Matching & more list functions

Transcription

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2 Matching & more list functions Dan S. Wallach and Mack Joyner, Rice University Copyright 2016 Dan Wallach, All Rights Reserved

3 When you do the same thing over and over If statements start to get ugly If empty-list then XXX else YYY If tree-leaf then XXX else YYY What if you forget a case? Weird bugs crop up. Hard to fix.

4 Wednesday s ogenerate v1 static <T> IList<T> ogenerate(supplier<option<t>> supplier) { Option<T> oresult = supplier.get(); if (oresult.issome()) { return make(oresult.get(), () -> ogenerate(supplier)); else { return makeempty(); This if / then / else block is easy to get wrong. Easy to forget a case. The call to oresult.get() would throw an exception if you got it backwards.

5 Wednesday s ogenerate v2 static <T> IList<T> ogenerate(supplier<option<t>> supplier) { return supplier.get().map(result -> make(result, () -> ogenerate(supplier))).getorelse(makeempty()); This map / getorelse pattern is just another way of writing an if / then / else block. Better than v1 because you won t call get() on an Option.none.

6 The War on IfStatements!

7 Traditional solution: methods in subclasses List.Cons.limit: public IList<T> limit(int n) { if (n < 1) { return makeempty(); return make(headval, tailval.limit(n - 1)); List.Empty.limit (actually IList.Empty.limit): default IList<T> limit(int n) { return this;

8 Traditional solution: methods in subclasses List.Cons.limit: public IList<T> limit(int n) { if (n < 1) { return makeempty(); return make(headval, tailval.limit(n - 1)); Cleverly moving common logic for empty List and LazyList into a single interface that both can share. Don t repeat yourself! List.Empty.limit (actually IList.Empty.limit): default IList<T> limit(int n) { return this;

9 Problems with the subclass approach 1) Related program logic spread across the program Totally different files have code that implements IList.limit() What if you change it one place and forget to change the other? 2) You can t always add new code to somebody else s classes You can t edit the core java.util packages. Your patches to other libraries might not be taken by the maintainer.

10 Pre-Java8 solution: The Visitor Pattern Hypothetical code inside IList<T>: interface IVisitor<T,U> { U acceptnonempty(ilist<t> list); U acceptempty(ilist<t> list); ; default <U> U visit(ivisitor<t,u> visitor) { if(empty()) return visitor.acceptempty(this); else return visitor.acceptnonempty(this); IVisitor defines two methods: acceptempty and acceptnonempty The user creates a whole new class with these two methods defined.

11 Visitor pattern usage Create an anonymous inner class with two method bodies Here s a recursive list-length function, implemented outside of IList: static <T> int listlength(ilist<t> inputlist) { return inputlist.visit(new IVisitor<T, Integer>() public Integer acceptnonempty(ilist<t> list) { return 1 + public Integer acceptempty(ilist<t> list) { return 0; );

12 Visitor pattern usage Create an anonymous inner class with two method bodies Here s a recursive list-length function, implemented outside of IList: static <T> int listlength(ilist<t> inputlist) { return inputlist.visit(new IVisitor<T, Integer>() public Integer acceptnonempty(ilist<t> list) { return 1 + listlength(list.tail()); Anonymous inner classes don t names, they just implement an interface. public Integer acceptempty(ilist<t> list) { return 0; );

13 Visitor pattern usage Create an anonymous inner class with two method bodies Here s a recursive list-length function, implemented outside of IList: static <T> int listlength(ilist<t> inputlist) { return inputlist.visit(new IVisitor<T, Integer>() public Integer acceptnonempty(ilist<t> list) { return 1 + public Integer acceptempty(ilist<t> list) { Standard recursive call. ); return 0;

14 Visitor pattern usage Create an anonymous inner class with two method bodies Here s a recursive list-length function, implemented outside of IList: static <T> int listlength(ilist<t> inputlist) { return inputlist.visit(new IVisitor<T, Integer>() public Integer acceptnonempty(ilist<t> list) { return 1 + public Integer acceptempty(ilist<t> list) { return 0; Bug opportunity: inputlist vs. list say the wrong thing and get ); the wrong answer. (Anonymous inner classes capture variables just like lambdas.)

15 Pluses / minuses of the Visitor pattern Code for both cases is side-by-side Easier to see all at once, convince yourself it s correct. Method arguments are the correct type. And method names are descriptive of the cases. You can t forget a case. You re required to write two methods for this visitor interface. You don t need to be inside IList or the List class Visitor pattern is general-purpose, useful for all kinds of things. - Ugly, ugly, ugly! IntelliJ at least will write a lot of the boilerplate for you.

16 Java8 solution: matching Actual code in IList; you can use this today: default <Q> Q match(function<ilist<t>, Q> emptyfunc, BiFunction<T, IList<T>, Q> nonemptyfunc) { if (empty()) { return emptyfunc.apply(this); else { return nonemptyfunc.apply(head(), tail()); Two lambdas: one for empty-list, one for non-empty lists No anonymous inner classes, no visitor interface

17 Same example: recursive list-length But now with lambdas! static <T> int listlength(ilist<t> list) { return list.match( emptylist -> 0, (head, tail) -> 1 + listlength(tail));

18 Same example: recursive list-length But now with lambdas! static <T> int listlength(ilist<t> list) { return list.match( emptylist -> 0, (head, tail) -> 1 + listlength(tail)); Deconstruction : The head and tail are given to us already.

19 Same example: recursive list-length But now with lambdas! static <T> int listlength(ilist<t> list) { return list.match( emptylist -> 0, (head, tail) -> 1 + listlength(tail)); Standard recursive call.

20 Which looks better to you? Java7 visitors: Java8 matching with lambdas: static <T> int listlength(ilist<t> input) { return input.visit(new IVisitor<T, Integer>() public Integer acceptnonempty(ilist<t> list) { return 1 + public Integer acceptempty(ilist<t> list) { return 0; ); static <T> int listlength(ilist<t> list) { return list.match( emptylist -> 0, (head, tail) -> 1 + listlength(tail));

21 Which looks better to you? Java7 visitors: Java8 matching with lambdas: static <T> int listlength(ilist<t> input) { return input.visit(new IVisitor<T, Integer>() public Integer acceptnonempty(ilist<t> list) { return 1 + public Integer acceptempty(ilist<t> list) { return 0; ); static <T> int listlength(ilist<t> list) { return list.match( emptylist -> 0, (head, tail) -> 1 + listlength(tail)); λ

22 If you ever use a real functional language The matching features are typically built into the language And you can match deeply into the list Scala example: if you want to match a list with at least two elements: result = list match { case List(val1, List(val2, tail)) =>... case... Neither Java7 visitors nor our Java8 matching can do this. Take Comp311 and you ll get to do this and more.

23 If you ever use a real functional language The matching features are typically built into the language And you can match deeply into the list Scala example: if you want to match a list with at least two elements: result = list match { case List(val1, List(val2, tail)) =>... case... Not in Java Neither Java7 visitors nor our Java8 matching can do this. Take Comp311 and you ll get to do this and more.

24 But we can still do really useful things Here s an example from next week: measure maximum tree depth A tree is either an empty-tree, or a node (element, left-tree, right-tree) default int maxdepth() { return match( emptytree -> 0, (elem, lefttree, righttree) -> Integer.max(leftTree.maxDepth(), righttree.maxdepth()) + 1); Each lambda takes different arguments, deconstructs the tree If you get the order of the lambdas wrong, it won t compile (a feature!)

25 Wednesday s ogenerate" v3 static <T> IList<T> ogenerate(supplier<option<t>> supplier) { return supplier.get().match( () -> makeempty(), result -> make(result, () -> ogenerate(supplier))); Option also has a match method!

26 Wednesday s ogenerate" v3 static <T> IList<T> ogenerate(supplier<option<t>> supplier) { return supplier.get().match( LazyList::makeEmpty, result -> make(result, () -> ogenerate(supplier))); Option also has a match method! () -> LazyList.makeEmpty() LazyList::makeEmpty equivalent code

27 So start using the match methods! You have them this week in IList and Option. Use match rather than if/then/else when writing your Queue methods Finish sooner, fewer bugs, cleaner code

28 A brief note about multiple inheritance

29 Implementing multiple interfaces You ll notice code like this (List.Empty and LazyList.Empty): class Empty<T> implements List<T>, IList.Empty<T> { See how it s implementing more than one interface? You can bring in default methods from more than one place! Not possible in Java7 (This is why we re not doing class inheritance much in Comp215.)

30 Diamond dependencies C++ hackers complain about multiple inheritance. Java does better. interface A { default void foo() { System.out.println("A"); interface B extends A { default void foo() { System.out.println("B"); interface C extends A {... // no mention of foo() class D implements B, C {... foo(); // Does this print A, B, or get a compiler error?

31 Diamond dependencies C++ hackers complain about multiple inheritance. Java does better. interface A { default void foo() { System.out.println("A"); interface B extends A { default void foo() { System.out.println("B"); interface C extends A {... // no mention of foo() class D implements B, C {... foo(); // Does this print A, B, or get a compiler error? The rules: If two interfaces provide the same default method directly? Compiler error. Else, closest definition wins. Disambiguation: Name the one you want. A.super.foo() B.super.foo()

32 When should I use default methods? First write an interface with no code, only method signatures. Make sure it makes sense. Write the class(es) that implement it. Hide the constructors, use factory methods (make, makeempty, etc.) Zen programming rule #1: don t repeat yourself If you have identical code across two classes: Promote it to a default method on the interface Requires that you only use other methods on the interface Cannot see member variables directly

33 Default methods + match When you ve got one liners, match is your friend public interface IList<T> { default Option<T> ohead() { return match( emptylist -> Option.none(), (head, tail) -> Option.some(head)); Keep it simple, keep it all in one place When in doubt, do whatever makes your code cleanest.

34 Cool features in this week s IList / List / LazyList classes

35 IList.nth You can pretend that a list is like an array and fetch something. IList<String> names = List.of("Alice","Bob","Charlie","Dorothy"); assertequals("alice", names.nth(0)); assertequals("bob", names.nth(1)); assertequals("charlie", names.nth(2)); This costs O(n), so if you do it a lot, your performance will suffer.

36 IList.join You can convert a list to a string, joined with any delimiter This shows up all the time, so it s worth having it built-in. This will call the tostring method on each element Works on lists of any type, not just IList<String>! IList<String> names = List.of("Alice","Bob","Charlie","Dorothy"); assertequals("alice:bob:charlie:dorothy", names.join(":"));

37 IList.sublist You can extract a range of a list IList<String> names = List.of("Alice","Bob","Charlie","Dorothy"); assertequals("alice Bob", names.sublist(0,1).join(" ")); assertequals("charlie Dorothy", names.sublist(2,3).join(" "));

38 IList.toString Lists know how to convert themselves to strings Special handling of String (adding quotation marks) Other Comp215 classes, like Option, also have tostring methods IList<Option<String>> list = List.of( Option.some("Alice"), Option.some("Bob"), Option.none(), Option.some("Charlie")); System.out.println(list); Prints this: List(Option.Some("Alice"), Option.Some("Bob"), Option.None(), Option.Some("Charlie"))

39 Hint for this week s ListQueue project Your ListQueue includes a tostring method as well public String tostring() { return "Queue(" + tolazylist().map(strings::objecttoescapedstring).join(", ") + ")"; Maybe you might want to add another method to show the internals Then add logging (not just System.out.println) Watch your queues evolve!

40 Hint for this week s ListQueue project Your ListQueue includes a tostring method as well public String tostring() { return "Queue(" + tolazylist().map(strings::objecttoescapedstring).join(", ") + ")"; Maybe you might want to add another method to show the internals Then add logging (not just System.out.println) Watch your queues evolve! Helper function: put quotation marks around a string properly