why do you want to know about functional programming?ats/talks/linq-ieee/linq-ieee.pdf · 04/23/07...

04/23/07 linq-

Why do you want to know about functional programming?

A look at LinQ in C# and (perhaps) Java

Axel T. SchreinerRIT — Computer Science

Eric Meijer

http://www.cs.rit.edu/~ats/talks/linq-ieee/http://www.cs.rit.edu/~ats/cs-2006-1/14_linq.pdf

04/23/07 linq-

Principles

Patterns

Examples

Implementation

Conclusions

cs-2006-1 10/30/06 linq-

Principles

Map a query language to cascading method calls.

Implement the necessary methods for IEnumerable, XML- and database-access.

Sugar syntax liberally.

cs-2006-1 10/30/06 linq-

What if...

Syntactic sugar patterned after SQL.

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 });

cs-2006-1 10/30/06 linq-

What if...

Syntactic sugar patterned after SQL.

Actions converted into messages,Expressions converted into functions.

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 });

cs-2006-1 10/30/06 linq-

What if...

Functional style [Eric Meijer]:

function as arguments,function composition for actions.

Anonymous class new { field ,... }.6

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 }));

cs-2006-1 10/30/06 linq-

Typing

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-2006-1 10/30/06 linq-

Typing

cs-2006-1 10/30/06 linq-

Typing

cs-2006-1 10/30/06 linq-

Typing Pattern

cs-2006-1 10/30/06 linq-

Query Expression Pattern

from join... introduce iteration variable(s)

fromlet ...where

introduces iteration variable(s)introduces variablefilters

orderby rearranges sequence

selectgroup by

produces flat resultproduces structured result

into join...

produces iteration variable(s) to start over

cs-2006-1 10/30/06 linq-

Query Class Pattern

from join...

class C<T> { C<V> Join (...) C<V> GroupJoin (...)

fromlet ...where

C<T> Where (...)

orderby

O<T> OrderBy[Descending] (...) class O<T> : C<T> { O<T>ThenBy[Descending] (...) }

selectgroup by

C Select[Many] (...) C<G<Key,E>> GroupBy (...) class G<Key,E> : C<E> {...}

into join...

04/23/07 linq-

Exampleshttp://www.cs.rit.edu/~ats/cs-2006-1/code/adt/query/

http://www.cs.rit.edu/~ats/talks/linq-ieee/linq/

cs-2006-1 10/30/06 linq-

from let where ... / Where

from x in xs where f

xs.Where(x => f)

retains only records satisfying a condition.

let x = e

defines name for value.

04/23/07 linq-

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;}

Implementation

04/23/07 linq-

public 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;}

IEnumerable<T> Where<T> (IEnumerable<T> container, Func<bool,T> predicate) { foreach (var c in container) if (predicate(c)) yield return c;}

Implementation

04/23/07 linq-

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-

int[] xs = { 1, 2, 3, 4, 5 };

from x in xs where x % 2 == 0 select x

Where(xs, x => x % 2 == 0)

04/23/07 linq-

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; }})

int[] xs = { 1, 2, 3, 4, 5 };

from x in xs where x % 2 == 0 select x

Where(xs, x => x % 2 == 0)

Arrays are not Iterable.

cs-2006-1 10/30/06 linq-

Sugar: Extension Methods

A static class can define (static) extension methods.

If the class is imported, the methods can be called as instance methods:

static class X { static R method (this A parm, ...) { ... }}

A a;a.method(...)

cs-2006-1 10/30/06 linq-

Sugar: Lambda Syntax

An anonymous method definition can be significantly simplified.

Parameter and result types can be inferred.

Proposed for Java 7.

parameter => expression(type parameter, ...) => { body }

cs-2006-1 10/30/06 linq-

select / Select

from x in xs select f

xs.Select(x => f)

creates new records.

04/23/07 linq-

IEnumerable Select<U,T> (IEnumerable<T> container, Func<U,T> selector) { foreach (var c in container) yield return selector(c);}

Implementation

04/23/07 linq-

public 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;

IEnumerable Select<U,T> (IEnumerable<T> container, Func<U,T> selector) { foreach (var c in container) yield return selector(c);}

Implementation

04/23/07 linq-

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;} };} };}

cs-2006-1 10/30/06 linq-

select / 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.

04/23/07 linq-

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;}

Implementation➦

04/23/07 linq-

public 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;

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;}

Implementation➦

04/23/07 linq-

int[] 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 }))

cs-2006-1 10/30/06 linq-

Sugar: Tuple Types

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.

new { name = expression, ... }

04/23/07 linq-

int[] 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 }))

Review: Use

04/23/07 linq-28

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; } }); } })

cs-2006-1 10/30/06 linq-

orderby

e.OrderBy(x => f) // .ThenBy(x => g) ...

from x in e orderby f [ascending|descending] // , g ,...

defines sort key[s] and direction.

04/23/07 linq-

public<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)); } } );}

Implementation:primary key

Comparator is composed from key

04/23/07 linq-31

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

Implementation:secondary keys

04/23/07 linq-32

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); } // ...

protected List<T> getValues () { // use order List<T> values = super.getValues(); if (!ordered) { Collections.sort(values, order); ordered = true; } return values; }

Implementation:sort

get, remove, set, and size are also delegated; any use of values can be overridden:

04/23/07 linq-

sorts in dictionary order

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; } })

04/23/07 linq-

joincomputes for pairs: inner record with same key as outer record

group-joincomputes for group of inner records with same key as outer record

group bygroups values computed from elements with equal key values

etc.➦

cs-2006-1 10/30/06 linq-

from join...

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.

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

defines iteration variable.

cs-2006-1 10/30/06 linq-

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-2006-1 10/30/06 linq-

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-2006-1 10/30/06 linq-

group 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.

cs-2006-1 10/30/06 linq-

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-2006-1 10/30/06 linq-

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.

04/23/07 linq-

Think functional!

A computer scientist invents this...

Action sequence: compositionCondition/selection: lambdas as argumentsNested selections: application of lambda Sorting: compose Comparator based on lambda arguments

No chance to see this in Java [56]...

04/23/07 linq-

Think functional!

Action sequence: composition, lazy evaluationCondition/selection: lambdas as argumentsNested selections: application of lambda Sorting: compose Comparator based on lambda arguments

04/23/07 linq-

Think functional!

04/23/07 linq-

Think functional!

04/23/07 linq-

Think functional!

04/23/07 linq-

Think functional!

Action sequence: compositionCondition/selection: lambdas as argumentsNested selections: application of lambda Sorting: compose Comparator based on lambda arguments

04/23/07 linq-

References

Erik Meijerhttp://research.microsoft.com/~emeijer/

C#: LinQhttp://msdn.microsoft.com/netframework/future/linq/

C# 3.0http://msdn2.microsoft.com/en-us/vcsharp/aa336745.aspx

Java 7: Closureshttp://www.javac.info/closures-v03.html

why do you want to know about functional programming?ats/talks/linq-ieee/linq-ieee.pdf · 04/23/07...

Documents

language integrated query: an introduction - … ·...

linq programming in c# linq cse 459.24 prof. roger crawfis

fpga ieee.pdf

ganpat university...linq to objects: introduction to linq,...

c2006 ieee.pdf

linq & wcf

lambda expressions & linq

linq intro

linq 2013 session_green_1_trailer

custom linq providers

linq , take two realizing the linq to everything dream

mee t the future - caesars forum · linq lane linq lane...

linq 2013 session_blue_3_isip

professional - elte ik linq (program… · linq to xml...

linq introduction

linq documentation

linq 2013 session_green_3_organic.lingua

linq examples

linq bright

linq providers