Why do you want to know about functional programming?

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1 Why do you want to know about functional programming? A look at LinQ in C# and (perhaps) Java Axel T. Schreiner RIT Computer Science Eric Meijer 04/23/07 linq-1

2 Principles Patterns Examples Implementation Conclusions 04/23/07 linq-2

3 Principles Map a query language to cascading method calls. Implement the necessary methods for IEnumerable, XML- and database-access. Sugar syntax liberally. cs /30/06 linq-3

4 What if... struct Person { string first, last; int phone; IContainer<Person> staff =... Console.WriteLine( from x in staff where x.phone % 2 == 0 orderby x.last select new { x.first, x.phone ); Syntactic sugar patterned after SQL. cs /30/06 linq-4

5 What if... struct Person { string first, last; int phone; IContainer<Person> staff =... Console.WriteLine( from x in staff where x.phone % 2 == 0 orderby x.last select new { x.first, x.phone ); Syntactic sugar patterned after SQL. Actions converted into messages, Expressions converted into functions. cs /30/06 linq-5

6 What if... struct Person { string first, last; int phone; IContainer<Person> staff =... Console.WriteLine( staff.where(x => x.phone % 2 == 0).OrderBy(x => x.last).select(x => new { x.first, x.phone ) ); Functional style [Eric Meijer]: function as arguments, function composition for actions. Anonymous class new { field,.... cs /30/06 linq-6

7 Typing struct Person { string first, last; int phone; IContainer<Person> staff =... Console.WriteLine( staff.where(x => x.phone % 2 == 0).OrderBy(x => x.last).select(x => new { x.first, x.phone ) ); class C<T> { delegate R Func<R, T> (T arg); C<T> Where (Func<bool, T> predicate); C<T> OrderBy<K> (Func<K, T> keyselector); C Select (Func<U, T> selector); cs /30/06 linq-7

8 Typing struct Person { string first, last; int phone; IContainer<Person> staff =... Console.WriteLine( staff.where(x => x.phone % 2 == 0).OrderBy(x => x.last).select(x => new { x.first, x.phone ) ); class C<T> { delegate R Func<R, T> (T arg); C<T> Where (Func<bool, T> predicate); C<T> OrderBy<K> (Func<K, T> keyselector); C Select (Func<U, T> selector); cs /30/06 linq-8

9 Typing struct Person { string first, last; int phone; IContainer<Person> staff =... Console.WriteLine( staff.where(x => x.phone % 2 == 0).OrderBy(x => x.last).select(x => new { x.first, x.phone ) ); class C<T> { delegate R Func<R, T> (T arg); C<T> Where (Func<bool, T> predicate); C<T> OrderBy<K> (Func<K, T> keyselector); C Select (Func<U, T> selector); cs /30/06 linq-9

10 Typing Pattern struct Person { string first, last; int phone; IContainer<Person> staff =... Console.WriteLine( staff.where(x => x.phone % 2 == 0).OrderBy(x => x.last).select(x => new { x.first, x.phone ) ); class C<T> { delegate R Func<R, T> (T arg); C<T> Where (Func<bool, T> predicate); C<T> OrderBy<K> (Func<K, T> keyselector); C Select (Func<U, T> selector); cs /30/06 linq-10

11 Query Expression Pattern from join... introduce iteration variable(s) from let... where introduces iteration variable(s) introduces variable filters orderby rearranges sequence select group by into join... produces flat result produces structured result produces iteration variable(s) to start over cs /30/06 linq-11

12 Query Class Pattern from join... class C<T> { C<V> Join (...) C<V> GroupJoin (...) from let... where C<T> Where (...) orderby O<T> OrderBy[Descending] (...) class O<T> : C<T> { O<T>ThenBy[Descending] (...) select group by C Select[Many] (...) C<G<Key,E>> GroupBy (...) class G<Key,E> : C<E> {... into join... cs /30/06 linq-12

13 Examples /23/07 linq-13

14 cs /30/06 linq-1from let where... / Where let x = e defines name for value. from x in xs where f xs.where(x => f) retains only records satisfying a condition.

15 Implementation 04/23/07 linq-1delegate R Func<R,A> (A a); IEnumerable<T> Where<T> (IEnumerable<T> container, Func<bool,T> predicate) { foreach (var c in container) if (predicate(c)) yield return c;

16 Implementation delegate R Func<R,A> (A a); IEnumerable<T> Where<T> (IEnumerable<T> container, Func<bool,T> predicate) { foreach (var c in container) if (predicate(c)) yield return c; 04/23/07 linq-1public interface Func<R,A> { R f (A a); public<t> Iterable<T> where (Iterable<T> container, Func<Boolean,T> predicate) { List<T> result = newlist(); for (T c: container) if (predicate.f(c)) result.add(c); return result;

Lazy Evaluation public<t> IterableImpl<T> where (final Func<Boolean,T> predicate) { return new IterableImpl<T>(this) { public Iterator<T> iterator () { return new IteratorImpl<T> () { final

17 Lazy Evaluation public<t> IterableImpl<T> where (final Func<Boolean,T> predicate) { return new IterableImpl<T>(this) { public Iterator<T> iterator () { return new IteratorImpl<T> () { final Iterator<T> c = delegate.iterator(); protected void getnext () { while (c.hasnext()) if (predicate.f(next = c.next())) { hasnext = true; return; hasnext = false; eof = true; ; ; 04/23/07 linq-16

18 Use 04/23/07 linq-1int[] xs = { 1, 2, 3, 4, 5 ; from x in xs where x % 2 == 0 select x Where(xs, x => x % 2 == 0)

19 int[] xs = { 1, 2, 3, 4, 5 ; from x in xs where x % 2 == 0 select x Where(xs, x => x % 2 == 0) Use List<Integer> xs = Arrays.asList(new Integer[]{ 1, 2, 3, 4, 5); where(xs, new Func<Boolean,Integer>() { public Boolean f (Integer x) { return x % 2 == 0; ) Arrays are not Iterable. 04/23/07 linq-17

20 Sugar: Extension Methods static class X { static R method (this A parm,...) {... A static class can define (static) extension methods. If the class is imported, the methods can be called as instance methods: A a; a.method(...) cs /30/06 linq-18

21 Sugar: Lambda Syntax parameter => expression (type parameter,...) => { body An anonymous method definition can be significantly simplified. Parameter and result types can be inferred. Proposed for Java 7. cs /30/06 linq-19

22 cs /30/06 linq-2select / Select from x in xs select f xs.select(x => f) creates new records.

23 Implementation 04/23/07 linq-2delegate R Func<R,A> (A a); IEnumerable Select<U,T> (IEnumerable<T> container, Func<U,T> selector) { foreach (var c in container) yield return selector(c);

24 Implementation delegate R Func<R,A> (A a); IEnumerable Select<U,T> (IEnumerable<T> container, Func<U,T> selector) { foreach (var c in container) yield return selector(c); 04/23/07 linq-2public interface Func<R,A> { R f (A a); public<u,t> Iterable select (Iterable<T> container, Func<U,T> selector) { List result = newlist(); for (T c: container) result.add(selector.f(c)); return result;

25 Lazy Evaluation public<u,t> IterableImpl select (final Func<U,T> selector) { return new IterableImpl(this) { public Iterator iterator () { return new IteratorImpl () { final Iterator<T> c = delegate.iterator(); protected void getnext () { if (hasnext = c.hasnext()) next = selector.f(c.next()); else eof = true; ; ; 04/23/07 linq-22

cs-2006-1 10/30/06 linq-2select / Select[Many] from x in xs select f xs.select(x => f) creates new records.

26 cs /30/06 linq-2select / Select[Many] from x in xs select f xs.select(x => f) creates new records. from x in xs from y in ys select f xs.selectmany(x => ys.select(y => f)) creates new records notice scoping.

27 Implementation 04/23/07 linq-2delegate R Func<R,A> (A a); IEnumerable SelectMany<U,T> (IEnumerable<T> container, Func<IEnumerable,T> selector) { foreach (var c in container) foreach (var u in selector(c)) yield return u;

28 Implementation delegate R Func<R,A> (A a); IEnumerable SelectMany<U,T> (IEnumerable<T> container, Func<IEnumerable,T> selector) { foreach (var c in container) foreach (var u in selector(c)) yield return u; 04/23/07 linq-2public interface Func<R,A> { R f (A a); public<u,t> Iterable selectmany (Iterable<T> container, Func<Iterable,T> selector) { List result = newlist(); for (T c: container) for (U u: selector.f(c)) result.add(u); return result;

29 Use 04/23/07 linq-2int[] xs = { 1, 2, 3, 4, 5 ; double[] ys = { 6.0, 7.0, 8.0, 9.0, 10.0 ; from x in xs where x > 3 from y in ys where y < 8.0 select new { y, x SelectMany(Where(xs, x => x > 3), x => Select(Where(ys, y => y < 8.0), y => new { y, x ))

30 Sugar: Tuple Types new { name = expression,... A type can be created on the fly. It has a read/write property for each field (and ToString). The field types (and names) are inferred. cs /30/06 linq-26

31 Review: Use 04/23/07 linq-2int[] xs = { 1, 2, 3, 4, 5 ; double[] ys = { 6.0, 7.0, 8.0, 9.0, 10.0 ; from x in xs where x > 3 from y in ys where y < 8.0 select new { y, x SelectMany(Where(xs, x => x > 3), x => Select(Where(ys, y => y < 8.0), y => new { y, x ))

32 List<Integer> xs = Arrays.asList(new Integer[]{ 1, 2, 3, 4, 5); final List<Double> ys = Arrays.asList(new Double[]{ 6.0, 7.0, 8.0, 9.0, 10.0 ); selectmany( where(xs, new Func<Boolean,Integer>() { public Boolean f (Integer x) { return x > 3; ), new Func<Iterable<HashMap<String,Object>>,Integer>() { public Iterable<HashMap<String,Object>> f (final Integer x) { return select( where(ys, new Func<Boolean,Double>() { public Boolean f (Double y) { return y < 8.0; ), new Func<HashMap<String,Object>,Double>() { public HashMap<String,Object> f (Double y) { HashMap<String,Object> result = new HashMap<String,Object>(); result.put("y", y); result.put("x", x); return result; ); ) 04/23/07 linq-28

33 cs /30/06 linq-2orderby from x in e orderby f [ascending descending] //, g,... e.orderby(x => f) //.ThenBy(x => g)... defines sort key[s] and direction.

34 Implementation: primary key 04/23/07 linq-3public<k extends Comparable<K>,T> Ordered<T> orderby (Iterable<T> container, final Func<K,T> key) { // need a copied list for sorting List<T> values = newlist(); for (T c: container) values.add(c); return new Ordered<T>(values, new Comparator<T>() { public int compare (T a, T b) { return key.f(a).compareto(key.f(b)); ); Comparator is composed from key

35 Implementation: secondary keys public static class Ordered<T> extends Base<T> { protected final Comparator<T> order; protected boolean ordered; // true once sorted public Ordered (List<T> values, Comparator<T> order) { super(values); this.order = order; public<k extends Comparable<K>> Ordered<T> thenby (final Func<K,T> key) { // add to order return new Ordered<T>(super.getValues(), new Comparator<T>() { public int compare (T a, T b) { int c = order.compare(a, b); return c!= 0? c : key.f(a).compareto(key.f(b)); ); next Comparator is composed from order and key 04/23/07 linq-31

36 Implementation: sort protected static class Base<T> extends AbstractList<T> { private final List<T> values; protected List<T> getvalues () { return values; public Base (List<T> values) { this.values = values; public void add (int index, T element) { getvalues().add(index, element); //... get, remove, set, and size are also delegated; any use of values can be overridden: protected List<T> getvalues () { // use order List<T> values = super.getvalues(); if (!ordered) { Collections.sort(values, order); ordered = true; return values; 04/23/07 linq-32

37 Use public static void main (String... _args) { List<String> args = Arrays.asList(_args); orderby(args, new Func<String,String>() { // s => s.tolowercase public String f (String s) { return s.tolowercase(); ).thenby(new Func<String,String>() { // s => s public String f (String s) { return s; ) sorts in dictionary order 04/23/07 linq-33

38 etc. 04/23/07 linq-3join computes for pairs: inner record with same key as outer record group-join computes for group of inner records with same key as outer record group by groups values computed from elements with equal key values

39 cs /30/06 linq-3from join... from x in e defines iteration variable. from x in e join y in f on g equals h //... z e.join(f, x => g, y => h, (x,y) => z) defines anonymous iteration over pairs. from x in e join y in f on g equals h into i //... z e.groupjoin(f, x => g, y => h, (x,i) => z) defines anonymous iteration over groups.

40 Join delegate R F<R,A> (A a); delegate R F<R,A,B> (A a, B b); C<V> Join<U,K,V,T> (C inner, F<K,T> outerkey, F<K,U> innerkey, F<V,T,U> selector) { var result = new C<V>(); foreach (var t in this) // from's container foreach (var i in inner) if (outerkey(t).equals(innerkey(i))) result.add(selector(t, i)); return result; selector combines records with matching keys. cs /30/06 linq-36

GroupJoin C<V> GroupJoin<U,K,V,T> (C<U> inner, F<K,T> outerkey, F<K,U> innerkey, F<V,T,C<U>> selector) { var result = new C<V>(); foreach (var t in this) { var group = new C<U>(); foreach (var i in

41 GroupJoin C<V> GroupJoin<U,K,V,T> (C inner, F<K,T> outerkey, F<K,U> innerkey, F<V,T,C> selector) { var result = new C<V>(); foreach (var t in this) { var group = new C(); foreach (var i in inner) if (outerkey(t).equals(innerkey(i))) group.add(i); result.add(selector(t, group)); return result; selector combines outer record with group of inner records and matching keys. cs /30/06 linq-37

42 cs /30/06 linq-3group by from x in e group x by g e.groupby(x => g) groups elements by key value. from x in e group f by g e.groupby(x => g, x => f) groups new values by key value.

43 GroupBy C<Group<K,T>> GroupBy<K,T> (F<K,T> key) { return GroupBy(key, (F<T,T>)delegate (T t) { return t; ); C<Group<K,E>> GroupBy<K,E,T> (F<K,T> key, F<E,T> selector) { var dictionary = new Dictionary<K,Group<K,E>>(); var result = new C<Group<K,E>>(); foreach (var t in this) { var k = key(t); Group<K,E> group; try { group = dictionary[k]; catch { group = new Group<K,E>(k); result.add(group); dictionary[k] = group; group.add(selector(t)); return result; cs /30/06 linq-39

44 Group class Group<K,T> : C<T> { readonly K key; public Group (K key) { this.key = key; public K Key { get { return key; Group preserves the common key value. cs /30/06 linq-40

45 Think functional! A computer scientist invents this... Action sequence: composition Condition/selection: lambdas as arguments Nested selections: application of lambda Sorting: compose Comparator based on lambda arguments No chance to see this in Java [56]... 04/23/07 linq-41

46 Think functional! A computer scientist invents this... Action sequence: composition, lazy evaluation Condition/selection: lambdas as arguments Nested selections: application of lambda Sorting: compose Comparator based on lambda arguments No chance to see this in Java [56]... 04/23/07 linq-42

47 Think functional! A computer scientist invents this... Action sequence: composition, lazy evaluation Condition/selection: lambdas as arguments Nested selections: application of lambda Sorting: compose Comparator based on lambda arguments No chance to see this in Java [56]... 04/23/07 linq-43

48 Think functional! A computer scientist invents this... Action sequence: composition, lazy evaluation Condition/selection: lambdas as arguments Nested selections: application of lambda Sorting: compose Comparator based on lambda arguments No chance to see this in Java [56]... 04/23/07 linq-44

49 Think functional! A computer scientist invents this... Action sequence: composition, lazy evaluation Condition/selection: lambdas as arguments Nested selections: application of lambda Sorting: compose Comparator based on lambda arguments No chance to see this in Java [56]... 04/23/07 linq-45

50 Think functional! A computer scientist invents this... Action sequence: composition Condition/selection: lambdas as arguments Nested selections: application of lambda Sorting: compose Comparator based on lambda arguments No chance to see this in Java [56]... 04/23/07 linq-46

51 References Erik Meijer C#: LinQ C# Java 7: Closures 04/23/07 linq-47

LinQ Why we have to teach functional programmming

LinQ Why we have to teach functional programmming Axel T. Schreiner http://www.cs.rit.edu/~ats/talks/linq/ http://www.cs.rit.edu/~ats/cs-2006-1/14_linq.pdf Principles Map a query language to cascading