pure function DI pattern

example.go

package example

// This is the function you want to test. It has as a depdendency a pure
// function, foo, that you would like to control for tests.
func doSomeWork() int {
  y := 0
  // ...
  x := fooFunc(y)
  // ...
  return x
}

// This is the dependency
func foo(x int) int {
  // ...
  // do the thing for real
  // ...
  return x
}

// This is the seam, you swap it out in tests.
var fooFunc = foo

example_test.go

package example

func fakeFoo(x int) int {
  // The fake/mock/stub functionality goes here
  return 0
}

func TestDoSomeWork() {
  oldFooFunc := fooFunc
  fooFunc = fakeFoo
  defer func() { fooFunc = oldFooFunc }()

  // Here write tests for doSomeWork, which now uses fakeFoo
  assert.True(doSomeWork() == ...)
}