Course of Raku / Functional, concurrent, reactive, and web programming / Web programming / Cro 101

Fetching from a public API

The client from the previous page reaches any server on the web, not only your own. Many public APIs reply with JSON, and Cro parses that for you, so .body gives back a ready-made data structure rather than text. Here we ask a free weather service for the current temperature in Berlin:

use Cro::HTTP::Client;

my $url = 'https://api.open-meteo.com/v1/forecast'
        ~ '?latitude=52.52&longitude=13.41&current=temperature_2m,wind_speed_10m';

my $response = await Cro::HTTP::Client.get($url);
my %current  = (await $response.body)<current>;

say "Berlin right now:";
say "  temperature: %current<temperature_2m> °C";
say "  wind speed:  %current<wind_speed_10m> km/h";

which prints something like:

Berlin right now:
  temperature: 21.9 °C
  wind speed:  8.4 km/h

The exact numbers change with the weather, of course. The two numbers in the URL are geographic coordinates — 52.52° N, 13.41° E is Berlin — so put your own city’s latitude and longitude there to get your local weather. Either way, one request and .body (or .body-text) gives you the answer — no sockets, no status parsing, no JSON handling by hand.

Plain text for the terminal

JSON is not the only shape an answer can take. The weather service wttr.in can serve JSON too, but its speciality is plain text drawn for the terminal — and it decides the format by looking at who is asking: command-line clients get text, browsers get a web page. Cro lets you set request headers, so introduce yourself as one of the terminal folk:

use Cro::HTTP::Client;

my $response = await Cro::HTTP::Client.get(
    'https://wttr.in/Berlin?format=3',
    headers => [ User-Agent => 'curl' ],
);
print await $response.body-text;

This prints a one-line weather report, ready for a shell prompt or a status bar:

Berlin: 🌤️  +25°C

Here the body is plain text, so it is read with .body-text. Drop the ?format=3 from the URL and the service replies with its full, multi-line ASCII-art weather report instead.

These examples need Cro installed (zef install cro) and a working network connection.

Requests and responses are not the whole story, though. The final topic opens a WebSocket — a live, two-way channel where the reactive tools of this part get their grand finale.

← Using a client module | WebSockets →

💪 Or jump directly to the exercises in this section.

Fetching from a public API

Plain text for the terminal

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