[Caml-list] wrapping a string in an in_channel

[Caml-list] wrapping a string in an in_channel

[Martin DeMello]

Does OCaml have any equivalent to ruby's StringIO, to wrap an
in_channel around a string and then treat it as if it were a file?

martin

Re: [Caml-list] wrapping a string in an in_channel

[Gerd Stolpmann]

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Am Sonntag, den 05.01.2014, 00:23 -0800 schrieb Martin DeMello:
> Does OCaml have any equivalent to ruby's StringIO, to wrap an
> in_channel around a string and then treat it as if it were a file?

With the standard library this is not possible.

When I need such a feature, I normally use OCamlnet's channel objects:

http://projects.camlcity.org/projects/dl/ocamlnet-3.7.3/doc/html-main/Netchannels_tut.html

http://projects.camlcity.org/projects/dl/ocamlnet-3.7.3/doc/html-main/Netchannels.html

http://www.ocaml-programming.de/rec/IO-Classes.html

Gerd
-- 
------------------------------------------------------------
Gerd Stolpmann, Darmstadt, Germany    gerd@gerd-stolpmann.de
My OCaml site:          http://www.camlcity.org
Contact details:        http://www.camlcity.org/contact.html
Company homepage:       http://www.gerd-stolpmann.de
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Re: [Caml-list] wrapping a string in an in_channel

[Hezekiah M. Carty]

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On Jan 5, 2014 3:23 AM, "Martin DeMello" <martindemello@gmail.com> wrote:
>
> Does OCaml have any equivalent to ruby's StringIO, to wrap an
> in_channel around a string and then treat it as if it were a file?
>
> martin
>

Batteries provides something similar with its BatIO.input_string function.
It returns the Batteries equivalent to an in_channel.

Hez

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Re: [Caml-list] wrapping a string in an in_channel

[John Whitington]

Martin DeMello wrote:
> Does OCaml have any equivalent to ruby's StringIO, to wrap an
> in_channel around a string and then treat it as if it were a file?

Here's another example of wrapping IO around strings, bigarrays, etc as 
a record of functions:

https://github.com/johnwhitington/camlpdf/blob/master/pdfio.ml

Here's a type:

type input =
   {pos_in : unit -> pos;
    seek_in : pos -> unit;
    input_char : unit -> char option;
    input_byte : unit -> int;
    in_channel_length : pos;
    set_offset : pos -> unit;
    caml_channel : in_channel option;
    source : string}

It's good to add the 'source' field - which is an arbitrary string for 
debugging purposes. Of course, if the input came from a file, we would 
use the file name.

The caml_channel field optionally stores a channel this input was 
derived from. So, we can still drill down and use fast functions from 
the standard library such as Pervasives.really_input.

(The set_offset field is a nasty PDF-related hack: a PDF file can have 
garbage bytes preceding the header, and PDF absolute byte offsets are 
offsets from the header, so every read and write might have to be offset)

Functions in records as an abstraction don't seem to be a speed problem. 
The use of option-returning functions like input_char above rather than 
exception-raising ones or minus-one-as-exception functions like 
input_byte above is big, though, if the data processed is large enough, 
due to garbage collection.

-- 
John Whitington
Director, Coherent Graphics Ltd
http://www.coherentpdf.com/

Re: [Caml-list] wrapping a string in an in_channel

[Stéphane Glondu]

Le 05/01/2014 09:23, Martin DeMello a écrit :
> Does OCaml have any equivalent to ruby's StringIO, to wrap an
> in_channel around a string and then treat it as if it were a file?

Technically, it should be possible by using low-level Unix machinery
like in the following:

let wrap_string s =
  let infd, outfd = Unix.pipe () in
  let inc = Unix.in_channel_of_descr infd in
  let outc = Unix.out_channel_of_descr outfd in
  let writer () =
    output_string outc s;
    flush outc;
    close_out outc
  in Thread.create writer ();
  inc

... but the other suggestions are probably better...

Cheers,

-- 
Stéphane