Fable.Logging 0.12.1

Fable.Logging

A cross-platform logging framework for Fable. It mirrors the .NET Microsoft.Extensions.Logging pattern with ILogger, ILoggerFactory, and ILoggerProvider interfaces, letting you write idiomatic logging code in F# that works across JavaScript, Python, and Erlang/BEAM.

Packages

Package	NuGet	Description
Fable.Logging		Core interfaces, LoggerFactory, ConsoleLogger, JS console logger
Fable.Logging.Structlog		Python structlog provider
Fable.Logging.Beam		Erlang/OTP logger provider

Quick Start

Create a logger factory, configure it with providers, and start logging:

open Fable.Logging

let factory =
    LoggerFactory.Create(fun builder ->
        builder.AddProvider(ConsoleLoggerProvider())
        builder.SetMinimumLevel(LogLevel.Debug))

let logger = factory.CreateLogger("MyApp.Service")

logger.LogInformation("Application started")
logger.LogDebug("Processing request for {UserId}", box 42)
logger.LogError("Something went wrong")

Platform-Specific Providers

JavaScript (console)

The built-in JS logger maps log levels to the appropriate console.* methods (console.debug, console.info, console.warn, console.error).

open Fable.Logging
open Fable.Logging.JS

let factory =
    LoggerFactory.Create(fun builder ->
        builder.AddProvider(LoggerProvider()))

let logger = factory.CreateLogger("MyApp")
logger.LogInformation("Hello from {Platform}!", box "JavaScript")

Python (structlog)

Uses structlog for structured logging with support for both console and JSON output.

open Fable.Logging
open Fable.Logging.Structlog

// Console output (human-readable)
let factory =
    LoggerFactory.Create(fun builder ->
        builder.AddProvider(ConsoleLoggerProvider()))

// JSON output (machine-readable)
let jsonFactory =
    LoggerFactory.Create(fun builder ->
        builder.AddProvider(JsonLoggerProvider()))

let logger = factory.CreateLogger("MyApp")
logger.LogInformation("User {Name} logged in", box "Alice")

Erlang/BEAM (OTP logger)

Bridges to the OTP logger module for applications targeting the BEAM runtime.

open Fable.Logging
open Fable.Logging.Beam

let factory =
    LoggerFactory.Create(fun builder ->
        builder.AddProvider(LoggerProvider()))

let logger = factory.CreateLogger("MyApp")
logger.LogWarning("Connection pool running low: {Available} remaining", box 3)

Log Levels

Log levels match the .NET LogLevel enum:

Filtering

Set a minimum log level to filter out less severe messages:

let factory =
    LoggerFactory.Create(fun builder ->
        builder.AddProvider(ConsoleLoggerProvider())
        builder.SetMinimumLevel(LogLevel.Warning))

let logger = factory.CreateLogger("MyApp")
logger.LogDebug("This is filtered out")
logger.LogWarning("This is logged")

Message Templates

Use named placeholders in log messages for structured logging. The placeholder names become keys in the structured log output, while values are substituted positionally:

logger.LogInformation("Order {OrderId} placed by {Customer}", box 1234, box "Alice")
// Output: MyApp - Order 1234 placed by Alice

Logging Exceptions

try
    failwith "Something broke"
with ex ->
    logger.LogError("Operation failed", ex)

Multiple Providers

The factory dispatches log messages to all registered providers:

let factory =
    LoggerFactory.Create(fun builder ->
        builder.AddProvider(consoleProvider)
        builder.AddProvider(jsonProvider))

// Messages are sent to both providers
let logger = factory.CreateLogger("MyApp")
logger.LogInformation("This goes to all providers")

Writing a Custom Provider

Implement ILoggerProvider and ILogger to create your own logging backend:

open Fable.Logging

type MyLogger(name: string) =
    interface ILogger with
        member _.Log(state: LogState) =
            printfn "[%A] %s: %s" state.Level name state.Format

        member _.IsEnabled(logLevel: LogLevel) = true
        member _.BeginScope(_) = failwith "Not implemented"

type MyLoggerProvider() =
    interface ILoggerProvider with
        member _.CreateLogger(name) = MyLogger(name)
        member _.Dispose() = ()

License

This project is licensed under the MIT License - see the LICENSE file for details.