Efs, as the pronounced plural of letter F (for function), is a simple mocking/stubing library for testing purpose.

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Kind of functions

✅ Passthrough functions

You can stub functions to just return simple values : in Efs, it’s called Passthrough.

// Based on this kind of injection:
struct Dependencies {
  typealias SaveToDisk = (User) -> User.SaveStatus

  let saveToDisk: SaveToDisk
  init(saveToDisk: @escaping SaveToDisk) {
    self.saveToDisk = saveToDisk

// Inject a simple passthrough stub:
let dependencies = Dependencies(
  saveToDisk: Efs.passthrough(.succeed)
dependencies.saveToDisk(user) // returns .succeed

❌ Failing functions

You can mock functions with automatic failing. It can alert you when some functions are called without intention of.

// Inject a simple failing stub:
let dependencies = Dependencies(
  saveToDisk: Efs.failing()
dependencies.saveToDisk(user) // will raise fatalError 


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