1 00:00:01,400 --> 00:00:07,310 So we left off last time with a simple Hello World program, and this time I want to make another hello 2 00:00:07,310 --> 00:00:11,840 world program, but I want to make it a little more complex this time. 3 00:00:11,840 --> 00:00:18,710 Instead of just printing Hello World to the screen, I want to create a Web application that listens 4 00:00:18,710 --> 00:00:25,250 on a particular part so I can hit that application with my favorite web browser and see Hello World 5 00:00:25,250 --> 00:00:26,730 printed to the browser window. 6 00:00:27,380 --> 00:00:33,200 So that is actually pretty simple and it doesn't sound that interesting, but it actually is a lot more 7 00:00:33,200 --> 00:00:34,190 interesting than you think. 8 00:00:34,190 --> 00:00:35,630 And you'll see why shortly. 9 00:00:36,140 --> 00:00:40,400 So I'm back in my Hello World program and I'm just going to modify this one. 10 00:00:40,760 --> 00:00:45,900 And this time, instead of just printing Hello World to the screen, I'm going to comment that oath, 11 00:00:46,910 --> 00:00:53,270 which automatically makes my idy remove that from FMT format, function or package that I don't need 12 00:00:53,270 --> 00:00:53,680 anymore. 13 00:00:53,960 --> 00:01:00,260 And this time I'm going to create a new function within my main function. 14 00:01:00,350 --> 00:01:02,080 And it's you'll see how it works here. 15 00:01:02,480 --> 00:01:08,230 So we're going to use a built in package that's part of the standard library for God called HTP. 16 00:01:08,390 --> 00:01:12,400 And as the name implies, it's one that deals with the web. 17 00:01:12,410 --> 00:01:14,450 So http dot. 18 00:01:15,200 --> 00:01:20,480 And then I get my idy giving me all of my things that I can possibly use from here. 19 00:01:20,480 --> 00:01:24,140 And you can see there's all kinds of things that are related to the Web. 20 00:01:24,380 --> 00:01:29,960 And that's not surprising because God was built from the ground up to be aware of the web and to be 21 00:01:29,960 --> 00:01:31,890 able to deal with it in a meaningful fashion. 22 00:01:32,330 --> 00:01:36,500 So I'm looking for one that starts with the letter H and it's called Handler Thunk. 23 00:01:36,830 --> 00:01:37,440 There it is. 24 00:01:37,910 --> 00:01:42,250 So that's part of the net HDB package, which I can see right here. 25 00:01:42,260 --> 00:01:47,360 So when I hit return to choose that, it should automatically import net HTTP. 26 00:01:47,360 --> 00:01:48,980 If it doesn't, you need to go type that in. 27 00:01:49,550 --> 00:01:54,030 Now this requires some arguments and the arguments it requires are pretty straightforward. 28 00:01:54,050 --> 00:01:59,720 The first thing it wants and we can see when I roll my mouse over it saying it needs a couple of things. 29 00:01:59,720 --> 00:02:04,060 It needs well to start with, I'm going to give it the pat name. 30 00:02:04,310 --> 00:02:10,090 That's the u r l, the uniform resource locator that I want to listen to. 31 00:02:10,100 --> 00:02:16,820 So when people go to my Web application and don't specify anything after the name of the host, here's 32 00:02:16,820 --> 00:02:18,050 what I want to give back to them. 33 00:02:18,500 --> 00:02:24,500 And what I'm going to give them is I'm actually going to inline a function here func and it's going 34 00:02:24,500 --> 00:02:29,030 to have W which is the HTTP response writer. 35 00:02:30,140 --> 00:02:34,700 This is the very first thing that you need to have because this is a handler function for the HTP. 36 00:02:35,090 --> 00:02:38,210 I need to be able to write responses to the user. 37 00:02:38,600 --> 00:02:40,490 So that's what the response writer does. 38 00:02:40,490 --> 00:02:43,940 And I've just called that w you can call it whatever you want to, you can call it yellow if you want 39 00:02:43,940 --> 00:02:45,020 to, but it's a writer. 40 00:02:45,020 --> 00:02:51,390 So I'm going to put W4 response writer and then I need the request and request starts with a letter. 41 00:02:51,590 --> 00:02:57,560 So I'm going to call my variable R this is my parameter for this function and it's a request now. 42 00:02:57,560 --> 00:02:59,870 It's not an EDP request. 43 00:03:00,440 --> 00:03:05,090 It's actually and this may be the very first time you've ever seen this in your entire programming career. 44 00:03:05,420 --> 00:03:09,770 It's a pointer and I can call it a pointer by putting a star in front of it. 45 00:03:10,070 --> 00:03:15,140 Now a pointer is nothing more than an address in memory where some value is stored. 46 00:03:15,590 --> 00:03:21,230 And the reason I have to use a pointer here is because the handler func requires the request to be a 47 00:03:21,230 --> 00:03:22,460 pointer to a request. 48 00:03:22,490 --> 00:03:28,400 So the request is what the Web server or web browser that's hitting my application sends. 49 00:03:28,400 --> 00:03:34,820 It sends a request and then I process that request, whatever it may be, and send back a response. 50 00:03:34,820 --> 00:03:39,620 And every HTTP handler function has to deal with responses and requests. 51 00:03:40,040 --> 00:03:42,970 So I have my my function partly written here. 52 00:03:43,010 --> 00:03:48,680 Let's get back between these two parentheses and actually do the body of the function and the body is 53 00:03:48,680 --> 00:03:53,540 going to be nothing more than I don't know why I have the letter in there. 54 00:03:53,540 --> 00:03:55,160 There should be no erm try that again. 55 00:03:55,160 --> 00:03:55,880 Handle func. 56 00:03:56,840 --> 00:04:01,910 I want to return something and what I want to return is just font which we saw before. 57 00:04:02,300 --> 00:04:03,020 Print line. 58 00:04:04,250 --> 00:04:05,600 Hello world. 59 00:04:06,350 --> 00:04:08,990 OK, so what reimported FMT for me. 60 00:04:08,990 --> 00:04:12,080 The format function and that's all that I have. 61 00:04:12,890 --> 00:04:15,710 So that is pretty straightforward. 62 00:04:15,950 --> 00:04:20,130 But I don't actually want to print a line here because printing a line is just going to print it to 63 00:04:20,150 --> 00:04:20,680 the console. 64 00:04:20,690 --> 00:04:21,920 So let's change that a little bit. 65 00:04:21,920 --> 00:04:29,450 Instead of using print line, let's use F print F and F print F actually requires a response writer 66 00:04:29,450 --> 00:04:30,110 to write to. 67 00:04:30,110 --> 00:04:36,350 Well I have one right here, the W, so let's put it W in there and now everything seems OK. 68 00:04:36,380 --> 00:04:40,010 The only danger is you notice there's a little squiggly line under this. 69 00:04:40,010 --> 00:04:43,520 It might be different in visual studio code but it's probably there in some format. 70 00:04:44,180 --> 00:04:47,530 If I roll over it, it says this is an unhandled error. 71 00:04:48,170 --> 00:04:53,420 Well, one of the cool things about the standard library is I can actually go and look at this actual 72 00:04:53,420 --> 00:04:57,290 built in function within the the the standard library. 73 00:04:57,290 --> 00:04:59,060 And I'm going to do that in. 74 00:05:00,190 --> 00:05:05,950 My idea, by hitting the car, holding down the Comanche and hitting or clicking on it, and that takes 75 00:05:05,950 --> 00:05:08,510 me right to this function f print f it returns. 76 00:05:08,530 --> 00:05:10,880 So here's the description of this function. 77 00:05:11,380 --> 00:05:14,500 The function is called F print F it's part of the package. 78 00:05:14,500 --> 00:05:16,990 If I scroll to the very top, I'm on line two hundred and two. 79 00:05:16,990 --> 00:05:18,910 I don't want to lose my line at the very top. 80 00:05:18,910 --> 00:05:22,770 It's part of the FMT package, so I'm going to go back to line two hundred and two. 81 00:05:23,830 --> 00:05:26,020 So it's a function as part of that package. 82 00:05:26,020 --> 00:05:33,670 It takes as arguments an IO writer and fortunately the response writer for that's part of the package 83 00:05:33,670 --> 00:05:35,070 satisfies this requirement. 84 00:05:35,080 --> 00:05:40,780 So it's a writer and then it has a format string and then optionally some other things after it, which 85 00:05:40,780 --> 00:05:41,890 I'm going to ignore for right now. 86 00:05:42,190 --> 00:05:43,600 So and it returns. 87 00:05:43,600 --> 00:05:44,660 This is the part I'm interested in. 88 00:05:44,660 --> 00:05:49,240 It returns an end, which is an entry and an error which is of type error. 89 00:05:49,540 --> 00:05:50,950 So let's read the description here. 90 00:05:51,280 --> 00:05:56,680 F print f format's according to a format specified and writes to W it returns the number of bytes written 91 00:05:56,830 --> 00:05:59,410 and any right error encountered. 92 00:05:59,440 --> 00:06:00,950 So I'm going to get the number of bytes written. 93 00:06:01,190 --> 00:06:07,930 I don't really want that, but I can look at it so I could say n that's a variable that's going to store 94 00:06:07,930 --> 00:06:13,510 the number of bytes written and the error, which is I'm going to call E R again. 95 00:06:13,510 --> 00:06:17,890 I can call it whatever I want to and then I think it might just go equals, but that's not going to 96 00:06:17,890 --> 00:06:18,260 work. 97 00:06:18,640 --> 00:06:22,120 Now that is actually a function we're going to use a lot this this equals sign. 98 00:06:22,120 --> 00:06:28,720 But right now we're going to use a special one that says automatically format these return types to 99 00:06:28,720 --> 00:06:30,940 whatever this function sends back. 100 00:06:30,940 --> 00:06:35,710 And we saw in the description that F print F returns and int and an error. 101 00:06:35,710 --> 00:06:39,610 So N will be an or an integer and error will be of type error. 102 00:06:40,240 --> 00:06:43,930 I still have red lines underneath these things indicating there's an error. 103 00:06:44,340 --> 00:06:52,780 Actually if I try to compile this, if I say in my terminal, go, run Maeng go, I'll get an error 104 00:06:54,220 --> 00:06:59,620 and the error is actually two errors and declared but not used an error declared but not used. 105 00:06:59,650 --> 00:07:05,890 So I actually am not allowed in go to declare variables unless I actually use them. 106 00:07:06,280 --> 00:07:09,210 I don't really care about the number of bytes written, but let's do something with it. 107 00:07:09,220 --> 00:07:15,610 OK, format f print line and then I'm going to put. 108 00:07:17,140 --> 00:07:27,170 Bytes written all by column and then a plus sign to join what I'm passing it next, and that would be 109 00:07:27,170 --> 00:07:28,290 when you think that would work. 110 00:07:28,290 --> 00:07:34,410 If you're coming from the AP world, that would make perfect sense, but it doesn't because AP is loosely 111 00:07:34,410 --> 00:07:40,500 typed, which means I can store anything I want in a variable, whereas GO is considerably more strict. 112 00:07:40,500 --> 00:07:45,320 It says M is an aunt and you can't print an event like that. 113 00:07:45,510 --> 00:07:49,440 Well, how am I going to print out the number of bytes to the terminal window, which is all I'm trying 114 00:07:49,440 --> 00:07:49,830 to do? 115 00:07:50,610 --> 00:07:51,570 Well, it's not that hard. 116 00:07:51,870 --> 00:07:55,650 Instead of format print line, I'm going to go format. 117 00:07:57,710 --> 00:08:01,580 Print line, I'm still going to use that, and I don't know your format again. 118 00:08:02,790 --> 00:08:09,920 S print F and s print F allows me to take different data types and return them as a string. 119 00:08:09,930 --> 00:08:16,480 So in here I'm going to put a number of bytes written colon and then I'm going to put a placeholder. 120 00:08:16,900 --> 00:08:23,850 D and D is a placeholder that F format s print F is going to replace with some value. 121 00:08:23,860 --> 00:08:27,460 And as you might guess, I want to put any of them there. 122 00:08:27,630 --> 00:08:28,590 OK, so that works. 123 00:08:28,590 --> 00:08:30,960 But I still have an error that I'm not doing anything with. 124 00:08:31,500 --> 00:08:33,170 All right, let's handle that. 125 00:08:34,370 --> 00:08:40,230 Since error is returned by the F print F function, and if there is no error, it doesn't have any value. 126 00:08:40,280 --> 00:08:45,950 I can actually do a simple if statement and an if statement is nothing more than checking one condition 127 00:08:45,950 --> 00:08:52,220 against another and then executing a particular set of commands if the conditions meet whatever criteria 128 00:08:52,220 --> 00:08:52,920 I specified. 129 00:08:52,940 --> 00:08:53,590 So let's try it. 130 00:08:53,600 --> 00:08:56,230 If error is not equal to nil. 131 00:08:56,420 --> 00:09:00,640 In other words, if there is an error, I don't know what it's going to be. 132 00:09:00,650 --> 00:09:04,430 But I know if there is no error, it's going to be nil that I want to do something. 133 00:09:04,430 --> 00:09:08,530 And all I'm going to do here is format, print, line error. 134 00:09:08,840 --> 00:09:14,090 I'm just going to write the error to the console, which is not terribly helpful in a real world program, 135 00:09:14,420 --> 00:09:16,850 but it's suitable for our requirements right now. 136 00:09:17,660 --> 00:09:20,270 So I've now solved the problem. 137 00:09:20,270 --> 00:09:25,730 I've got those two return values from F print F and I handle one. 138 00:09:25,730 --> 00:09:26,930 If there's an error, I print it. 139 00:09:26,930 --> 00:09:29,840 If there's not, nothing happens and then I just print the number of bytes written. 140 00:09:29,850 --> 00:09:30,800 So let's try that. 141 00:09:30,800 --> 00:09:31,720 Let's run this again. 142 00:09:31,730 --> 00:09:33,020 So back down to my terminal. 143 00:09:33,320 --> 00:09:39,020 I hit the up arrow to get my last command and I run this command and then it goes and says, OK, well, 144 00:09:39,620 --> 00:09:40,490 nothing happened. 145 00:09:40,490 --> 00:09:44,840 It started and it ended y well it's pretty straightforward. 146 00:09:44,840 --> 00:09:50,750 I have this HTP handler function which is listening for a request sent by a web browser. 147 00:09:51,140 --> 00:09:57,340 And yet at no point have I started any process that says listen for requests from Web browsers. 148 00:09:57,380 --> 00:10:02,450 So I actually have to do something else beyond this handler function, which in fact is never actually 149 00:10:02,450 --> 00:10:02,900 called. 150 00:10:03,200 --> 00:10:08,000 It's just sitting there embedded in my main function and it never executes. 151 00:10:08,570 --> 00:10:19,280 So now here's how easy it is to start a Web server that listens for requests in go http dot listen and 152 00:10:19,280 --> 00:10:19,940 serve. 153 00:10:20,510 --> 00:10:23,450 And I'm going to listen to a particular part of my computer. 154 00:10:23,690 --> 00:10:28,820 And because I don't want to use a privileged port and ports are just things that you can listen to from 155 00:10:28,820 --> 00:10:29,480 applications. 156 00:10:30,050 --> 00:10:35,150 The first one thousand and twenty four ports on any computer system are privileged. 157 00:10:35,150 --> 00:10:37,940 You have to be a super user to use those, which is no problem. 158 00:10:37,940 --> 00:10:40,160 I'm going to listen to Port and it has to start with the colon. 159 00:10:40,590 --> 00:10:41,170 Eighty, eighty. 160 00:10:42,050 --> 00:10:46,340 So I'm going to listen on port eighty eighty and then it expects another argument. 161 00:10:46,340 --> 00:10:46,940 A handler. 162 00:10:46,940 --> 00:10:48,500 But I'm going to say nil. 163 00:10:48,890 --> 00:10:49,940 I'm not going to pass it. 164 00:10:49,940 --> 00:10:51,650 A particular handler instead. 165 00:10:51,650 --> 00:10:54,350 That's what this HDB handle function does. 166 00:10:54,380 --> 00:10:56,000 I don't need this anymore so I'm going to delete it. 167 00:10:57,590 --> 00:11:01,430 And now when I run this notice, this also has an error. 168 00:11:01,430 --> 00:11:04,040 It's listening for and I'm going to ignore it explicitly. 169 00:11:04,040 --> 00:11:06,620 I'm going to do that because I don't want to handle this error. 170 00:11:06,620 --> 00:11:09,740 If the Web server doesn't start, I will know because the program will crash. 171 00:11:09,740 --> 00:11:11,060 So I don't need to look for this error. 172 00:11:11,450 --> 00:11:12,320 I'm going to say. 173 00:11:13,650 --> 00:11:20,970 Underline so the shift dash equals that and all this is saying is if there's an error, I don't care, 174 00:11:20,970 --> 00:11:23,840 just throw it in the trash bin, pay no attention to it whatsoever. 175 00:11:24,120 --> 00:11:26,480 So let's run this program again and see what happens. 176 00:11:26,700 --> 00:11:27,720 Go, run, go. 177 00:11:28,830 --> 00:11:32,990 And nothing seems to be happening, but it didn't kick me out of the application. 178 00:11:33,270 --> 00:11:40,410 What I have just done with one line of code is started a production quality web server to listen for 179 00:11:40,410 --> 00:11:41,100 requests. 180 00:11:41,340 --> 00:11:42,930 So let's make sure this actually works. 181 00:11:43,080 --> 00:11:45,780 What I expect to happen is I'm going to go to my Web browser. 182 00:11:46,110 --> 00:11:50,100 I'm going to go to this server, which is called localhost on any computer. 183 00:11:50,100 --> 00:11:53,130 That's the default localhost Colen eighty eighty. 184 00:11:53,460 --> 00:11:56,630 And what I want to see happen is two things I want to see. 185 00:11:56,640 --> 00:12:02,010 Hello, world printed to my Web browser window, and then I want to be able to come back in here and 186 00:12:02,010 --> 00:12:06,380 I should see a log entry right down here in the terminal that tells me how many bytes were written. 187 00:12:06,480 --> 00:12:07,410 Let's see if that works. 188 00:12:08,130 --> 00:12:14,280 So I go to my favorite Web browser and I go to the URL bar and I type in local host Colen eight zero 189 00:12:14,280 --> 00:12:15,990 eight zero and it return. 190 00:12:16,680 --> 00:12:17,910 And there is hello world. 191 00:12:17,910 --> 00:12:18,480 Perfect. 192 00:12:18,510 --> 00:12:20,340 Now let's go back and look at our terminal window. 193 00:12:21,420 --> 00:12:25,290 And there it is, number of bytes written, 13 number of bytes written 13. 194 00:12:25,320 --> 00:12:26,710 Now, why did that run it twice? 195 00:12:26,730 --> 00:12:27,450 I'm not sure. 196 00:12:27,490 --> 00:12:28,800 Don't really care about that right now. 197 00:12:29,040 --> 00:12:30,650 I'm going to let that do its thing. 198 00:12:31,380 --> 00:12:35,250 So now I have written a complete Web application. 199 00:12:35,290 --> 00:12:35,730 True. 200 00:12:35,730 --> 00:12:40,170 All it does actually serve the words hello, karma world exclamation mark. 201 00:12:40,560 --> 00:12:42,750 But it does it really, really well. 202 00:12:42,960 --> 00:12:50,010 And this is the basis for any web application and go now things will get more complex, but not that 203 00:12:50,010 --> 00:12:50,790 complex. 204 00:12:50,790 --> 00:12:57,480 One of the great things about Go is it's an extremely easy to learn language and everything functions 205 00:12:57,480 --> 00:13:02,670 in a really straightforward, organized and logical way, and you're not going to have any difficulty 206 00:13:02,670 --> 00:13:03,410 following along. 207 00:13:03,930 --> 00:13:05,190 So that's enough for this time. 208 00:13:05,400 --> 00:13:07,650 Next time will make it a little bit more complex.