witness

gleam add witness@1 logging@1

import gleam/io
import witness

pub fn main() -> Nil {
  configure_logging()

  // log a message
  witness.this(witness.Info, "Some log message", [
    witness.string("example", "you can add structured data"),
    witness.bool("also_bools", True),
    witness.float("and_floats", 1.23),
    witness.int("ints_as_well", 3),
  ])
}

/// Configure the logging. (Only needs to be run once at startup)
///
fn configure_logging() {
  // create a new config
  witness.empty_config()
  // log formatted text to stdout
  |> witness.with_sink(witness.Text, fn(level, message) {
    io.println(witness.level_to_badge(level) <> ": " <> message)
  })
  // log as json 
  |> witness.with_sink(witness.Json, fn(_, message) {
    // ideally you'd log this to a file using, for example, the erlang logger
    io.println(message)
  })
  // apply this config globally
  |> witness.set_config()
}

Will log this json line:

{"timestamp":"2026-07-22T17:13:17.100815963Z","level":"info","message":"Some log message","example":"you can add structured data","also_bools":true,"and_floats":1.23,"ints_as_well":3}

And this text to the console:

[INFO]: 19:13:17 Some log message
  example: you can add structured data
  also_bools: True
  and_floats: 1.23
  ints_as_well: 3

Further documentation can be found at https://witness.hexdocs.pm.