Through section 6.2

elm.json

@@ -23,7 +23,9 @@
         }
     },
     "test-dependencies": {
-        "direct": {},
+        "direct": {
+            "elm-explorations/test": "2.0.1"
+        },
         "indirect": {}
     }
 }

src/PhotoGroove.elm

@@ -1,4 +1,4 @@
-port module PhotoGroove exposing (main)
+port module PhotoGroove exposing (Model, Msg(..), initialModel, main, photoDecoder, update)
 
 import Browser
 import Html exposing (..)

tests/PhotoGrooveTests.elm

@@ -0,0 +1,42 @@
+module PhotoGrooveTests exposing (..)
+
+import Expect exposing (Expectation)
+import Fuzz exposing (Fuzzer, int, list, string)
+import Json.Decode as Decode exposing (decodeValue)
+import Json.Encode as Encode
+import PhotoGroove exposing (Model, Msg(..), initialModel, photoDecoder, update)
+import Test exposing (..)
+
+
+decoderTest : Test
+decoderTest =
+    fuzz2 string int "title defaults to (untitled)" <|
+        \url size ->
+            Encode.object
+                [ ( "url", Encode.string url )
+                , ( "size", Encode.int size )
+                ]
+                |> decodeValue photoDecoder
+                |> Result.map .title
+                |> Expect.equal
+                    (Ok "(untitled)")
+
+
+sliders : Test
+sliders =
+    describe "Slider sets the desired field in the Model"
+        [ testSlider "SlidHue" SlidHue .hue
+        , testSlider "SlidRipple" SlidRipple .ripple
+        , testSlider "SlidNoise" SlidNoise .noise
+        ]
+
+
+testSlider : String -> (Int -> Msg) -> (Model -> Int) -> Test
+testSlider description toMsg amountFromModel =
+    fuzz int description <|
+        \amount ->
+            initialModel
+                |> update (toMsg amount)
+                |> Tuple.first
+                |> amountFromModel
+                |> Expect.equal amount