Update README.md

Author: rentziass

README.md

@@ -1,18 +1,111 @@
-# eventually
+# Eventually
+
+[![PkgGoDev](https://pkg.go.dev/badge/github.com/rentziass/eventually)](https://pkg.go.dev/github.com/rentziass/eventually)
+
+Eventually provides support for running a test block that should eventually succeed, while still
+providing access to a `testing.TB` throughout the whole block. Eventually will keep trying re-running
+the test block until either a timeout or max attempts are reached. While this was created to test
+asynchronous systems I suppose it might help with flaky tests too :D. Here's an example:
 
 ```go
-// Must will keep trying until it succeeds or the test times out or reaches
-// max attempts. If it fails, it will call t.Fatalf.
-eventually.Must(t, func(t testing.TB) {
-	t.Fatal("but keep trying")
-}, eventually.WithMaxAttempts(10))
+func TestAsync(t *testing.T) {
+  asyncProcessSucceeded := false
+
+  go func() {
+    time.Sleep(100 * time.Millisecond)
+    asyncProcessSucceeded = true
+  }()
+
+  eventually.Should(t, func(t testing.TB) {
+    if !asyncProcessSucceeded {
+      t.Fail()
+    }
+  })
+}
+```
+
+Notice how within the function you pass to `eventually.Should` you have access to a `t testing.TB`, this
+allows you to still use your helpers and assertions that rely on the good old `t`. Here's another example,
+using [testify](https://github.com/stretchr/testify)'s `assert` and `require`:
+
+```go
+// code that sets up async consequences, e.g. writes some events on a queue
+
+eventually.Should(t, func(t testing.TB) {
+  events, err := readFromQueue()
+  require.NoError(t, err)
+  require.Len(t, events, 1)
+  assert.Equal(t, "event", events[0])
+})
+
+```
+
+Eventually has [`Should`](https://pkg.go.dev/github.com/rentziass/eventually#Should) and [`Must`](https://pkg.go.dev/github.com/rentziass/eventually#Must) functions, that 
+correspond to [`Fail`](https://pkg.go.dev/testing#T.Fail) and [`FailNow`](https://pkg.go.dev/testing#T.FailNow) respectively in case of failure.
+
+Behaviour can be customised with use of [`Options`](https://pkg.go.dev/github.com/rentziass/eventually#Option), for example:
+
+```go
+eventually.Should(t, func(t testing.TB) {
+  // your test code here
+},
+  eventually.WithTimeout(10*time.Second),
+  eventually.WithInterval(100*time.Millisecond),
+  eventually.WithMaxAttempts(10),
+)
+```
+
+And if you want to reuse your configuration you can do so by creating your very own `Eventually`. The example above would look something like:
+
+```go
+eventually := eventually.New(
+  eventually.WithTimeout(10*time.Second),
+  eventually.WithInterval(100*time.Millisecond),
+  eventually.WithMaxAttempts(10),
+)
 
-// Should will keep trying until it succeeds or the test times out or reaches
-// max attempts. If it fails, it will call t.Errorf.
 eventually.Should(t, func(t testing.TB) {
-	t.Fatal("but keep trying")
-}, eventually.WithTimeout(10*time.Second), eventually.WithInterval(1*time.Second))
+  // test code
+})
+
+eventually.Must(t, func(t testing.TB) {
+  // test code
+})
+```
+
+## Why does this exist?
+
+> TL;DR: I like `t` **a lot**
+
+Other testing libraries have solutions for this. Testify for instance has its own [`Eventually`](https://pkg.go.dev/github.com/stretchr/[email protected]/assert#Eventually), but
+the function it takes returns a `bool` and has no access to an "inner" `*testing.T` to be used for helpers and assertions.
+Let's say for example that you have a helper function that reads a file and returns its content as a string, failing the test if 
+it can't find the file (more convenient than handling all errors in the test itself). If the file you want to test is being
+created asynchronously using that helper within Eventually will halt the whole test instead of trying executing again. In Go code:
+
+```go
+func TestAsyncFile(t *testing.T) {
+  // setup
+
+  assert.Eventually(t, func() bool {
+    contents := readFile(t, "path") // <-- this halts the whole TestAsyncFile, not just this Eventually run
+    return contents == "expected"
+  })
+}
+
+func readFile(t *testing.T, path string) string {
+  f, err := os.Open(path)
+  require.NoError(t, err)
+
+  // reading the file
+}
 ```
 
-## TODO
-- [ ] Add documentation
+Another available alternative is Gomega's [`Eventually`](https://pkg.go.dev/github.com/onsi/gomega#Eventually) (yes, this package has a very original name), which can be very convenient to use but requires buying into Gomega as a whole, which is quite the commitment (and I don't find a particularly idiomatic way of writing tests in Go but hey, opinions). This also still doesn't give access to a `t` with its own scope, you can do assertions within the `Eventually` block but if you have code that relies on `*testing.T` being around you cannot use it:
+
+```go
+gomega.Eventually(func(g gomega.Gomega) {
+  contents := readFile(t, "path") // no t :(
+  g.Expect(contents).To(gomega.Equal("expected"))
+}).Should(gomega.Succeed())
+```