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Today's Topics:
1. State in IO monad (Dmitriy Matrosov)
2. Re: State in IO monad ([email protected])
----------------------------------------------------------------------
Message: 1
Date: Sat, 14 Nov 2020 16:53:35 +0300
From: Dmitriy Matrosov <[email protected]>
To: [email protected]
Subject: [Haskell-beginners] State in IO monad
Message-ID: <[email protected]>
Content-Type: text/plain; charset=utf-8; format=flowed
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
------------------------------
Message: 2
Date: Sat, 14 Nov 2020 14:36:06 +0000
From: [email protected]
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] State in IO monad
Message-ID:
<RHOjSMVDRyRLCT60sErhau5ddCy_OUVCVgCbP3HS2USME5KeslEWfsjcs6D8GFU6ERQrxsKrmHNvlWY-OhQXMaXGaaP5c5UyLbgHsbr7SQY=@coot.me>
Content-Type: text/plain; charset="utf-8"
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.
```
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)
```
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 <[email protected]>
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
>
> Beginners mailing list
>
> [email protected]
>
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
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