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Tutorial: Create a tool in a REPL

REPLs in Darklang are general-purpose coding blocks. They're typically to create reusable tools, experiments, one-off commands, or admin dashboards, in a similar way that you might use scripts or REPLs in other languages.

We'll run you through creating a REPL, experimenting with the HttpClient library to make API calls.

  1. Open the omnibox by pressing (Ctrl-K/Cmd-K), and create a new REPL.

  2. Type "httpclient" in the REPL (lowercase, no spaces)


This will show you all the standard library functions for HttpClient, their signatures, and their docstrings.

  1. Continue typing, until you have "httpclientpost". Darklang autocompletes on substrings so getting the exact text isn't important:


  1. Hit enter to add the function into the handler.


Darklang automatically creates spaces for the four arguments that HttpClient::post requires. We display a gray play button beside the function – it will turn green when all the arguments are complete, allowing you to run the function from within the editor.

  1. Let's call a new /test endpoint for the application we're developing. As we saw from the GET, our endpoints live at, so enter a string like "".


  1. Next let's add the arguments. The structured editor simplifies creation of the arguments we want:
  • For the body, we want to pass the value { test : "test1" }. To do so, type { to start a record, then hit enter and add the key and the value.

  • For the query, we want an empty dictionary. Type { to create one.

  • For the header, we want the JSON content-type, so type "json" and the autocomplete will offer you a function to use.


  1. Once we've filled in all the arguments, the play button (▶️) next to the function turns green, which means we can execute this code from within the editor.


  1. Hitting play we receive a 404 response. This is because there's no /test endpoint on our canvas (yet!)


  1. 404s in Darklang appear in the 404 section of the sidebar. We'll discuss that more later.


Congratulations! You've built a Darklang REPL and which calls the Darklang HTTP Client function.