json_read_dict example

[Bambang Purnomosidi D. P.]

Dear all,

This one is probably trivial but I don't know how to solve it. I try to read a JSON file and get one of its value:

file.json
-----------

{ "port": 8080 }

Then I want to get the value of "port", using swipl REPL below:

?- open('file.json',read,X).
X = <stream>(0x882d3f8).

?- use_module(library(http/json)).
true.

?- json_read_dict(X,Config).
ERROR: get_code/2: Arguments are not sufficiently instantiated
?-

Any help / pointer is appreciated.

Thank you.

[Jan Wielemaker]

Variables are not shared between interactive goals. So,

1 ?- use_module(library(http/json)).
true.

2 ?- open('file.json',read,X), json_read_dict(X,Config), close(X).
X = <stream>(0xf16b70),
Config = _G1894{port:8080}.

If you do this in a program, use something like this:

file_json(File, JSON) :-
setup_call_cleanup(
open(File, read, In),
json_read_dict(In, JSON),
close(In)).

Using setup_call_cleanup/3 you don't have to worry about leaking stream
handles, no matter what happens.

Cheers --- Jan

P.s. You can refer to old toplevel variables in a new query using
$X instead of plain X. This however is only a shorthand for
interactive usage.


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