Unexpected result using with_output_to and string(S)

Help!
1

Loosely related, since S is a string in the example below. Note that S ends up being empty:

?- with_output_to(string(S), license).
% SWI-Prolog is covered by the Simplified BSD license:
% 
% Redistribution and use in source and binary forms, with or without
% modification, are permitted provided that the following conditions
(...)
% additional conditions.  These components are:
% 
%     libgmp
S = "".

Is this a feature or a bug? I guess it has to do with print_message printing to user_error.

Workaround:

new_memory_file(M), 
open_memory_file(M, write, S), 
set_stream(S, alias(user_error)), 
license, 
close(S),
memory_file_to_atom(M, A),
free_memory_file(M).
2

If you check the documentation for with_output_to/2 it notes

Run Goal as once/1, while characters written to the current output are sent to Output.

If you look at the code for license using listing, i.e.

?- listing(license).

is shows

 print_message(informational,
                  license(GPL,
                          LGPL,
                          Permissive,
                          Proprietary)).

and the print_message/2 documentation notes

Printing messages through this indirection instead of using format/3 to the stream user_error

Since the license text is being output to user_error, with_output_to/2 will not capture it. :slightly_smiling_face:

3

There seems to be a with_output_to/3, but it is not found on my system (neither linux or windows), since library(streams) is not installed.

4

Since really shortly, there is library(streams) that defines with_output_to/3, so we can do

?- with_output_to(string(S), license, [capture([user_error])]).
S = "% SWI-Prolog is covered by the Simplified BSD license ... "

This is part of some ongoing work towards proper parallel testing and provide a ctest like --output-on-error

5

For those looking for the exact date it is Dec 2, 2022

I only knew of it from checking the latest commits every morning.

6

All right, found it, thanks!

The name of the library is obviously misleading. library(streams) suggests that it’s been there since the 1980ies, with their tell/1, told/0, you name it :slight_smile: One would really not expect to find it only in the recent swipl-devel.

7

You’d preferred “new_streams” :slight_smile: I think the name is fine. Stream handling is mostly built-in. This library is about more high level stream operations. Having a library with a single predicate is a bit of an overkill. We’ll see where it goes.

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