How do I do simple multithreading in Isabelle ML? - isabelle

Question

I've found a use for wanting to start multiple Isabelle_System.bash processes.
In this next source, I use 3 bash commands. For a simple example, I would like to start them in separate threads, so that they run concurrently, rather than sequentially.
ML {*
Isabelle_System.bash ("echo '1. Call script to compile in the PIDE console.'");
Isabelle_System.bash ("echo '2. Call script to compile in a Windows console.'");
Isabelle_System.bash ("echo '3. Maybe a third process.'");
(*In an outer syntax command, I have options to allow 1 and 2, so it might be
useful to allow starting both at the same time, to be able to terminate the
PIDE process, and let the Windows console keep running. But, unless
multithreading is used, their execution will be sequential, which is
useless.*)
*}
I did a grep, and I've found src/Pure/Concurrent/simple_thread.ML.
However, this is not a priority, and it wouldn't be best for me to use trial-and-error to try and figure out what needs to be done on my own.
If someone can give me a simple plug-n-play template to run the 3 bash commands above, I would appreciate it. Or, maybe someone can tell me why I can't or shouldn't do it.

Answer 1

"Simple multithreading" is an oxymoron, because threads are never simple. If you just want to do parallel programming, which is simple, then the first place to look is the structure Par_List, e.g. the Par_List.map combinator. In the Isabelle/Isar Implementation Manual there is a section on "Parallel skeletons" about it, and more information in the vicinity.

Answer 2

Here, I fill in a few details, based on Makarius Wenzel's answer.
It turns out, that with the Isabelle/ML library in src/Pure, running parallel, independent bash processes is extraordinarily easy:
ML {* (*Running this multiple times shows the order of execution varies.*)
Par_List.map : ('a -> 'b) -> 'a list -> 'b list;
val bash_strings = [
"echo 'Echo 1.'; echo 'Echo 1.'; echo 'Echo 1.'; echo 'Echo 1.' ",
"echo 'Bash 2, bro!'",
"echo 'Bash 3, hombre or señorita!'",
"echo 'Bash 4, dude!'"];
val _ = Par_List.map Isabelle_System.bash bash_strings
*} (*
OUTPUT:
Bash 4, dude!
Bash 2, bro!
Bash 3, hombre or señorita!
Echo 1.
Echo 1.
Echo 1.
Echo 1.
val it = fn: ('a -> 'b) -> 'a list -> 'b list
val bash_strings =
["echo 'Echo 1.'; echo 'Echo 1.'; echo 'Echo 1.'; echo 'Echo 1.' ",
"echo 'Bash 2, bro!'", "echo 'Bash 3, hombre or señorita!'",
"echo 'Bash 4, dude!'"]:
string list*)
A 5 minute solution to a multi-month implementation problem, not counting the overhead of asking the question, reading the answer, opening the PDF, firing up the PIDE, and typing various stuff before locking onto some source that shows a simple example that works.
Section 0.9.1 Parallel skeletons, The Isabelle/Isar Implementation, Isabelle2014 (PDF)
src/Pure/par_list.ML