Skip to main content
The console module gives Lion programs a way to emit output during evaluation. Both functions write to the console and then return the logged value, so they can be used inside a begin block without discarding a result you still need.

​
Functions

​
console/log

Logs a message string to the console and returns the message. 
["console/log", "hello"]
Output: hello Return value: "hello"

​
console/log-json

Pretty-prints any value as JSON with two-space indentation, then returns the formatted string. Useful for inspecting objects and arrays during development. 
["console/log-json", {"hello": "world"}]
Output: 
{
  "hello": "world"
}
Return value: "{\n \"hello\": \"world\"\n}"

​
Logging inside a sequence

Because both functions return the value they log, you can place them inside a begin block without interrupting the flow of your program: 
["begin",
  ["define", "result", ["number/add", 1, 2]],
  ["console/log-json", "result"],
  "result"]
console/log and console/log-json use Effect’s Console.log under the hood. In test environments that provide a custom Effect console layer, output may be captured rather than printed to stdout.
During development, wrap intermediate values with console/log-json to inspect them in place. Remove the wrapper once you are satisfied with the result — the surrounding expression is unaffected because both functions return the value they receive.