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
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LinQWhy we have to teach functional programmming
Axel T. Schreinerhttp://www.cs.rit.edu/~ats/talks/linq/
http://www.cs.rit.edu/~ats/cs-2006-1/14_linq.pdf
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.
2
cs-2006-1 10/30/06 linq-
What if...
Syntactic sugar patterned after SQL.
3
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.
4
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 ,... }.
5
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 }));
➦
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Typing
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 }));
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<U> Select<U> (Func<U, T> selector);}
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Typing
7
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<U> Select<U> (Func<U, T> selector);}
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
8
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<U> Select<U> (Func<U, T> selector);}
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 Pattern
9
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<U> Select<U> (Func<U, T> selector);}
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 }));
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Query Expression Pattern
10
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
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Query Class Pattern
11
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<U> Select[Many] (...) C<G<Key,E>> GroupBy (...) class G<Key,E> : C<E> {...}
into join...
➦
Exampleshttp://www.cs.rit.edu/~ats/cs-2006-1/code/adt/query/
http://www.cs.rit.edu/~ats/talks/linq/linq/
cs-2006-1 10/30/06 linq-
from let where ... / Where
13
from x in xs
where f
xs.Where(x => f)
retains only records satisfying a condition.
let x = e
defines name for value.
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
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;}
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
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; }})
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.
Use
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select / Select
16
from x in xs
select f
xs.Select(x => f)
creates new records.
delegate R Func<R,A> (A a);
IEnumerable<U> Select<U,T> (IEnumerable<T> container, Func<U,T> selector) { foreach (var c in container) yield return selector(c);}
Implementation
public interface Func<R,A> { R f (A a); }public<U,T> Iterable<U> select (Iterable<T> container, Func<U,T> selector) { List<U> result = newList(); for (T c: container) result.add(selector.f(c)); return result;
}
delegate R Func<R,A> (A a);
IEnumerable<U> Select<U,T> (IEnumerable<T> container, Func<U,T> selector) { foreach (var c in container) yield return selector(c);}
Implementation
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select / Select[Many]
18
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.
delegate R Func<R,A> (A a);
IEnumerable<U> SelectMany<U,T> (IEnumerable<T> container, Func<IEnumerable<U>,T> selector) { foreach (var c in container) foreach (var u in selector(c)) yield return u;}
Implementation➦
public interface Func<R,A> { R f (A a); }public<U,T> Iterable<U> selectMany (Iterable<T> container, Func<Iterable<U>,T> selector) { List<U> result = newList(); for (T c: container) for (U u: selector.f(c)) result.add(u); return result;
}
delegate R Func<R,A> (A a);
IEnumerable<U> SelectMany<U,T> (IEnumerable<T> container, Func<IEnumerable<U>,T> selector) { foreach (var c in container) foreach (var u in selector(c)) yield return u;}
Implementation➦
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 }))
Use
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; } }); } })
Teach 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]...
Teach 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]...
Teach 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]...
Teach 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]...
Teach 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]...
Teach 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]...
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
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