Logger

Flix provides Logger as a library effect for structured logging. The Logger effect has a default handler, so no explicit runWithIO call is needed in main. The key module is Logger.

The Logger Effect

The Logger effect has a single operation that logs a message at a given severity:

pub eff Logger {
    /// Logs the given message `m` at the given severity `s`.
    def log(s: Severity, m: RichString): Unit
}

The Severity enum defines five levels, from lowest to highest:

pub enum Severity with Eq, Order, ToString {
    case Trace
    case Debug
    case Info
    case Warn
    case Fatal
}

The Logger Module

The Logger module provides convenience functions:

mod Logger {
    /// Logs the message `m` at the Trace level.
    def trace(m: a): Unit \ Logger with Formattable[a]

    /// Logs the message `m` at the Debug level.
    def debug(m: a): Unit \ Logger with Formattable[a]

    /// Logs the message `m` at the Info level.
    def info(m: a): Unit \ Logger with Formattable[a]

    /// Logs the message `m` at the Warn level.
    def warn(m: a): Unit \ Logger with Formattable[a]

    /// Logs the message `m` at the Fatal level.
    def fatal(m: a): Unit \ Logger with Formattable[a]

Note: The logging functions accept any type that implements the Formattable trait. Most standard types (String, Int32, Bool, etc.) implement Formattable, so plain values can be logged directly. The trait converts values into RichString, which supports styled terminal output (colors, bold, etc.).

Logging Messages

The convenience functions accept any value that implements Formattable:

def main(): Unit \ { Logger } =
    Logger.info("Application started");
    Logger.debug("Loading configuration...");
    Logger.warn("Cache size exceeds threshold");
    Logger.fatal("Unrecoverable error")

The default handler prints each message to standard output with a colored severity prefix, for example: [Info] Application started.