[Haskell-beginners] Understanding recursion in Haskell.

[Thomas Davie]

On 18 Mar 2009, at 16:32, Zachary Turner wrote:

> A few others have given more complete walkthroughs / traces of the  
> executions, but just to add a little.  When it finally "clicked"  
> with me and recursion became easier to understand, it was when I  
> stopped trying to think about the entire execution chain.  For  
> example, let's say I run a ticket resale (scalping) shop and someone  
> calls me and says "hey I need two tickets for the show this  
> saturday."  I tell the guy sure no problem, I'll call you back in 2  
> hours.  But actually I don't have the tickets, so I call one of my  
> connections.   Unfortunately, he doesn't have the tickets either so  
> this process ends up repeating for a while.  When I talk to my  
> connection on the phone I don't think "ok he's going to call John,  
> and John's probably going to call William, and then William might  
> call Frank, etc".  I just think "he's either going to call me back  
> and say yes or no, regardless of how he arrives at that answer".   
> That's the key to recursion.

This, and the fact that Frank does not call you looking for tickets  
(i.e. we must always make some progress).

Bob