file system

[alkubaig]

Hello all!

Thank you in advance for any help!

I was wondering whether or not it is possible to read and write to files in Eff?

[matijapretnar]

Hi! No, this is currently not possible, but it would probably be easy to add in external.ml. What kind of operations do you imagine to have?

[alkubaig]

Read and write!

[matijapretnar]

So the signatures would be read : string -> string where the parameter would be the filename, and write : string * string -> string, where the first parameter is the filename and second one is the string? For simplicity, each operation would open the file, perform the IO and close the file?

[alkubaig]

Perfect! Thank you for your reply. It is absolutely appreciated

So are you saying I can use these read and write functions from external.ml?

is that the right one? https://github.com/ocaml/ocamlbuild/blob/master/testsuite/external.ml

and I am assuming I can write

#use "external.ml";;

in the top of my eff file to import it?

I followed these steps but it did not work, so I am not sure what I am doing wrong

FYI, I have Ocamal installed on my machine

[alkubaig]

I see that external.ml exist in eff/_build/src/runtime
But it gives a parse error when I use it

[alkubaig]

I am guessing I should do something like

external ( < ) : 'a -> 'a -> bool = "<"
external read : string -> string = "read"

[matijapretnar]

No, you should take a look at src/runtime/external.ml. But read and write don't exist there yet, someone would need to add them just like print.

[zigaLuksic]

You can also add such effects in the same way that #Print was added, in src/runtime/eval.ml.

For instance, for a basic version of #Read_file you need to:

• add to file pervasives.eff (around line 10 for instance) the effect type effect Read_file : string -> string

• add to file src/runtime/eval.ml at the bottom in the function top_handle another case for your effect.
  You need to be careful to correctly transform types.

| V.Call ("Read_file", v, k) ->
    let filename = V.to_str v in
    let channel = open_in filename in
    let text = ref "" in
    (
    try
      while true do
        text := !text  ^ (input_line channel) ^ "\n"   (* Gather text into a single string. *)
      done; 
      close_in channel;
      top_handle (k (V.Const (Const.of_string !text)))   (* This is only for typechecking. *)
    with
      End_of_file ->
        close_in channel;
        top_handle (k (V.Const (Const.of_string !text)))
    ) 

seems to work for me. You should adapt the function to your needs (as you can see this adds an
unnecessary "\n" at the end, also gathering all the text in a single string is a bad idea for large files).

Also, don't forget to make eff after you have added the changes.

[alkubaig]

Thanks a lot!
I will try it and let you know

[alkubaig]

That worked! Thank you so much
I appreciate your help

I will work now to add write_to_file and also read_line

[alkubaig]

This is my implementation of write_to_file

I added this effect to pervasives.eff

effect Write_file : string * string -> unit

I added these two to src/runtime/value.mli

val first : value -> value
val second : value -> value

and these two src/runtime/value.ml

let first = function
  | Tuple (x::xs) -> x
  | _ -> Error.runtime "first value expected."
  
let second = function
  | Tuple (x::y::xs)-> y
  | _ -> Error.runtime "second value expected."

and added this case to top_handle in src/runtime/eval.ml

| V.Call ("Write_file", v , k) ->
	
     let filename =  V.to_str (V.first v) in
     let text =  V.to_str (V.second v) in
     let channel = open_out filename in
	fprintf channel "%s\n" text;
     top_handle (k V.unit_value)

I can now see why read_line might be complicated because we need to keep track of the file pointer

[matijapretnar]

Yes, you'd need a separate constant for file pointers (just like for booleans, strings, …)

[andrejbauer]

I think this issue presents a very realistic and nice test case for effects: what is the Correct Way of working with files?