Logging

• Introduction
  • Sending messages
  • How it works
  • Query timing
  • Suppressing query logging

Introduction

IHP uses fast-logger for logging. A FastLogger (which is just LogStr -> IO ()) is available via ?context.logger in controllers and ?modelContext.logger in model code.

Sending messages

In any controller or model code, you can log a message by calling the logger directly:

action TopPerformancesAction {collection} = do
    ?context.logger (toLogStr ("starting TopPerformancesAction" :: Text))
    let n = paramOrDefault 5 "numPerformances"
    band <- fetchBand collection
    topPerformances <- fetchTopPerformances collection n
    ?context.logger (toLogStr (show (length topPerformances) <> " top performances received." :: Text))
    render TopPerformancesView {..}

Make sure you have System.Log.FastLogger imported:

import System.Log.FastLogger (toLogStr)

How it works

IHP creates a FastLogger at startup using withFastLogger from the fast-logger package. This logger writes to stdout by default. It is stored in FrameworkConfig.logger and ModelContext.logger, and is accessible via implicit parameters in controllers and models.

The FastLogger type is just a function:

type FastLogger = LogStr -> IO ()

You convert text to LogStr using toLogStr and append a newline with <> "\n".

Query timing

In development mode (when the DEBUG environment variable is set), IHP automatically logs query timing information. This is controlled by the queryLoggingEnabled field on ModelContext, which is set based on the DEBUG environment variable.

Suppressing query logging

Use withoutQueryLogging to suppress query logs for a specific block:

users <- withoutQueryLogging (sqlQuery "SELECT * FROM users" ())