Implement withModel function in ProgramTest to perform simulations using model data

src/ProgramTest.elm

@@ -24,6 +24,7 @@ module ProgramTest exposing
     , routeChange
     , update
     , expectModel
+    , withModel
     , expectLastEffect, ensureLastEffect
     , simulateLastEffect
     , fail, createFailed
@@ -179,6 +180,7 @@ but you may find them useful to test things that are not yet directly supported
 
 @docs update
 @docs expectModel
+@docs withModel
 
 
 ## Low-level functions for effects
@@ -1917,6 +1919,33 @@ expectModel assertion =
         >> done
 
 
+{-| A way to use the current model in your simulations.
+
+When possible, you should not rely on your internal model but on the view to check that your program is behaving as expected.
+This function can be very helpful if your model contains data that you need in order to compute a value to test against.
+
+For example, let's say that you are storing the currentDay on your model, and you want to compute a date in the future to assert that a certain behaviour occurs.
+This way you can access the data inside the current model coming from the current state of your program.
+
+-}
+withModel :
+    (model -> ProgramTest model msg effect -> ProgramTest model msg effect)
+    -> ProgramTest model msg effect
+    -> ProgramTest model msg effect
+withModel fn programTest =
+    case programTest of
+        Created created ->
+            case created.state of
+                Err { reason } ->
+                    FailedToCreate reason
+
+                Ok state ->
+                    fn state.currentModel programTest
+
+        FailedToCreate failure ->
+            FailedToCreate failure
+
+
 {-| Simulate the outcome of the last effect produced by the program being tested
 by providing a function that can convert the last effect into `msg`s.