[Haskell-cafe] exceptions, errors, random

Dennis Raddle dennis.raddle at gmail.com
Fri Jun 16 20:45:22 UTC 2017

I am wondering what exception/error and random classes I should use for my

The application will be doing Monte Carlo backtracking search. I want it to
be multithreaded and run on all four cores on my MacBook.

I need detailed error messages in order to investigate errors. So I will
throw errors or exceptions, then catch them in lower functions and annotate
them with the parameters or computations within the lower function, then
re-throw until finally the exception pops into the terminal.

Because it's multi-threaded, at least some parts will needs to be in IO.
Also, because it's a Monte Carlo algorithm it will need to make
pseudorandom choices.

Perhaps I should put everything in the IO monad? This will give me access
to threads and the standard generator seed. But how about throwing,
catching/rethrowing exceptions in IO? I have done this in pure code before:
not sure if it's better to do it that way, in which case I could put some
kind of state monad to hold the random seed together with an error monad
and IO at the core of the monad stack.

Any advice welcome.
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