[Haskell-beginners] State in IO monad

[Dmitriy Matrosov]

On 11/14/20 5:36 PM, coot at coot.me wrote:
> Hi,
> 
> 
> First you should ask yourself if you need concurrent access to that state.  If not then you can thread this state as argument, e.g.

No, for the time being i don't need concurrent access. Just make it work.

> ```
> data State = State
> 
> someAction :: State -> IO a
> someOtherAction :: State -> IO b
> ```
> This functions compose perfectly well, if you need to update the state then each action should also return it:
> 
> ```
> someAction :: State -> IO (State, a)
> someOtherAction :: State -> IO (State, b)
> ```

Thanks, but the problem is how this library works. [Here][1] is example
provided by library author.

So, essentially, first i need to initialize library with

     telnetInit :: [OptionSpec] -> [Flag] -> EventHandler -> IO Telnet

and pass it callback function with type

     type EventHandler = TelnetPtr -> Event -> IO ()

and this callback function will be called upon each telnet event, including
data reception. But because each time the same function is called and it is in
IO monad, i don't see a way how to pass something (i.e. state) from one call
to the second. In fact, i don't even see a way how to save result of the
single call to use later in the rest of the program. Unless i use global
state in IO (using 'IORef' or 'MVar' (didn't tried yet)).

Alternatively, (looking at [example][1]) i may parse received data after
'recv' call before telling telnet library that data is received with 'telnetRecv',
but then callback function passed to 'telnetInit' does not needed at all
(well, at least in my simple case). And this looks a little strange: why do i
ever need this callback function, if i can't parse data there and save it
for use in the rest of the program?

[1]: https://git.sr.ht/~jack/libtelnet-haskell/tree/master/example/Main.hs

> You still can compose those.  This is `StateT` monad transformer in disguise (from `transformers` / `stm` package combo), but I personally prefer to pass arguments explicitly and only use `StateT` or `RederT` monad if they bring more clarity.
> 
> If you need concurrent access from different threads, then I'd use `MVar` or `stm` for holding a mutable variable, rather than `IORef` (which are useful if the code needs to be as fast as possible, but they present less guarantees).
> 
> If you go with mutable cell to hold the state, then there's no need to create it using `unsafePerformIO`, you can directly create it in `main :: IO ()` and pass it around as an argument. `unsafePerformIO` is really rarely needed and most of the time there are well established patterns to avoid using it.
> 
> Best regards,
> Marcin Szamotulski
> 
> ‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
> 
> On Saturday, November 14th, 2020 at 14:53, Dmitriy Matrosov <sgf.dma at gmail.com> wrote:
> 
>> Hi.
>>
> 
>> I want to use [telnet library][1] to run some
>>
> 
>> commands on cisco switches and then parse output.
>>
> 
>> Thus, first i need to login (enter username and
>>
> 
>> password, etc) and then run commands and collect
>>
> 
>> output. As far as i understand that library API,
>>
> 
>> all of output parsing should be done in
>>
> 
>> EventHandler
>>
> 
>> of type
>>
> 
>> TelnetPtr -> Event -> IO ()
>>
> 
>> But, these (mine) operations require some state
>>
> 
>> (to track where am i now (entered username,
>>
> 
>> entered password, etc) and to collect output). But
>>
> 
>> the monad of this event handler is IO, so i can't
>>
> 
>> see any simple way of adding state to it apart
>>
> 
>> from ['unsafePerfromIO' trick][2], like
>>
> 
>> telnetRef :: IORef TelnetRef
>>
> 
>> {-# NOINLINE telnetRef #-}
>>
> 
>> telnetRef = unsafePerformIO $ newIORef (TelnetRef undefined Unauth M.empty)
>>
> 
>> ('undefined' is 'Telnet' pointer, which is
>>
> 
>> returned by 'telnetInit' and will be initialized
>>
> 
>> later..)
>>
> 
>> Is there a better a way to do this?
>>
> 
>> Thanks.
>>
> 
>> [1]: https://hackage.haskell.org/package/libtelnet
>>
> 
>> [2]: https://wiki.haskell.org/Top_level_mutable_state
>>
> 
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> 
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