SWI-Prolog 7.4.2 Reference Manual: Section A.14

## A.14 library(iostream): Utilities to deal with streams

See also
  `library(archive)`, `library(process)`, `library(zlib)`, `library(http/http_stream)`

This library contains utilities that deal with streams, notably originating from non-built-in sources such as URLs, archives, windows, processes, etc.

The predicate open_any/5 acts as a broker between applications that can process data from a stream and libraries that can create streams from diverse sources. Without this predicate, processing data inevitally follows the pattern below. As call_some_open_variation can be anything, this blocks us from writing predicates such as `load_xml(From, DOM)` that can operate on arbitrary input sources.

    setup_call_cleanup(
        call_some_open_variation(Spec, In),
        process(In),
        close(In)).

Libraries that can open streams can install the hook iostream:open_hook/6 to make their functionality available through open_any/5.

open_any(+Specification, +Mode, -Stream, -Close, +Options)
  Establish a stream from Specification that should be closed using Close, which can either be called or passed to close_any/1. Options processed:
  encoding(Enc)
    Set stream to encoding Enc.

  Without loaded plugins, the open_any/5 processes the following values for Specification. If no rule matches, open_any/5 processes Specification as `file(Specification)`.

  Stream
    A plain stream handle. Possisible post-processing options such as encoding are applied. Close does not close the stream, but resets other side-effects such as the encoding.
  stream(Stream)
    Same as a plain Stream.
  FileURL
    If Specification is of the form =file://...=, the pointed to file is opened using open/4. Requires `library(uri)` to be installed.
  file(Path)
    Explicitly open the file Path. Path can be an Path(File) term as accepted by absolute_file_name/3.
  string(String)
    Open a Prolog string, atom, list of characters or codes as an input stream.

  The typical usage scenario is given in the code below, where <process> processes the input.

      setup_call_cleanup(
          open_any(Spec, read, In, Close, Options),
          <process>(In),
          Close).

  Currently, the following libraries extend this predicate:

  library(http/http_open)
    Adds support for URLs using the `http` and `https` schemes.
close_any(+Goal)
  Execute the Close closure returned by open_any/5. The closure can also be called directly. Using close_any/1 can be considered better style and enhances tractability of the source code.
[semidet,multifile]open_hook(+Spec, +Mode, -Stream, -Close, +Options0, -Options)
  Open Spec in Mode, producing Stream.

  Close | is unified to a goal that must be called to undo the side-effects of the action, e.g., typically the term `close(Stream)`
  Options0 | are the options passed to open_any/5
  Options | are passed to the post processing filters that may be installed by open_any/5.