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open System
type Logger = {
Log : string -> unit
}
type HelloService = {
SayHelloTo : string -> unit
}
// This is the constructor of a logger that adds a timestamp in the specified
// format to each log message it writes.
let dateTimeLogger (dateFormat : string) =
{ Log =
fun msg ->
let timestamp = DateTime.Now.ToString dateFormat
printfn "[%s]: %s" timestamp msg }
let logger = dateTimeLogger "yyyy-MM-dd hh:mm:ss"
// This implementation of Logger ignores all messages
let noopLogger =
{ Log = ignore }
let doSomething logger =
logger.Log "Starting to do something"
let x = 1 + 1
sprintf "Did something! Result: %d" x
|> logger.Log
// Implementations can be injected to application code
doSomething logger
doSomething noopLogger
// Implement HelloService. Our implementation depends on a Logger to write
// messages to.
let helloService logger =
{ SayHelloTo =
fun user ->
sprintf "Saying hi to %s" user
|> logger.Log
printfn "Hi %s!" user
logger.Log "Operation complete" }
// Create an instance of helloService.
//
// We don't want to do anything with the messages so we discard them by using
// injecting noopLogger as the implementation of Logger.
let instance = helloService noopLogger
instance.SayHelloTo "John"
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