add notes on golang testing

1 files changed, 102 insertions, 0 deletions

diff --git a/learn/golang/README.md b/learn/golang/README.md
index 2062a3c..ead94b6 100644
--- a/learn/golang/README.md
+++ b/learn/golang/README.md
@@ -140,3 +140,105 @@ type Handler interface {
 
 This means that any `type` that implements the function `ServeHTTP(w, *r)` can
 be used as a `Handler` for the HTTP Mux.
+
+## Testing
+
+The Go stdlib includes a [testing package](https://go.dev/doc/code#Testing).
+These tests can be run using `$ go test`. To run all the tests in a project
+folder run `$ go test ./...`. The `./...` is the way that go expresses the idea
+of "just find all the go files and do the thing".
+
+To write a test you must declare a function name with a prefex of `Test` e.g.
+`func TestMyThing`. This function must accept a parameter of type `testing.T`.
+
+```go
+package x
+
+import "testing"
+
+func TestNew(t *testing.T) {
+  item := x.New()
+  // ...
+}
+```
+
+The `testing.T` type can be used to declare nested blocks of tests to be able to
+group associated tests together.
+
+```go
+package x
+
+import "testing"
+
+func TestNew(t *testing.T) {
+  item := x.New()
+  t.Run("when creating a new x", func(t *testing.T){
+    t.Run("it returns a pointer to a new instance", func(t testing.T){
+      // ...
+    })
+  })
+}
+```
+
+This allows you to write tests in the form of 'Context/Specifications',
+'Given/When/Then', 'Arrange/Act/Assert' or however you want.
+
+### Assertions
+
+The `testing` package does not include an assertion library so the canonical way
+to fail a test is to call `t.Error` or `t.Fail`. There are many different
+assertion libraries out there.
+[testity](https://github.com/stretchr/testify#assert-package) is a popular one.
+
+```go
+package x
+
+import (
+  "testing"
+  "github.com/stretchr/testify/assert"
+)
+
+func TestNew(t *testing.T) {
+  item := x.New()
+  assert.NotNil(t, item)
+}
+```
+
+### Arrange, Act, Assert
+
+1. Arrange: set the context for the test.
+2. Act: execute some behaviour on the [subject under test](http://xunitpatterns.com/SUT.html).
+3. Assert: assert that the desired result or interaction was achieved.
+
+```go
+package calculator
+
+import (
+  "testing"
+  "github.com/stretchr/testify/assert"
+)
+
+func TestCalculator(t *testing.T) {
+  // arrange
+  calc := calculator.New()
+
+  // act
+  result := cacl.Add(2, 2)
+
+  // assert
+  assert.NotNil(t, 4, result)
+}
+
+func TestCalculator(t *testing.T) {
+  // arrange
+  engine := // stub TODO:: figure out how to stub interfaces
+  calc := calculator.NewWithEngine(engine)
+
+  // act
+  result := cacl.Add(2, 2)
+
+  // assert
+  // engine.Compute("+", 2, 2) TODO:: figure out how to assert interactions on a mock or stub.
+  // e.g. expect(engine).to have_received(:compute).with("+", 2, 2)
+}
+```