WEBVTT 00:00.110 --> 00:00.620 Okay guys. 00:00.620 --> 00:02.240 So pure functions. 00:02.240 --> 00:04.760 That's an example of a pure function. 00:06.590 --> 00:12.050 When you run a pure function with the same arguments, you always get the same result. 00:13.040 --> 00:16.700 So the first round, second round it is exactly the same. 00:19.040 --> 00:22.190 Now an example of a non pure function. 00:23.180 --> 00:29.240 So this one has some dependency on a shared state and has side effect. 00:30.320 --> 00:33.260 So it modifies the global state. 00:33.770 --> 00:37.850 When this run for the first time the result is five. 00:37.880 --> 00:41.600 When it's run for the second time the result is ten. 00:41.630 --> 00:48.650 Even though you supply the same arguments, that's because it has side effects and it is dependent on 00:48.650 --> 00:50.030 the shared state. 00:52.400 --> 00:54.560 So let's actually write this code. 00:54.560 --> 00:59.810 Let's create a pure PHP file, open it right away. 00:59.810 --> 01:05.720 Are the PHP tag and implement this add function, which is pure. 01:05.990 --> 01:08.090 It has two arguments. 01:08.090 --> 01:10.070 We can also type them as. 01:10.100 --> 01:11.090 Why not? 01:11.630 --> 01:13.250 So those are integers. 01:13.280 --> 01:19.070 The return type is also an integer and we return a plus b. 01:22.760 --> 01:23.390 Like that. 01:23.390 --> 01:31.880 Now if I echo or vardump the result of calling this function, I should get the same result every single 01:31.880 --> 01:32.660 time. 01:33.170 --> 01:37.400 So I'm calling add with those two parameters. 01:38.030 --> 01:42.590 Maybe let's do that again and just var dump this immediately. 01:42.830 --> 01:48.050 I need to jump to functions folder and run pure PHP. 01:48.830 --> 01:53.210 Now as you see we've got two exactly the same results. 01:55.130 --> 01:57.620 Now let's do the second example from the slide. 01:58.280 --> 02:04.230 So define the function called add to total that just accepts a value. 02:05.850 --> 02:06.630 But this one. 02:06.630 --> 02:11.370 This is using a global variable that I'm going to define right here. 02:13.530 --> 02:18.180 And to use this variable I need to use the global keyword. 02:18.210 --> 02:23.940 And now I have access to the original total variable from outside this function. 02:25.650 --> 02:30.300 So if I add something to total. 02:32.640 --> 02:41.490 And I return it then as you've seen on the slide, this actually is influencing some outside state, 02:41.490 --> 02:45.120 some variables that are outside this function. 02:45.120 --> 02:47.370 And this is called a side effect. 02:47.970 --> 02:55.410 So if you are calling a function and you would like to actually calculate something, but instead a 02:55.440 --> 03:00.970 global state is modified which you might be aware of or you might not be. 03:01.000 --> 03:04.570 Then this function is not considered pure. 03:08.950 --> 03:10.300 Now let's run that. 03:12.040 --> 03:17.320 So I'm going to var dump two calls to this function with the same parameters. 03:17.350 --> 03:19.390 So this is maybe. 03:19.750 --> 03:21.550 Sorry that's just one parameter. 03:21.550 --> 03:26.590 Let's maybe call with three and another call with the same one. 03:26.590 --> 03:29.590 So we can see the different output. 03:29.980 --> 03:32.020 So we are calling pure PHP. 03:32.260 --> 03:34.060 The first call returns three. 03:34.090 --> 03:36.430 The second call returns six. 03:38.170 --> 03:40.030 Okay so what's the point. 03:40.030 --> 03:42.850 Well the pure functions they are predictable. 03:42.850 --> 03:47.110 You always know that you're going to get the same output for the same input. 03:47.680 --> 03:52.900 It's easier to test such functions and they are easily reusable. 03:53.680 --> 03:57.190 Also the result of such functions can be cached.