1 00:00:01,060 --> 00:00:07,390 So we left off here last time we wrote a very simple hello world web application, everything is built 2 00:00:07,390 --> 00:00:13,390 into one function, the main function, and all it does is it doesn't matter what page you go to, it 3 00:00:13,390 --> 00:00:14,460 just returns. 4 00:00:15,040 --> 00:00:16,560 Hello, world to the screen. 5 00:00:16,690 --> 00:00:21,240 So let's make this a little more useful and we're going to do this really gradually. 6 00:00:21,310 --> 00:00:25,330 First of all, if you have the application running in your terminal window, you need to quit out of 7 00:00:25,330 --> 00:00:25,480 it. 8 00:00:25,630 --> 00:00:31,630 Control seat will stop it and then I'll hide my terminal and let's have a look at what we have here. 9 00:00:31,630 --> 00:00:32,080 So far. 10 00:00:32,090 --> 00:00:38,300 Right now, we have the main function, plus this handler function, which is built in. 11 00:00:38,320 --> 00:00:41,750 It has a built-In anonymous function with the does everything we want it to do. 12 00:00:42,460 --> 00:00:45,870 So this works, but it might be a little more useful. 13 00:00:45,880 --> 00:00:51,490 Let's pretend we're building a website that consists of two pages, so we're going to have the home 14 00:00:51,490 --> 00:00:51,940 page. 15 00:00:51,940 --> 00:00:57,790 So I'm going to create a new function called Func Home, and this is going to be a handler function. 16 00:00:58,090 --> 00:01:03,430 And this is something you need to remember in order for a function to respond to a request from a Web 17 00:01:03,430 --> 00:01:06,190 browser, it has to handle two parameters. 18 00:01:06,640 --> 00:01:14,050 A response writer, which I'm going to call w http response writer, and it has to handle a request 19 00:01:14,090 --> 00:01:18,850 r star, dot star, dot request. 20 00:01:19,720 --> 00:01:21,370 And then of course it has to have the Brace's. 21 00:01:21,370 --> 00:01:24,270 So there is a function that will actually handle requests. 22 00:01:24,310 --> 00:01:28,780 Right now it doesn't do anything, but it does in fact handle requests. 23 00:01:29,050 --> 00:01:34,120 And what I can do is instead of this handler func with an embedded function, I can delete that entirely 24 00:01:34,690 --> 00:01:38,800 and say I'm going to make a http dot handle func. 25 00:01:39,720 --> 00:01:45,660 And it's going to respond to the root level of my Web application, in other words, the homepage, 26 00:01:46,020 --> 00:01:48,410 and then it's going to call a function home. 27 00:01:49,020 --> 00:01:53,580 Well, Hung is actually this function up here and this doesn't do anything. 28 00:01:54,000 --> 00:02:04,830 But I can, of course, do my standard response and just say format F print F F print F W for my response 29 00:02:04,830 --> 00:02:06,260 writer and then just return. 30 00:02:07,080 --> 00:02:13,920 This is the home page and then I'm going to have a second function and this is going to be an about 31 00:02:13,920 --> 00:02:14,250 page. 32 00:02:14,250 --> 00:02:17,610 So I'm just going to duplicate this line just to save copying and pasting. 33 00:02:18,090 --> 00:02:24,450 And I'm going to say for any request that comes to about, I want to go to a function called the boat, 34 00:02:25,050 --> 00:02:26,150 which doesn't exist yet. 35 00:02:26,310 --> 00:02:31,540 But again, I can copy this, paste it here, change it to a boat. 36 00:02:31,800 --> 00:02:33,960 So now this is linked to this. 37 00:02:34,350 --> 00:02:36,630 And I'm going to say instead of this is the home page. 38 00:02:36,630 --> 00:02:38,430 This is the about page. 39 00:02:39,060 --> 00:02:39,770 This is clear. 40 00:02:39,960 --> 00:02:46,260 Now, I have really short lines in my main function that point to external functions, but we can make 41 00:02:46,260 --> 00:02:47,610 a little more complex than that. 42 00:02:47,610 --> 00:02:52,040 Instead of just saying this is the about page, let's have it return a computed function. 43 00:02:52,320 --> 00:02:54,480 So I'm going to have another function func. 44 00:02:54,870 --> 00:02:58,020 And this doesn't actually have it's not going to be a handler. 45 00:02:58,020 --> 00:03:02,550 It's just going to be a standard function that I can do anything with and I'll call it add values. 46 00:03:03,060 --> 00:03:08,520 And this is going to take two parameters, X and Y, both of which are integers. 47 00:03:08,520 --> 00:03:14,210 And it's going to return an integer, which is just not integer. 48 00:03:15,060 --> 00:03:17,220 And I'm going to do is return. 49 00:03:19,770 --> 00:03:22,490 X plus one there. 50 00:03:22,920 --> 00:03:27,960 So now this function isn't called yet, hence my ID, hopefully Graser to let me know that it's unused 51 00:03:28,440 --> 00:03:33,840 but it's going to take to values X and Y, it's going to add them and return that value. 52 00:03:33,870 --> 00:03:43,560 Now, I could have done this in two lines and said some is assigned the value of X plus Y and then return 53 00:03:43,560 --> 00:03:47,370 some instead or and you just get used to this. 54 00:03:47,370 --> 00:03:51,120 There's multiple ways of doing things in certain parts of go. 55 00:03:51,480 --> 00:03:55,530 I could declare a variable sum that's an ENT and then make some. 56 00:03:55,710 --> 00:03:57,770 I'm not creating and assigning the type here. 57 00:03:57,780 --> 00:03:58,800 This is the shorthand. 58 00:03:59,220 --> 00:04:02,170 I could just say some equals X plus Y and that works. 59 00:04:02,190 --> 00:04:03,020 All of those work. 60 00:04:03,300 --> 00:04:09,420 Of course I like to be concise, so the way I'm actually going to do it is to return X plus what. 61 00:04:10,540 --> 00:04:14,350 Great, now you've got to start thinking like a programmer. 62 00:04:14,740 --> 00:04:19,790 Are there any situations where this function might not work? 63 00:04:20,260 --> 00:04:26,080 What if I pass it X and Y, but one of those is not an integer? 64 00:04:26,500 --> 00:04:28,210 Well, actually, it wouldn't compile. 65 00:04:28,210 --> 00:04:35,470 But for the sake of argument, I probably want to say don't just return an ink type, return an integer. 66 00:04:35,470 --> 00:04:40,950 And if there's an error, return that which means I'm returning to types. 67 00:04:41,410 --> 00:04:44,370 So here I can't do the shorthand of X plus one. 68 00:04:44,380 --> 00:04:45,550 I need to add them together. 69 00:04:46,150 --> 00:04:53,480 So I could say if I was throwing an error, I would have to return my values so I would declare a variable. 70 00:04:53,500 --> 00:04:56,790 I'm actually not going to do this because it's impossible to throw an error here easily. 71 00:04:57,100 --> 00:05:06,570 I could say var some sum equals X plus Y and then return that and nil because there's no error. 72 00:05:06,670 --> 00:05:11,530 But we actually don't have returns some and nil and some is in it. 73 00:05:13,100 --> 00:05:15,430 Like I need more caffeine anyway. 74 00:05:15,430 --> 00:05:18,430 That's what I would normally do and I would check for an error here. 75 00:05:18,430 --> 00:05:22,030 I'm just adding to since I know they're going to be ense because it's a strongly typed language, I 76 00:05:22,030 --> 00:05:23,830 can't accept anything that's not an end. 77 00:05:24,130 --> 00:05:27,460 But I can return my value and an error if there isn't any. 78 00:05:27,520 --> 00:05:36,130 Of course, this is a really simple, straightforward function, so I'll just go return X plus Y and 79 00:05:36,130 --> 00:05:41,830 make this return just an int, but we'll create a third page in a minute where it's more possible to 80 00:05:41,830 --> 00:05:42,370 have an error. 81 00:05:42,490 --> 00:05:44,800 OK, so now I've got this function. 82 00:05:44,800 --> 00:05:46,470 I actually want to return it. 83 00:05:46,780 --> 00:05:52,210 So let's go up here and call it, let's say, my sum. 84 00:05:52,210 --> 00:05:59,770 And here the scope of this sum is only within this variable that's going to be equal to add values and 85 00:05:59,770 --> 00:06:02,600 I'll pass it to integers two and two. 86 00:06:03,760 --> 00:06:06,160 And I can't just not use this variable. 87 00:06:06,160 --> 00:06:06,850 I've created it. 88 00:06:06,850 --> 00:06:08,830 So Go says we've got that variable. 89 00:06:08,980 --> 00:06:11,510 I don't want variables hanging around that don't do anything. 90 00:06:11,770 --> 00:06:13,680 So if you're going to create it, you better use it. 91 00:06:13,990 --> 00:06:18,850 So let's change this instead of just returning a string here, let's cut that out there and format as 92 00:06:18,850 --> 00:06:27,970 print F and now we're going to say this is the about page and two plus two is and now I'm going to put 93 00:06:27,970 --> 00:06:31,750 it in there and remember first print F which I better spell right. 94 00:06:33,400 --> 00:06:39,670 S print F actually wants a placeholder and the placeholder for it is percent d if it was a string it 95 00:06:39,670 --> 00:06:44,380 would be percent s but it's a we'll learn more about that as we use it right now. 96 00:06:44,380 --> 00:06:47,950 I need to put it at integer value in there and I just calculated one. 97 00:06:47,950 --> 00:06:52,510 Am I calculated integer is something so I can put that in there and write it out. 98 00:06:52,840 --> 00:06:55,840 And this throws an error which I'm going to ignore right now. 99 00:06:55,840 --> 00:06:59,890 And remember, the shorthand for ignoring and ignoring something that's return by function is to use 100 00:06:59,890 --> 00:07:00,610 the underscore. 101 00:07:01,750 --> 00:07:04,300 So F productive actually returns to things. 102 00:07:04,480 --> 00:07:04,810 Right? 103 00:07:04,820 --> 00:07:07,290 So I'll ignore both of them. 104 00:07:08,110 --> 00:07:13,960 So now I've created a home handler, I've created an about handler and I have these which point to those 105 00:07:13,960 --> 00:07:17,460 functions and this function about actually calls add values. 106 00:07:18,370 --> 00:07:23,980 Now, here's another thing you have to get used to when a function name or a variable name, which will 107 00:07:23,980 --> 00:07:30,030 get to later on, begins with a capital letter, it means it's accessible outside a given package. 108 00:07:30,040 --> 00:07:33,700 It's not true for the main package because you can't call the main package from an external package, 109 00:07:34,270 --> 00:07:37,300 but you may as well get in the habit of doing things properly. 110 00:07:37,570 --> 00:07:42,550 I am never going to call this function from anywhere else, so I'm going to give it a lowercase name 111 00:07:42,850 --> 00:07:44,020 and that hides it. 112 00:07:44,320 --> 00:07:49,600 If you come from object oriented background, if you've done some programming like in Java or even for 113 00:07:49,600 --> 00:07:53,530 that matter, you can think of this as making the function private. 114 00:07:54,100 --> 00:07:58,120 The only places that this function can be called from are within this package. 115 00:07:58,330 --> 00:07:59,500 And the way that go does it. 116 00:07:59,500 --> 00:08:05,800 If it has an upper area uppercase letter for the first letter of the function name, then it's visible 117 00:08:05,800 --> 00:08:06,940 outside of the package. 118 00:08:07,180 --> 00:08:09,240 If it has a lowercase one, it is not. 119 00:08:09,310 --> 00:08:11,020 So I'm going to leave it like that right now. 120 00:08:11,530 --> 00:08:16,150 And we may also get into the habit of giving comments to things as well because they're quite helpful. 121 00:08:16,600 --> 00:08:21,430 A comment is something in a source file that is ignored by the compiler, and it's just there for people 122 00:08:21,430 --> 00:08:22,630 like you and me to read. 123 00:08:22,990 --> 00:08:27,490 And you can start you can write a function with two slashes and the convention in go. 124 00:08:27,520 --> 00:08:31,360 Is that the very first thing that should appear in the comment above? 125 00:08:31,360 --> 00:08:38,250 A function is the function name about is the about the page scandal. 126 00:08:38,860 --> 00:08:45,060 And the same thing with Home Home is the home page handler. 127 00:08:47,230 --> 00:08:52,600 Now, you don't have to give things comments, of course, but you probably should, even though this 128 00:08:52,600 --> 00:08:54,580 one's not going to be visible outside of the package. 129 00:08:54,910 --> 00:08:56,170 I'm going to give it a comment. 130 00:08:57,670 --> 00:09:05,620 Add values, adds to integers and returns the sum and then main, I'll just call. 131 00:09:05,620 --> 00:09:08,350 This is the main. 132 00:09:10,670 --> 00:09:16,070 Application of the entry point, in other words, OK, so there we are. 133 00:09:16,250 --> 00:09:17,480 Now I've got this all built. 134 00:09:17,720 --> 00:09:19,880 Let's compile it and see what happens. 135 00:09:19,910 --> 00:09:24,410 So all I do is open my terminal and go run the main domain. 136 00:09:25,240 --> 00:09:25,570 Go. 137 00:09:25,640 --> 00:09:26,060 That's it. 138 00:09:26,630 --> 00:09:27,470 So it starts. 139 00:09:27,680 --> 00:09:28,820 It seems to be running. 140 00:09:29,060 --> 00:09:32,150 Nothing strange is happening, but I'm going to make it even even simpler. 141 00:09:32,540 --> 00:09:34,280 I'm going to stop this and come down here. 142 00:09:34,280 --> 00:09:41,090 And just before I call my HDB, listen and serve, I'm going to write to the console and I'm going to 143 00:09:41,090 --> 00:09:42,140 do that like this. 144 00:09:42,920 --> 00:09:47,300 I'm going to come to the top just after my import statements and I'm going to say, well, the port 145 00:09:47,390 --> 00:09:50,890 is something I might want to change and I might use it more than one place. 146 00:09:51,140 --> 00:09:57,200 So rather than just putting Port 80, 80 like this, I'm going to create a variable called Port No. 147 00:09:59,420 --> 00:10:03,530 Which doesn't exist yet, and I could declare it inside the main function if I wanted to, but I might 148 00:10:03,530 --> 00:10:04,910 want to use it another function. 149 00:10:04,910 --> 00:10:13,400 So to do that, I'm going to change where I declare this variable and I'm going to say var porked number 150 00:10:13,430 --> 00:10:15,960 equals Colen eighty eight. 151 00:10:16,490 --> 00:10:20,700 And now that I've done that, I can actually use that where I need to. 152 00:10:20,960 --> 00:10:24,900 So I've got my number now and I can use that down here and I already have. 153 00:10:24,920 --> 00:10:25,610 So that's great. 154 00:10:25,640 --> 00:10:28,100 So the simple thing I can do here is say Vanport. 155 00:10:28,100 --> 00:10:30,530 No, I can actually get better than that. 156 00:10:30,560 --> 00:10:34,850 I never want this number to be changed by another part of the application. 157 00:10:35,180 --> 00:10:39,440 So I can say instead of VAR, which is a variable which can you can just overwrite it with a different 158 00:10:39,440 --> 00:10:40,130 string value. 159 00:10:40,550 --> 00:10:41,750 I can call it a constant. 160 00:10:42,110 --> 00:10:47,280 And the nice thing about a constant is it can't be changed by anything else in the application. 161 00:10:48,020 --> 00:10:53,690 So now that I have my number specified, I can come down here and I want to write to the console and 162 00:10:53,690 --> 00:10:57,350 I'm just going to say format, print, line. 163 00:10:59,190 --> 00:11:01,700 And then again in here, I want to use that part number. 164 00:11:01,700 --> 00:11:04,220 So I'm going to use format as a print F. 165 00:11:07,490 --> 00:11:15,140 Starting, not staring, starting application on port, and this is a placeholder, but it's a string 166 00:11:15,410 --> 00:11:18,610 and it's not an int, which is the only thing we've used so far. 167 00:11:19,340 --> 00:11:21,230 So Percent s is the placeholder. 168 00:11:21,560 --> 00:11:24,610 And now I need to say, what do I want to replace that placeholder with? 169 00:11:24,620 --> 00:11:25,930 I want to replace it with Port. 170 00:11:25,940 --> 00:11:26,220 No. 171 00:11:26,810 --> 00:11:27,260 All right. 172 00:11:27,860 --> 00:11:29,710 So let's run the application again. 173 00:11:29,720 --> 00:11:34,640 And now when I start it up, it's going to say starting application on Port 80 80. 174 00:11:34,670 --> 00:11:39,070 So let's go over to my Web browser and refresh this page. 175 00:11:39,560 --> 00:11:41,030 And this is the home page. 176 00:11:41,060 --> 00:11:47,270 So when I get this is the home page up here, that's because I didn't specify anything after the URL. 177 00:11:47,270 --> 00:11:49,370 If I put a slash there, I should get the same thing. 178 00:11:50,210 --> 00:11:51,320 This is the home page. 179 00:11:51,680 --> 00:11:57,620 And down here it says in these handlers, aha, if it has nothing or a slash, go to home. 180 00:11:57,920 --> 00:12:01,250 And the home function just says this is the home page. 181 00:12:02,360 --> 00:12:03,740 Let's try the about page. 182 00:12:04,190 --> 00:12:12,230 So here back in my Web browser about this is the about page and two plus two is for exactly what I wanted. 183 00:12:12,230 --> 00:12:13,150 And what did it do. 184 00:12:13,160 --> 00:12:17,240 It said, well when you made the request, it wasn't just something to ignore that. 185 00:12:17,660 --> 00:12:19,670 And under here we have slash about. 186 00:12:19,820 --> 00:12:26,810 So now return whatever the about function gives us and the about function calls this function add values 187 00:12:27,170 --> 00:12:33,530 adds two plus to the parameters I specified here and stores that in the variable sum, which is an int 188 00:12:33,530 --> 00:12:38,090 because we use the shorthand assignment, this returns and therefore this is an Int. 189 00:12:38,840 --> 00:12:44,720 And down here I just say print to the response rate or w this computed statement. 190 00:12:44,840 --> 00:12:46,910 This is the which I spelled wrong. 191 00:12:46,910 --> 00:12:47,990 So I means we'll fix that. 192 00:12:48,260 --> 00:12:55,910 This is the about page and two plus two is percent D replace percent D with some and two plus two is 193 00:12:55,910 --> 00:12:56,390 four. 194 00:12:56,900 --> 00:12:57,590 So there you go. 195 00:12:57,980 --> 00:13:02,810 Now we can handle two pages but of course we can handle many pages, as many pages as we want to. 196 00:13:03,200 --> 00:13:05,800 But this is sufficient for our purposes for the moment. 197 00:13:06,230 --> 00:13:07,350 So let's stop now. 198 00:13:07,410 --> 00:13:09,290 We'll pick it up in the next lecture.