[Haskell-cafe] Combine `StateT` and `InputT` to maintain state on Ctrl-C interrupt

[mikael.brockman at gmail.com]

haskell at stefan-klinger.de writes:

> On 2015-Aug-12, mikael.brockman at gmail.com wrote with possible deletions:
>> But could one get away with using `handle` at the particular sites where
>> one expects to be interrupted, e.g. around the call to `threadDelay`?
>
> Yeah, I've had that, but I don't like it: When the user's wetware
> decides that somethig takes longer than expected, it decides to press
> Ctrl-C.  Now the calculation might just terminate an instant before
> the keypress, yielding a race condition between whether the interrupt
> is caught, or not (or by which exception handler).  Also, pressing C-c
> (un)intentionally at the prompt would either terminate the program, or
> reset all settings made in the interactive session, thats way more
> than I'd like to happen.
>
> But thanks for the idea...
> Stefan

Maybe keeping the state in an I/O variable is the most viable way. I
can't explain clearly why that would be the case, but it seems tricky to
work "externally" with the state carried by StateT, since it only
exists in the transient form of a value threaded through lambdas...