if !closed then
      None
    else
      try
        Some (Scanf.fscanf in_channel "%[^\r\n]\n" (fun x -> x))
      with
        End_of_file ->
          let _ = close_in_noerr in_channel in
          let _ = closed := true in
          None in
  {read_next = read_next_line};;

Usage:

let r = make_reader "input.txt";;
let next_line = r.read_next ();;

Hope this helps!

Answer 13

Std.input_list apparently requires Extlib, which you should install on your system (libextlib-ocaml and libextlib-ocaml-dev on Debian systems).