Reputation: 6462
I'm trying to create 2 logging functions: one is to directly print to stdout, another is to print with an antecedent string [Error]
.
let log message =
printfn message
let twError (tw:System.IO.TextWriter) message =
tw.Write("[Error] {0}", message)
let error (message:Printf.StringFormat<_>) =
let format = Printf.TextWriterFormat<_>(sprintf "[Error] %A" message)
log format
let date = System.DateTime.Now
log "%A" twError date
error "%s" <| date.ToString()
As you can see, in the error
function I need to make a new string, then convert it to a TextWriterFormat
type. But this will be less efficient than writing directly to the stream, how should I change the error
function to make it more efficient?
Upvotes: 3
Views: 272
Reputation: 36738
You want the Printf.kprintf
family of functions. Their documentation is very hard to understand, but the concept is simple:
printf
takes a format string (and arguments), formats it, and then prints it to the consolesprintf
takes a format string (and arguments), formats it, and then returns it as a stringkprintf
takes a format string (and arguments), formats it, and then passes it (as a string) to a function that you provide. That function should take in a string and do whatever you want with it.For example:
open Printf
let addDateTime (s : string) =
sprintf "%A: %s" DateTime.UtcNow s
sprintf "%A" [1; 2; 3] // Returns "[1; 2; 3]"
kprintf addDateTime "%A" [1; 2; 3] // Returns "2017-05-26 06:05:00: [1; 2; 3]"
let printDateTime (s : string) =
printfn "%A: %s" DateTime.UtcNow s
printfn "%A" [1; 2; 3] // Prints "[1; 2; 3]"
kprintf printDateTime "%A" [1; 2; 3] // Prints "2017-05-26 06:05:00: [1; 2; 3]"
So your error
function could simply become:
let errorLogger (s : string) = printfn "[Error] %s" s
let error msg = kprintf errorLogger msg
error "Oops: %A" [1; 2; 3] // Prints "[Error] Oops: [1; 2; 3]"
Experiment with kprintf
a bit more and see what you can do with it. It's a very powerful function, and it's a shame that more people don't know about it.
Upvotes: 3