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Chatbots are basically a clunky commandline interface to things that sometimes really need a custom UI.
But what if I do want to make something that uses some of the features (a GPS receiver, a camera, a microphone) that a modern phone has? I'm too lazy to write device-specific code and dig around in the intrinsics of Android/iOS APIs.
So after doing some research on modern messenger apps (I kind of fell behind on what was going on after the WhatsApp acquisition and turns out billions more have popped up since then) I stumbled upon the Telegram bot API. And it's actually pretty simple. In a nutshell, you create a bot (by messaging another bot) which gives you a token. The token is the only thing your bot needs to communicate with the Telegram servers (so no coding up handshakes or managing session keys): it makes up your REST endpoint that you can throw queries at. The connection is over SSL, so that takes care of your ISP or a kid with a WiFi dongle and Wireshark grabbing hold of your token. Bot chats aren't end-to-end encrypted though, so Telegram are still able to read whatever you talk to the bot about.
With that in mind, receiving messages is easy: just shoot a GET request at
https://api.telegram.org/bot(TOKEN)/getUpdates (reference) and it will come back with a JSON-serialised list of events (message sent, message edited etc) that happened to your bot. Each one has a unique sequence number which you can use to seek around in the update log (to say get updates only starting from the last one you processed). Updates related to messages have in them a chat ID identifying your conversation with a given user -- and you include that chat ID in your POST requests to
https://api.telegram.org/bot(TOKEN)/sendMessage (reference) in order to send messages back to that user.
You can also send around various other things besides text messages, like locations (latitude-longitude pairs), photos (your bot gets some links to various-sized thumbnails of the photo), contacts etc.
So I managed to write Indiana, a treasure hunt bot that comes up with a random location inside Hyde Park (well, the rectangle whose all 4 points lie within Hyde Park and yes, that means it can sometimes put the treasure in the water or in some restricted areas and I take no responsibility for you ending up there) and, when sent a location, replies back with a rough estimate of how far the treasure is. Sort of like Pokemon Go without having to lug around an extra power pack. Note you also can send the bot a manual location -- it can't distinguish between that and a physical location read from GPS (thankfully).