A.22 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.