Recursive lambda expressions

Y combinators anyone?

Luca Bolognese's WebLog — Sat, 12 May 2007 02:17:37 GMT

From Mads, our proud C# language PM: http://blogs.msdn.com/madst/archive/2007/05/11/recursive-lambda-expressions.aspx

re: Recursive lambda expressions

Senior .NET Developer — Sat, 12 May 2007 05:41:57 GMT

My head hurts. I think I'll go become a manager.

re: Recursive lambda expressions

Tom Kirby-Green — Sat, 12 May 2007 22:08:05 GMT

I love this stuff. More of the same please!

re: Recursive lambda expressions

j marlowe — Sat, 12 May 2007 22:53:18 GMT

awesome.

and all this included in language that can actually pay the mortgage.

3.5 cannot be released fast enough!

how do you plan to deal with the inevitable problem of unifying the existing bcl apis with the more functional-style possible in 3.5 and beyond? extension methods only go so far in this regard. how will you avoid a "split" into apis that prefer a functional-style and those that do not?

A different point of view.

Community Blogs — Sun, 13 May 2007 01:40:05 GMT

I've met both the author (Mads T), and Luca in person. This post is not an impersonal, anonymous attack.

re: Recursive lambda expressions

David Travis — Mon, 14 May 2007 08:13:44 GMT

Hmmm... It is good know we can do it. But why not stick with the good old methods we know to implement recursions? The readability trade-off here is huge.

Either way, thank you for the thorough explanation.

Recursive lambda expressions

DotNetKicks.com — Mon, 14 May 2007 08:58:29 GMT

You've been kicked (a good thing) - Trackback from DotNetKicks.com

Community Convergence XXVII

Charlie Calvert's Community Blog — Mon, 14 May 2007 09:14:13 GMT

Welcome to the 27th Community Convergence. I use this column to keep you informed of events in the C#

re: Recursive lambda expressions

Matthieu MEZIL — Wed, 23 May 2007 01:05:32 GMT

Why don't you do this?

Func<int, int> fac = null;

fac = x => x-- * (x == 0 ? 1 : fac(x));

re: Recursive lambda expressions

Matthieu MEZIL — Wed, 23 May 2007 10:30:17 GMT

Sorry, I had bad read your post :-(

Recursive lambda expressions

A world apart from the everday ... — Thu, 24 May 2007 10:15:43 GMT

I feel it very neccessary at this point to warn you that this is most certainly not for the faint hearted

re: Recursive lambda expressions

Ryan CrawCour — Thu, 24 May 2007 10:17:40 GMT

Beyond words .... but thx for the entertaining read! :D

re: Recursive lambda expressions

Fduch — Fri, 08 Jun 2007 19:22:32 GMT

How about this?

I've never siin so complicated brain****ing stuff so I may be wrong:

delegate int SelfApplicableInt<Tvoid>(SelfApplicableInt<Tvoid> self, int x);

SelfApplicableInt<int> F = (fac, x) => x == 0 ? 1 : x * fac(fac, x - 1);

Func<int, int> factorial = (x) => F1(F1, x);

var z = factorial2(8);

re: Recursive lambda expressions

Fduch — Fri, 08 Jun 2007 19:24:32 GMT

spelling errors...

delegate int SelfApplicableInt<Tvoid>(SelfApplicableInt<Tvoid> self, int x);

SelfApplicableInt<int> F = (fac, x) => x == 0 ? 1 : x * fac(fac, x - 1);

Func<int, int> factorial = (x) => F(F, x);

var z = factorial(8);

re: Recursive lambda expressions

Jeff Lebowski — Thu, 26 Jul 2007 19:13:56 GMT

This is worse than regular expressions... but almost as entertaining. Thanks!

Taking LINQ to Objects to Extremes: A fully LINQified RayTracer

LukeH's WebLog — Tue, 02 Oct 2007 05:34:11 GMT

Not too long ago I blogged about a C# raytracer which took advantage of a lot of C#3.0 language constructs.

re: Recursive lambda expressions

Fan Shi — Mon, 26 Nov 2007 14:40:52 GMT

Actually, VB can define a pure lambda founded Y combinator, no neeed to define any helper types.

Dim Y1 = Function(f) _

(Function(h) Function(x) f.Invoke(h.Invoke(h)).Invoke(x)) _

(Function(h) Function(x) f.Invoke(h.Invoke(h)).Invoke(x))

It is a late binding version. Typed version is also be able to created.

Recursive Lambda with Fixed Point Generator

TripLeZoNe Technology Playground — Tue, 11 Dec 2007 00:10:41 GMT

Reading Mads Torgersen old blog post on Recursive Lambda Expressions , I decided it was time for me to

Recursive Lambda with Fixed Point Generator

Justin Lee's Technology Blog — Tue, 11 Dec 2007 16:33:48 GMT

.csharpcode, .csharpcode pre { font-size: small; color: black; font-family: consolas, "Courier New",

��ư��

assari (PukiWiki/TrackBack 0.3) — Sat, 26 Jan 2008 09:53:29 GMT

WikiPedia:��ư�� sumii�� - ��ư��黻�Ҥդ�� ٥��ӥ͡��äƲ��˻Ȥ��Ρ� - sumii�� ͸�� Lazy ��롣�ֳ��ԡ�Y��ӥ͡�� - ��쥲�� SICP 1.3.3 Procedures as General Methods:Finding fixed points of functions Fixed point combinato...

Fun with recursive Lambda functions

Fri, 28 Mar 2008 01:59:53 GMT

This post was originally published here . I saw a couple of posts on recursive lambda expressions , and

re: Recursive lambda expressions

gugoxx (giuseppe.ugo@integres.it) — Tue, 01 Apr 2008 02:27:06 GMT

I can't really understand why the following wouldn't be considered enough for the solution.

Func<int, int> fac = null;

fac = x => (x == 0) ? 1 : x * fac(x - 1);

Isn't enough for declaring that it's recursive?

re: Recursive lambda expressions

MattManela — Sun, 27 Apr 2008 10:25:35 GMT

Yes, but then you are no longer dealing with lambda expression in the mathamtical sense. In lambda calculus there is no concept of storing the expression in a 'variable'. Thus a function has no way to refer to itself. This fact leads to the need for a fixpoint combinator to create recursion in a more round-a-bout way.

re: Recursive lambda expressions

PeteS — Tue, 10 Jun 2008 21:37:27 GMT

Aside from LINQ is there any use for lambda expressions in C#? I mean it seems great academic fun, but if that's the idea why not use LISP or ML and such? Since C# is supposed to be a real world language one wonders what's this stuff doing there.

Other than that it IS fun! :)

re: Recursive lambda expressions

David Conrad — Wed, 30 Jul 2008 22:35:13 GMT

I know this is a very old post, but I hope you'll see this. I tried to "put Y and Fix as static members of a generic class", as you said, but got errors I couldn't understand. I assume I still need the SelfApplicable delegate, and I need to put it inside the Fun<T> class? This is what I tried, but it's wrong:

public class Fun<T> {

public delegate Q SelfApplicable<Q>(SelfApplicable<Q> self);

// The Y combinator

public static SelfApplicable<Func<Func<T,T>,T>> Y =

y => f => x => f(y(y)(f))(x);

// The fixed point generator

public static Func<Func<T, T>, T> Fix = Y(Y);

}

Can you expand on the ellipses that you used at that point in your post?

Book Links

Programming Linq Book Log — Tue, 19 Aug 2008 20:24:28 GMT

Chapter Page Url 4 111 http://msdn2.microsoft.com/en-us/library/bb386907.aspx 4 111 http://msdn2.microsoft.com/en-us/library/bb399400.aspx

New and Notable 167

Sam Gentile's Blog — Wed, 03 Dec 2008 05:50:16 GMT

You have to love a hotel where you have the amazing Lavazza coffee in your hotel room. CLR/Silverlight/Orcas/C# One of the things that has been a bit fustrating over the years with the CLR is the limitation of one CLR per process. Specifically, where

.NET Lambda Expressions – Resources

Scott Dorman's Blog — Sun, 11 Jan 2009 02:30:38 GMT

Note : This entry was originally posted on 11/28/2008 11:58:09 AM. I present at a lot of the local Florida

re: Recursive lambda expressions

Paul Batum — Mon, 12 Jan 2009 16:23:51 GMT

This post inspired me to do my own series on how the y combinator can be derived. I used C# and tried to go about it using very small steps. The series starts here:

http://www.paulbatum.com/2009/01/refactoring-towards-y-combinator-part-1.html

Recursive Lambdas

Steve's Tech Talk — Fri, 01 May 2009 23:09:56 GMT

At PDC several years ago, I attended the first talk on C# 3.0 by Anders Hejlsberg wherein he demonstrated

re: Recursive lambda expressions

sdclctb — Sun, 26 Jul 2009 06:59:20 GMT

YN7PXf <a href="http://msfumjakhgiv.com/">msfumjakhgiv</a>, [url=http://pperkffacltq.com/]pperkffacltq[/url], [link=http://tzxafxjhiwoc.com/]tzxafxjhiwoc[/link], http://mgzyrhsvzbuk.com/

re: Recursive lambda expressions

serge — Wed, 26 Aug 2009 14:56:02 GMT

Just for completeness. Here's what the Y combinator factorial looks like in Erlang. Simple and clear:

fun(0, _) -> 1; (X, F) -> X * F(X-1, F) end.

re: Recursive lambda expressions

Glenn Slayden — Thu, 17 Sep 2009 01:22:01 GMT

Great post.

I understand the geeky theory, and the fact that the compiler gives "error CS0165: Use of unassigned local variable" proves that it, too, is a geek.

But in the real world, assigning null first, as others have pointed out, works.

Func<int, int> f = null;

f = x => (x == 0) ? 1 : x * f(x - 1);