eyeprolog

HTTP client

How does a Prolog program fetch and read a JSON document?

This example is a three-line relation: fetch a URL over HTTP(S) and parse the body as JSON into an ordinary Prolog term. It depends on a live network endpoint, so it has no built-in query; you supply the URL.

Run it in the playground · Source

The question

Given a URL that returns JSON, what does the document say, and what HTTP status code came back?

The point is the boundary: network I/O happens in one predicate, and everything after it works on structured terms, not strings.

The program

:- use_module(library(http)).
:- use_module(library(json)).
:- use_module(library(dcgs)).

fetch_json(URL, JSON, Code) :-
    http_get(URL, Body, [status_code(Code), header("accept", "application/json")]),
    phrase(json_chars(JSON), Body).

http_get/3 performs the request and returns the body as a list of characters, with the status code as an option. json_chars//1 is a grammar (a DCG), so phrase/2 parses the characters into a JSON term.

The JSON term

The JSON grammar works in both directions. Parsing {"status":"ok","count":3} gives:

pairs([string("status") - string("ok"), string("count") - number(3)])

Objects become pairs/1, strings string/1 and numbers number/1, so later rules can match on the structure directly.

The answer

eyeprolog --goal 'fetch_json("https://example.org/api/status", JSON, Code)' examples/http-client.pl

The answer depends on the endpoint, so the example declares no goal of its own and its golden output file is empty. library(http) is backed by Node’s HTTP client, so run the request from the command line rather than the browser playground.

The reason

With no declared goal, the run makes no claims, so its proof is empty. Run your own goal with --proof and the proof shows fetch_json/3 resting on the http_get/3 and phrase/2 calls.

The network call cannot be replayed by a proof checker. Treat the fetched document as input that the host has to vouch for.

The check

condition('C1', resolution, ok, 0).
condition('C4', coverage, ok, 0).
condition('C7', relevance, ok, 0).
verdict(checked).

Every condition holds trivially: zero claims and zero steps. An empty proof certifies nothing, and the check says so honestly.

Try it

Fetch a public JSON endpoint and extract one field by matching on pairs/1.
Separate the network from the logic: save a response body to a file and parse it with phrase/2 instead of http_get/3.
See json for JSON parsing and generation without the network.

Learn more in The Art of EyeProlog, Chapter 15.

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