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Today's Topics:
1. Re: Understanding reason for Monad (Pietro Grandinetti)
2. Re: Understanding reason for Monad (Travis Cardwell)
----------------------------------------------------------------------
Message: 1
Date: Tue, 28 Feb 2023 14:01:15 +0000
From: Pietro Grandinetti <[email protected]>
To: Travis Cardwell <[email protected]>, The
Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Understanding reason for Monad
Message-ID:
<db9p191mb1514d1d85824036f56bb24bdfc...@db9p191mb1514.eurp191.prod.outlook.com>
Content-Type: text/plain; charset="iso-8859-1"
Travis,
Thank you very much. I have a question about the `putLanguage` function below.
________________________________
module Main (main) where
-- https://hackage.haskell.org/package/base
import Control.Monad.IO.Class (MonadIO(liftIO))
import Data.Bool (bool)
import System.Environment (getArgs)
-- https://hackage.haskell.org/package/transformers
import Control.Monad.Trans.Reader (ReaderT(runReaderT), asks)
data Locale = En | It
class HasLocale a where
getLocale :: a -> Locale
instance HasLocale Locale where
getLocale = id
class MonadLocale m where
askLocale :: m Locale
instance (HasLocale r, Monad m) => MonadLocale (ReaderT r m) where
askLocale = asks getLocale
putLanguage :: (MonadIO m, MonadLocale m) => m ()
putLanguage = do
locale <- askLocale
liftIO . putStrLn $ case locale of
En -> "English"
It -> "Italiano"
I understand that the result type, in this case `MonadLocale m => m ()`,
determines in what context the function `askLocale` is resolved. But what would
happen if the function type was
putLanguage' :: (MonadOut m, MonadIn v) => v () -> m () -- both MonadOut and
MonadIn are instances of MonadLocale
putLanguage' = do
locale <- askLocale
... -- other things
which `askLocale` function would be used?
putHelloWorld :: (MonadIO m, MonadLocale m) => m ()
putHelloWorld = do
locale <- askLocale
liftIO . putStrLn $ case locale of
En -> "Hello world!"
It -> "Ciao mondo!"
app :: (MonadIO m, MonadLocale m) => m ()
app = do
putLanguage
putHelloWorld
main :: IO ()
main = do
locale <- bool En It . elem "--it" <$> getArgs
runReaderT app locale
In this example, the state/context is simply a `Locale` value, which
defaults to `En`. The `main` function checks if string `--it` is passed
as an argument and configures the locale to `It` in that case.
The final line runs the `app` function using a `ReaderT` monad
transformer with the locale as the "environment." The `app` function,
as well as all functions that it calls in the same monad, have access to
this environment.
Type class `HasLocale` just provides a `getLocale` function for getting
a `Locale` value from some possibly larger value. The instance is the
trivial case of `Locale` itself.
Type class `MonadLocale` provides a locale API, just `askLocale` in this
case. In a monad that implements `MonadLocale`, the `askLocale`
function is able to get the locale. The instance provides a way to do
this in a Reader monad that has an environment with a `HasLocale`
instance. In this minimal example, the Reader environment is a `Locale`
value, so that trivial `HasLocale` instance is used.
The remaining three functions implement the example application. They
do not specify a concrete monad; they instead specify constraints on the
monad, allowing them to run in any monad that meets those constraints.
The `MonadIO m` constraint is required to use `liftIO . putStrLn` in
order to print to the screen, and the `MonadLocale m` constraint is
required to get the configured locale. In this example, they are run
in concrete monad `ReaderT Locale IO`, but note that they could also be
run in different monads as long as the constraints are satisfied.
The `app` function calls `putLanguage` and then `putHelloWorld`, and
both of these functions are able to use `askLocale` to get the
configured locale.
$ minimal-context
English
Hello world!
$ minimal-context --it
Italiano
Ciao mondo!
The architecture/design of a project/program depends on the needs. In
some programs, explicitly passing context as arguments is the best
approach. In others, even `MonadIO` should be avoided, since `IO` makes
anything possible. Getting back to your original question, the use of
type classes allows a library author to implement functions that work
across a wide variety of coding styles.
Cheers,
Travis
On Sun, Feb 26, 2023 at 6:35 PM Pietro Grandinetti
<[email protected]> wrote:
> Hi Travis,
>
> Thanks. This was indeed helpful. I think I haven't grasped the concept of
> "context" yet. Do you know any minimal example that shows this?
>
> Thanks.
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Message: 2
Date: Wed, 1 Mar 2023 18:59:02 +0900
From: Travis Cardwell <[email protected]>
To: Pietro Grandinetti <[email protected]>
Cc: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Understanding reason for Monad
Message-ID:
<CACaJP_QSi85-LDLE5bzJJv=U7U-C1z8co0_eY=lhtz7hk2k...@mail.gmail.com>
Content-Type: text/plain; charset="UTF-8"
Hi Pietro,
On Tue, Feb 28, 2023 at 11:01 PM Pietro Grandinetti wrote:
> Thank you very much. I have a question about the `putLanguage`
> function below.
You are welcome!
putLanguage :: (MonadIO m, MonadLocale m) => m ()
putLanguage = do
locale <- askLocale
liftIO . putStrLn $ case locale of
En -> "English"
It -> "Italiano"
> I understand that the result type, in this case
> `MonadLocale m => m ()`, determines in what context the function
> `askLocale` is resolved.
Correct. This function runs in monad `m`: it runs in any monad with
both `MonadIO` and `MonadLocale` instances, returning `()` (pronounced
"unit"). Function `askLocale` can be used here because the monad has a
`MonadLocale` instance.
> But what would happen if the function type was
putLanguage' :: (MonadOut m, MonadIn v) => v () -> m ()
putLanguage' = do
locale <- askLocale
...
> which `askLocale` function would be used?
This function runs in monad `m`: it runs in any monad with a `MonadOut`
instance, returning `()`. Function `askLocale` cannot be used here
because there is no `MonadLocale m` constraint. To answer the gist of
your question, however, the functions available to use are determined by
the `m` monad, *not* `v`.
In this function, `v ()` is a function that is passed as an argument.
Such a monadic argument is generally called an "action." You can
execute that action if you can create the appropriate monadic context.
In this case, there is no `MonadIn m` constraint, so it is not possible
to execute the action within `putLanguage'` (given just the above
information).
Here is a minimal example:
module Main (main) where
-- https://hackage.haskell.org/package/base
import Control.Monad.IO.Class (MonadIO(liftIO))
actionDemo
:: MonadIO m
=> (String -> IO ())
-> m ()
actionDemo trace = liftIO $ trace "Hello!"
main :: IO ()
main = actionDemo putStrLn
Function `actionDemo` runs in monad `m`: it runs in any monad with a
`MonadIO` instance, returning `()`. It accepts argument `trace`, which
is an action that accepts a `String` argument, runs in the `IO` monad,
and returns `()`. The `main` function passes `putStrLn`, which has this
type.
Since `actionDemo` runs in monad `m`, it cannot execute an action in the
`IO` monad directly. `MonadIO` provides a `liftIO` function to execute
actions in the `IO` monad, however, so `liftIO` is used here to execute
`trace` in the `IO` monad.
Cheers,
Travis
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