Imperative vs. Declarative

In order to start our journey on thinking functionally, one really important concept 0:00 to dial in on is the difference between imperative and declarative coding styles. 0:05 The imperative style focuses specifically on how to perform the operations. 0:11 Typically at a low level, taking the system it is running on into account. 0:15 Declarative style focuses more on 0:19 what specifically you are attempting to accomplish. 0:22 The imperative style is definitely more difficult to read as there are more lines 0:26 of code and declarative is usually very clear as to what is happening. 0:30 Let's walk through an example one that you've probably already seen and 0:35 it will help demonstrate the differences. 0:38 For those of you with people in your lives that have food allergies, 0:41 you know how difficult it is to find something that they can eat. 0:44 My daughter has an egg allergy. 0:48 It's a bummer. 0:50 It's pretty horrible. 0:50 It seems like everything has eggs in it. 0:51 Cupcakes, taffy, ice cream. 0:54 You know, everything a kid likes to eat. 0:56 Now as a parent, I am constantly needing to check 0:59 a list of ingredients to see if it indeed contains eggs. 1:01 Let's write a quick simulation of this in code, in both styles. 1:05 So here I have the current version of IntelliJ IDEA open and 1:10 I'm just going to make a new project. 1:13 I'm gonna click create new project. 1:15 Cool. And I'm gonna just leave 1:18 the defaults here. 1:19 I'm gonna click next and I do want to just make a quick command line. 1:20 Let's go ahead and call this exploration. 1:26 Okay so that's set everything up for us. 1:31 Let's go ahead and let's start write our code here and suggest it. 1:33 Oops I indent with two space I'm using the Google style. 1:39 So we'll say list the string and those are ingredients, right? 1:42 And we'll make a new Arrays.asList. 1:50 And we'll just go ahead and we'll enter those guys, that's what we want. 1:53 So let's see, we have flour, what else do we have? 1:59 We've got some salt. 2:03 Delicious cupcakes here, some baking powder, butter, 2:06 I don't know how to cook really, eggs and here comes the milk. 2:11 All right, and we will close off with a semicolon here. 2:16 Okay, so let's solve our issue of checking whether or 2:22 not our ingredients has eggs, okay. 2:25 I'm gonna go ahead and give us more space. 2:28 Get rid of all the windows that are open, Here we go. 2:30 Okay, so first thing's first. 2:33 We'll need a variable to store our state, right? 2:35 So we'll wanna say boolean hasEggs, and we'll just go ahead, and 2:38 we don't know yet. 2:42 We don't know if it has, and so we said false. 2:42 And now, what we wanna do is we wanna loop through each of those ingredients. 2:45 So we can do that by starting to define a for loop, right? 2:49 So we'll say for and now we need another one of those temporary 2:51 variables to store in the loop where we're gonna currently process. 2:56 So we'll say int i for index, kind of standard thing there, right? 2:58 And we'll make another temporary variable, right? 3:05 So now we've got 2, we've got hasEggs and i, two temporary variables. 3:09 And now we wanna make sure that we go through all of the ingredients. 3:14 So we wanna see if i is less than the ingredients. 3:16 And we wanna see if it's as long as it is which was size. 3:20 And we've gotta remember that it's zero-based, right? 3:24 It starts at zero, so it's gotta be less, 3:27 not less than or equal to, always have to think about that there. 3:29 And then we're gonna go ahead and we're gonna increment. 3:32 Then we need to check if that specific ingredient, so 3:35 what we need to do is we need to pull it out, right? 3:38 We need to pull that one out. 3:41 So we'll say String.ingredient equals ingredients.get, and 3:42 we're gonna pass in that index that we've been running through the loop with, right? 3:48 Okay so then we wanna see does this ingredient that we pulled out, 3:52 does that equal eggs, right? 3:59 Okay, and if it does we will for sure. 4:06 We'll say hasEggs, we're gonna set that to true. 4:09 You know what since we did know it hasEggs, 4:13 we don't need to look at the other stuff right? 4:16 We can finish here, so we can just go ahead and say break. 4:18 Let's look at this code here for a second. 4:23 That's a lot of code and brainpower needed just to understand what's happening. 4:25 Now we've done this a lot and probably see code like this quite a bit. 4:30 So it's probably feeling kind of familiar. 4:34 But just imagine for a second if you hadn't. 4:37 There's a whole lot of what is this doing going on, right. 4:40 So now that we have our variable, we can go ahead and see if it has eggs. 4:44 Gotta break the news to my daughter and say, sorry sweetheart. 4:50 It has eggs. 4:56 Sad face emoji. 4:58 Now of course there's been some evolution in looping. 5:00 So we can of course clean up this cognitive overload a little bit of all 5:04 these things, right? 5:07 So we can, we can just go ahead we can say for 5:08 the ingredients in ingredients, right? 5:11 This enhanced four loop. 5:17 So we can lose this index cause each one is gonna have a specific time, right? 5:19 We don't need to know how long the list is anymore. 5:24 This reads nicer, but it's still an awful lot 5:27 when all we really want to know is, does ingredients contain eggs. 5:31 We're still saying give me each of the ingredients and 5:36 I'll tell you when to stop. 5:39 So the more declarative way to ask this question, 5:40 is just to ask the question, right? 5:44 So, let's do this. 5:47 If ingredients.contains, 5:49 Eggs, Right? 5:55 We don't need this. 5:59 And there's nothing new here. 6:01 Contains has been around since the collections framework was introduced. 6:03 But it's declarative. 6:07 Look at how it explains what it is that we're trying to do. 6:09 It's not specifying how. 6:12 Now this can be optimized efficiently behind the scenes. 6:14 And this is how it works in the real world, right. 6:17 If I'm at a restaurant, and I wanna know about a possible cupcake for 6:20 my daughter, I don't go, can you please tell me every ingredient in this cupcake. 6:23 Now don't worry I tell you when to stop listing them and 6:28 the case that I happen to hear what I'm concerned about, right. 6:30 I simply say, does this contain eggs? 6:33 And the waiter or waitress has their own method of knowing. 6:36 How they get the answer isn't really something I need to concern myself with. 6:39 I'm really just concerned with the answer to my question. 6:43 So ready for it? 6:47 This is the start of the shift of your thinking. 6:48 Functional programming is declarative. 6:51 It'll take a little bit of shifting in you programming mind, but 6:53 we do it in real life. 6:56 So it should feel more normal than how we already force our imperative mind to work. 6:57 One more example of how this looks in something you've probably seen. 7:03 Is SQL or Structured Query Language. 7:07 It's all declarative. 7:10 Even if you haven't seen SQL before, this is how you write the statements. 7:11 The language allows you to declare 7:15 very specifically what you would like to get back. 7:18 In this case, it's some specific fields like first name, last name, and email. 7:21 For a student with the id of 123. 7:25 Notice how it doesn't specify how. 7:28 This allows the database to do whatever indexing or 7:32 caching it needs on whatever platform that implements the language specification. 7:35 Let's stay in the declarative mindset and introduce our first function. 7:42