dotSwift 2018 - Paul Hudson - Elements of Functional Programming

[Music] so yes functional programming but first some absolute basics Jumma bell ball you got beat the number there yeah Jake guitars are in theory there we go now the French speakers in the room will know I'm not 14 years old but this tells you a how long ago I learned French a long time and B how bad my French is I hope it's at the bar suitably low for the rest of this talk so yes functional coding but just I'm really pleased to begin with you because right now apparently there's the only place in the world where it rains more than the UK it's flooding outside this is crazy has to be great here and even worse if the floods weren't bad enough there are riots over Nutella what's wrong with you so at the party tonight you'll keep me safe from the floods and then the teller rioters please so functional coding functional coding there we go when people see these words they can sometimes get a little bit of freight the hair functionally thing a house is not for me it's very boring not applicable I don't think so okay I get that but in this talk I want to try and give you some real quick wins quick wins because sometimes you get words like monads and funk doors and applicative thrown around and it can seem very dry and very academic and not for you when it really is so this concept of quick wins I have shamelessly stolen from an ex Apple engineer called and EEMA to Shack and the idea is he talks about lightweight encounters you dip in get some quick wins and run away for it gets too hard but the real question is why bother what you're doing right now works right your current declarative approach is fine why change well it turns out there are a few informal conventions you know rules to functional programming what makes good functional code one of them is we rely very strongly on immutability we like using constants everywhere and of course swift and forces is for us you try and change a constant you get nothing at all we haven't compiled good functional code avoids state at all those little properties you store away settings and so forth it might start like this but after a few months if not years what you normally get a much messy situation so good functional code avoid states you pass values into a function change them inside the function and return new values you avoid external stuff and of course this makes a code significantly easier to test you know that function a will always return one two three for the same values every single time regardless of any properties you might be using good functional code is composable you write small functions that do one maybe two things then combine together to make bigger functions like Lego bricks but most importantly good functional code allows us to express our intent clearly it clarifies our intent use your quote stolen from xavier soto from twitch he says it allows us to express what we want to achieve rather than how it's implemented smart man now enough intro let's get to the code somethings map map works great on collections and optionals and we're starting easy collections okay where does it transform stuff values come in get transformed by a closure rights and emitted as transformed values r example one two three four map can double that to be two four six eight nice and easy play some English words camping parking in weekend we can say hey map make that French as Kumar said English is just French badly pronounced map could even try and make olympic marseille it's a good football team but probably not now as an example imagine a function which accepts an array of names John Paul George and Ringo the onus function to transform those names into the links of host names nice and simple we know that John has four Paul has four George has six Ringo has five array of strings in int array of counts back you're doing that declaratively you would say length of string string array going in return int array make a new empty int array loop over the strings add white count to the intr a and the loop and return the results that is your declarative function throw a comparator throughout that functionally you would say step one get rid of all that code step two you'd write strings dot map dollar zero dot count step three well there is no step three that's all your code now and the best bit is the function signature hasn't changed all the code that calls this function can be as nasty as it is right now this code can be improved so you can go ahead and chisel away at your bad code base and improve it as you go now if you don't use close as much dollar zero just means this element being passed in so John then Paul then George then Ringo now this code is shorter of course this code shorter but that's not the real win here okay this code is immutable that variable integer array done but it also clarifies what we're trying to do are intense clear when you call map on a sequence like I did there it must transform all the items in there you can't bail out partway through four laughs internally to Swift it looks a bit like this I've simplified its slight this is map internally to Swift and standard library map transforms some generic thing or turned the generic array of T create a new empty array of T loops over the items itself and then transforms your item puts into the return value then ends a loop returns turn value and returns a function that's exactly the same code you would written except now it's Swift's problem now it's Ben's problem through Apple to let that work better which means they can optimize that over time to be better and faster and you don't care your code just improves you can map all sorts of things here's an integer array 108 85 we could say map that into strings your score was dollar 0 your score 100 score 80 or 85 we could have yeah I'm Strama cram come on whist applause my friend che thank you we can say I want to map that to be uppercase and I'll just return those things upper case we could say his array of doubles two four nine and sixteen we could pass that straight to the square root function and get back two three four map all sorts of things and actually earlier Daniel Steinberg gave me a lovely analogy for how map works similar to you I've used in UIKit we're very comfortably the idea of saying here's a view I want you to animate with a 5 second duration from where you are down to the bottom right corner and it will do so we haven't going to tell it exactly where to be in every frame of the animation we just say the start point and the end point the transformation and let you Ike it sort it out very similar map that is map move on flat map in a smaller type compact map it's being renamed because of course you know Swift or R in Swift for one which is command next code and I put three years ago it's now compact map this precise you just I'm talking about but it's very similar to map please already map and your brain you think you've more the transformation happening here of course there is but a second part the flat / compact part it does something really cool after the map transformation completes it will unwrap your optionals then throw away any Nils for you and nil occurs in all sorts of places and Swift so if loves nil X variable initializes or try question mark or conditional typecast and so forth all of these when they fail return nil so when you say create the integer from this string number 5 a will actually be an optional integer because you may have said not as your number and of course the French don't you anyway you get nil back for that so nil brilliant good food to flatmap let's see that the flatmap thinks of that we can say here's an array of one five fish deer flatmap please put that through the initializer and what get back is this numbers array will be an int array not optionality here whatsoever it's a real int array it'll work out like this we have one five fish it goes through the International Iser which is available it comes back as optional one optional five Neil the flattening they're compacting part kicks off unwraps the optionals tosses away the nails out comes one five beautiful so anywhere you have failed but initialized you should be thinking flat map for example got an array of strings that might be URLs might not be let's find out try and make your else out of them for lucky there we go it fails black my posters my way an array of UI views some of which might be your image use great let's do a conditional typecast the ones you get back our image juice you can even just do this there's no transformation happening here you're just saying please give me your lovely compacting behavior please under optionals and toss away the nails that's black map now both of these things work on optionals too you can do an optional map and an optional flat map and they work in slightly different ways i think you an option of being a box map will open the box it'll extract the contents of a box some the teller and it of course transform that into vast amounts of money right now for the rioters and then it will put that thing back in the box so it rear a pit back in and optional again get an optional you end up with an optional so in code if you imagine a method like this get user ID keep an ID number 97 if it exists you get a string back if it doesn't get nil back there's an optional string return value we can pass that thing through map and if name is bill nothing happens you get back nil immediately but if name is not nil you'll get high user name hi jean-pierre and then you can say go ahead and print the greeting nil coalescing unknown user now it's the same thing declaratively you might say the same court start then an empty constant do a quick unwrap put it into the variable the end of an else block lucky there we go unknown user anything else and then pronounce routing crazy there we go now easy enough but what if what's in the box isn't what you thought you open the box out comes your prize transform to big cash but this time it is optional what happens then find out so here we have an optional string containing five we say okay let's try and map that to an integer what type is result spoiler it is not an integer it's not even an optional integer what you've got here is the lesser known and completely never wanted optional optional integer which really screws with your head no one wants to see these things now when we said map what we meant to say really was flat map and now result will be a regular optional int that we feel much more comfortable with you see when you call map or an optional and your transformation closure returns an optional map just put them together you get an optional optional flat map is able to say it's the same container let's join them together it can turn that into a single optional either the whole thing is there or the whole thing is not there so your transformation closure returns an optional use flat map move on filter you like coffee and France don't you still Turing filtering is where you can say here is a condition I want to check pass this collection or sequence through this filter and see what comes out for example I have an array of red and blue boxes I have a filter looking for blue boxes I can now say run it through the Reds are thrown away balloons come back that's my return value so time for some examples and let's of course play to the crowd a little bit let goodwine equals wind-up filter origins France let band equals player stop filter has a prefix Kevin the band he'll called Kevin from the thing great you can tilt whatever you like let's filter these numbers base where they are odd or not using modulus and there's so many more Swift packed these lovely functional functions you can call on I love the first method this will find the first score over 85 like that you've got array of numbers 9 5 3 5 and your cat Hoff and they appear trivial you can put it through map make an array of tuples so it'll produce an array like this nine one five one two three one five one then swift for there's a new dictionary initializer it will treat that array of tuples as key value pairs so keys one values one keys three values one and so forth it's called unique and keys with and when it finds a collision in the keys like it will four five and five it'll add the one on the one together so wrote result will be nine appears once five appears twice three appears once it's trivial to do there's a classic one you'll see on so many talks how to add an array of numbers using reduce and plus it's shown so many times because it's beautiful it shows you how swift just totally blurs the lines between functions methods and operators and closures one big thing of beautifulness and swift you can even take it's ready to more french you can even take non-functional things you know already and apply them isn't of rain we wait the answer you can say exactly saying we you know this there's a contains method and arrays you know this and there's a stream contains we can say marathon said we look at saying we we put the two together you can now say here's my array of words we web beyond suit here's my string and then say yes doc contains where replied or contains do any of those words appear in that string done now there are some challenges to switching to a more functional approach of course there are first up muscle memory we sit down at your desk ready to face the same problems we face before you naturally reach for the same solutions you use before switching to map or flat map or reduce or whatever you want to do is a mental jump but it's worth it second up some hope to tell you Swift is not a functional language or a true functional language and the you know I've semantics it's functional its protocol oriented it's the clarity of its object-oriented it's good but one of the biggest ones is that state is comfortable we like storing things in properties right and then referring to them later on so getting away from that get into what you might call a pure function where you only refer to the values inside the function takes a work and that's okay so let's wrap up first up I hope you learn soon you I hope you impressed by my french suck it up you weren't you scared I did try and keep it as simple as possible quite as applicable as possible and thirdly I hope that tomorrow you can go back to your desk and apply what you've learned she just learned functional program look at happen she is right that could be you merci beaucoup [Applause] [Music] [Applause] [Music]

Channel: dotconferences

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Id: OgU8d_E1K14

Length: 16min 19sec (979 seconds)

Published: Wed Feb 07 2018