Lesson 25 rename and add comments

Author: dmitriz

examples/25-apply-natural-transformations.js

@@ -0,0 +1,96 @@
+const { Box, Either, Right, Left, fromNullable } = require('../examples/lib')
+
+const { List } = require('immutable-ext')
+
+const Task = require('data.task')
+
+// 'chain' does not exist on the array,
+// so we apply natural transformation into the List first
+const res = List(['hello', 'world'])
+.chain( x => List(x.split('')) )
+
+console.log(res)
+
+
+// natural tranformation from array to Either
+// safe first element
+const first = xs =>
+  fromNullable(xs[0])
+
+
+const largeNumbers = xs =>
+  xs.filter( x => x > 100 )
+
+const larger = x =>
+  x * 2
+
+
+const app = xs =>
+
+  // map, then first
+  first(
+    largeNumbers(xs)
+    .map(larger)
+  )
+
+console.log(app([2, 400, 5, 1000]))
+
+const app1 = xs =>
+
+  // first, then map
+  first(
+    largeNumbers(xs)
+  )
+  .map(larger)
+
+console.log(app1([2, 400, 5, 1000]))
+
+
+// fake user returned by id for testing purposes
+const fake = id => ({
+  id: id,
+  name: 'user1',
+  best_friend_id: id + 1
+})
+
+const Db = ({
+  find: id => new Task(
+    (rej, res) =>
+      // simulate error 'not found' for id <= 2
+      res( id > 2 ? Right(fake(id)) : Left(fake('not found')) )
+  )
+})
+
+// natural transform Either to Task
+const eitherToTask = e =>
+  e.fold(Task.rejected, Task.of)
+
+
+// valid user (id > 2)
+// -> Task returns Either
+Db.find(3)
+
+// Task result is 'Either' but we want to chain with plain function,
+// so apply natural transformation Either to Task first,
+.chain(eitherToTask)
+
+// now we have Task returning plain value,
+// errors were already sent to Task.rejected,
+// now map the result into another Task
+.chain(user => Db.find(user.best_friend_id))
+
+// the new result is again an Either,
+// so we map it again from Either to Task
+.chain(eitherToTask)
+.fork(console.error, console.log)
+
+
+// invalid user (id = 2)
+Db.find(2)
+.chain(eitherToTask)
+.chain(user => Db.find(user.best_friend_id))
+.chain(eitherToTask)
+.fork(console.error, console.log)
+
+
+