add: rough mock koan

Author: tnelson

F25/koans/brain_in_a_jar/mocks.ts

@@ -0,0 +1,84 @@
+
+/**
+ *  0320 Koan: "How do you know you're not a brain in a jar?"
+ * 
+ *  This one is messy at the moment, sorry. But ask yourself that 
+ *  question. Mocking is all about asking that question for a function,
+ *  class, or method that you write. 
+ */
+
+
+export const providence_lat = 41.8240;
+export const providence_lon = -71.4128;
+
+interface TemperatureResult {
+    success: boolean,
+    degrees?: number,
+    scale?: 'F' | 'C'
+}
+
+/** Make an API call to the National Weather Service. I'm leaving out 
+ *  Zod / careful validation here, because that's not the point of this 
+ *  example. The point is that this function _calls a real web api_.
+ */
+async function getRealTemperature(lat: number, lon: number): Promise<TemperatureResult> {
+    // As a demo, I'm using then/catch here instead of await.
+    return await fetch(`https://api.weather.gov/points/${lat},${lon}`)
+        .then(res => {
+            if (!res.ok) return {success: false}
+            return res.json();
+        })
+        .then(data => {
+            const forecastUrl = data.properties.forecast;
+            return fetch(forecastUrl);
+        })
+        .then(res => {
+            if (!res.ok) return {success: false}
+            return res.json();
+        })
+        .then(data => {
+            const timePeriodsData = data?.properties?.periods;
+            // This isn't actually right; it'll return the temp
+            // for the _next time period_, not the current temp.
+            // See: https://github.com/tnelson/pvd-weather/blob/main/src/App.tsx
+            //console.log(`${timePeriodsData[0].temperature}`)
+            return {success: true, 
+                    degrees: timePeriodsData[0].temperature, 
+                    scale: timePeriodsData[0].temperatureUnit} 
+        })
+        .catch(err => {
+            return {success: false}
+        });
+}
+
+/** But what if we want to do something with that temperature? */
+async function weatherReportProvidence(source: WeatherSource) {
+    const report = await source(providence_lat, providence_lon)
+    console.log(`It's ${report.degrees} ${report.scale} out!`)
+}
+
+/** Any function with this shape works as a weather source. */
+type WeatherSource = (latarg: number, lonarg: number) => Promise<TemperatureResult>
+
+/** Build a "temperature source" that never actually asks the NWS, and just
+ *  returns the provided constant value. */
+function buildTemperatureMock(temp: number, scale: 'F' | 'C'): WeatherSource {
+    // This builder returns an async function indistinguishable from the real source.
+    // (Note: we need the explicit return type here to get TypeScript to infer...)
+    return async function(lat: number, lon: number): Promise<TemperatureResult> {
+        return {success: true, degrees: temp, scale: scale}
+    }
+}
+
+const alwaysSunny = buildTemperatureMock(72, 'F')
+
+/** Note: don't try this outside of an async function.*/
+async function demo() {
+    // weatherReportProvidence can't tell the difference.
+    console.log(`real data:`)
+    await weatherReportProvidence(getRealTemperature)
+    console.log(`mock data:`)
+    // for prototyping/testing
+    await weatherReportProvidence(alwaysSunny)
+}
+demo()