Formatting

Type-safe, functional string formatting in Swift.

Inspired by Chris Done’s excellent Haskell library.

import Formatting

format("Hello, " % string % "!", "world")
// "Hello, world!"

Introduction

Traditional string formatting methods (interpolation, printf, and template strings) can lead to subtle (and not so subtle) runtime bugs.

print("Hello, \(thing)!")              // Hello, nil!
print("Hello, \(thing)!")              // Hello, Optional("world")!

NSLog(@"Hello, %@!", thing);           // Hello, (null)!

Formatting brings compile-time checks.

print("Hello, " % string % "!", thing) // Value of optional type String? not unwrapped

And composability.

let greet =
  format("Hello, " % string % "!")

greet("world")                         // Hello, world!

Composing formatters

Use % to build a formatter with strings and other formatters.

format(string % " is " % int % "years old.", "Alice", 25)
// "Alice is 25 years old."

Use <> to pass the previous formatter argument to the next formatter.

format(yyyy % "-" <> MM % "-" <> dd, Date())
// "2016-06-28"

Use .% to feed the result of one formatter into another.

format(left(2, "0") .% hex, 10)
// "0a"

Call format without arguments to return a curried formatter function.

let log =
  format(right(5) % " -- [" % iso8601 % "] " % string)

let infoLog = log("INFO")
let debugLog = log("DEBUG")

infoLog(Date())("Logging in...")
// "INFO  -- [2016-06-28T12:34:56Z] Logging in..."
debugLog(Date())("Logged in successfully!")
// "DEBUG -- [2016-06-28T12:34:56Z] Logged in successfully!"

License

Formatting is available under the MIT license. See the LICENSE
file for more information.