F# MailboxProcessor questions

Question

I've created a console program using the code from http://fssnip.net/3K . And I found that

1. I'd to add "System.Console.ReadLine() |> ignore" at the end to wait for the finish of threads. Is it possible to tell all the MailBoxProcessors are done and the program can exit itself?

2. I tried to change the test url "www.google.com" to something invalid url and I got the following output. Is it possible to avoid the "outputting race"?

     http://www.google.co1m crawled by agent 1.  
     AgAAAent gent 3 is done.  
     gent 2 is done.  
     5 is done.  
     gent 4 is done.  
     Agent USupervisor RL collector is done.  
     is done.  
     1 is done.

[Edit]

The last output/crawling is still terminated after using Tomas's update http://fssnip.net/65 . The following is the output of the program after I changed the "limit" to 5 and added some debugging messages. The last line shows the truncated URL. Is it a way to detect if all the crawlers finish their execution?

[Main] before crawl
[Crawl] before return result
http://news.google.com crawled by agent 1.
[supervisor] reached limit
http://www.gstatic.com/news/img/favicon.ico crawled by agent 5.
Agent 2 is done.
[supervisor] reached limit
Agent 5 is done.
http://www.google.com/imghp?hl=en&tab=ni crawled by agent 3.
[supervisor] reached limit
Agent 3 is done.
http://www.google.com/webhp?hl=en&tab=nw crawled by agent 4.
[supervisor] reached limit
Agent 4 is done.
http://news.google.com/n

I changed the main code to

printfn "[Main] before crawl"
crawl "http://news.google.com" 5
|> Async.RunSynchronously
printfn "[Main] after crawl"

However, the last printfn "[Main] after crawl" is never executed, unless I add a Console.Readline() at the end.

[Edit 2]

The code runs fine under fsi. However it will have the same problem if it was run using fsi --use:Program.fs --exec --quiet

Answer 1

I created a snippet that extends the previous one with the two features you asked about: http://fssnip.net/65 .

1. To solve this, I added Start message that carries AsyncReplyChannel<unit> . When the supervisor agent starts, it waits for this message and saves the reply channel for later use. When it completes, it sends a reply using this channel.

   The function that starts the agent returns asynchronous workflow that waits for the reply. You can then call crawl using Async.RunSynchronously , which will complete when the supervisor agent completes.

2. To avoid race when printing, you need to synchronize all prints. The easiest way to do this is to write a new agent:-). The agent receives strings and prints them to the output one by one (so that they cannot be interleaved). The snippet hides the standard printfn function with a new implementation that sends strings to the agent.