Add test.ErrIsWanted for table driven tests (#4)

README.md

@@ -101,6 +101,77 @@ Will give you:
           }
 ```
 
+### Table Driven Tests
+
+Table driven tests are great! But when you test errors too it can get a bit awkward, you have to do the `if (err != nil) != tt.wantErr` thing and I personally
+*always* have to do the boolean logic in my head to make sure I got that right. Enter `test.ErrIsWanted`:
+
+```go
+func TestTableThings(t *testing.T) {
+    tests := []struct {
+        name    string
+        want    int
+        wantErr bool
+    }{
+        {
+            name:    "no error",
+            want:    4,
+            wantErr: false,
+        },
+        {
+            name:    "yes error",
+            want:    4,
+            wantErr: true,
+        },
+    }
+
+    for _, tt := range tests {
+        t.Run(tt.name, func(t *testing.T) {
+            got, err := SomeFunction()
+
+            test.ErrIsWanted(t, err, tt.wantErr)
+            test.Equal(t, got, tt.want)
+        })
+    }
+}
+```
+
+Which is basically semantically equivalent to:
+
+```go
+func TestTableThings(t *testing.T) {
+    tests := []struct {
+        name    string
+        want    int
+        wantErr bool
+    }{
+        {
+            name:    "no error",
+            want:    4,
+            wantErr: false,
+        },
+        {
+            name:    "yes error",
+            want:    4,
+            wantErr: true,
+        },
+    }
+
+    for _, tt := range tests {
+        t.Run(tt.name, func(t *testing.T) {
+            got, err := SomeFunction()
+
+            if tt.wantErr {
+                test.Err(t, err)
+            } else {
+                test.Ok(t, err)
+            }
+            test.Equal(t, got, tt.want)
+        })
+    }
+}
+```
+
 ### Credits
 
 This package was created with [copier] and the [FollowTheProcess/go_copier] project template.

test.go

@@ -63,19 +63,31 @@ func Err(t testing.TB, err error) {
 	}
 }
 
+// ErrIsWanted fails if you got an error and didn't want it, or if you
+// didn't get an error but wanted one.
+//
+// It simplifies checking for errors in table driven tests where on any
+// iteration err could either be nil or not.
+func ErrIsWanted(t testing.TB, err error, want bool) {
+	t.Helper()
+	if (err != nil) != want {
+		t.Fatalf("\nGot error:\t%v\nWanted error: %v\n", err, want)
+	}
+}
+
 // True fails if v is false.
 func True(t testing.TB, v bool) {
 	t.Helper()
 	if !v {
-		t.Fatalf("\nGot:\t%+v\nWanted:\t%+v", v, true)
+		t.Fatalf("\nGot:\t%v\nWanted:\t%v", v, true)
 	}
 }
 
 // False fails if v is true.
 func False(t testing.TB, v bool) {
 	t.Helper()
 	if v {
-		t.Fatalf("\nGot:\t%+v\nWanted:\t%+v", v, false)
+		t.Fatalf("\nGot:\t%v\nWanted:\t%v", v, false)
 	}
 }
 

test_test.go

@@ -86,6 +86,8 @@ func TestPass(t *testing.T) {
 			test.Diff(tb, struct{ Name string }{Name: "dave"}, struct{ Name string }{Name: "dave"})
 		},
 		func(tb testing.TB) { test.DeepEqual(tb, []string{"hello"}, []string{"hello"}) },
+		func(tb testing.TB) { test.ErrIsWanted(tb, errors.New("uh oh"), true) },
+		func(tb testing.TB) { test.ErrIsWanted(tb, nilErr(), false) },
 	}
 
 	for _, fn := range passFns {
@@ -155,6 +157,8 @@ func TestFail(t *testing.T) {
 			test.Diff(tb, struct{ Name string }{Name: "dave"}, struct{ Name string }{Name: "john"})
 		},
 		func(tb testing.TB) { test.DeepEqual(tb, []string{"hello"}, []string{"world"}) },
+		func(tb testing.TB) { test.ErrIsWanted(tb, errors.New("uh oh"), false) },
+		func(tb testing.TB) { test.ErrIsWanted(tb, nilErr(), true) },
 	}
 
 	for _, fn := range failFns {