[hota_mazi]

Actually, `Reader` adds a lot of boiler plate that's not present with traditional @Inject injection:

- All your functions now need to return a Reader[C,A] instead of just A

- You need to pass all the parameters explicitly in each method signature as opposed to passing just the ones that don't need to be injected.

[thinkpad20]

There is a very small amount of boilerplate. And I would argue strongly that it's a good thing; it indicates to the reader of the code that an object's behavior reads from some initialized value, or equivalently that its behavior depends on some initial value which remains fixed through the computation. The reader monad gives you a simple language to express this common pattern, as well as the ability to easily set the behavior.

    -- This function will always return the same thing given the same input
    function1 :: Int -> String
    
    -- This function depends on reading some configuration which 
    -- needs to be provided upstream
    function2 :: Int -> Reader Configuration String

You don't need to pass parameters explicitly in each signature; indeed this is exactly what the reader monad obviates: the details of what is being read are not expressed inside the function (until the point that they they are actually used). This is hardly an onerous burden, in my opinion. And if typing `Reader X Y` is too annoying, you can just make a type alias.

[hota_mazi]

What if `function2` needs to log something? Without dependency injection, you need to pass that logger to the function. With dependency injection, that logger is available without having to pollute the method signature with an implementation detail.

[thinkpad20]

If `function2` needs to log something, then it should have a type signature which reflects that.

Logging is a side-effect. Logging requires configuration to be passed in; it means having access to some file descriptor or other object to interact with, it could potentially fail to connect, or cause a computation to hang, or cause a service to trigger, or make a disk run out of space, etc. If a function wants to log something it's not a simple reader anymore but something more complex. The fact that in Haskell this is reflected in the type signature of the function is again a good thing. It's not "polluting" the method signature; it's putting more information in the method signature. Not letting you hide side effects in a computation that appears to have no externalities is a strength of Haskell, not a weakness.

[hota_mazi]

> If `function2` needs to log something, then it should have a type signature which reflects that.

Yes if you value referential transparency.

No if you value encapsulation.

The fact that `function2` is logging stuff is an implementation detail that callers shouldn't care about. They should certainly not be forced to pass that function a logger.

What if that function decides that on top of logging, it wants to store stuff in a database. Should all callers suddenly find some kind of database to pass to that function too?

[thinkpad20]

> The fact that `function2` is logging stuff is an implementation detail that callers shouldn't care about.

On the contrary, I think it definitely matters. If a function is going to log something, I want to know about it. Those logs could cause me problems (e.g. polluting my stdout or attempting to write to a file they don't have permissions on), or I might want to control where those logs go, what the log level is, what the format is, et cetera. This is absolutely something I want to know about.

> What if that function decides that on top of logging, it wants to store stuff in a database. Should all callers suddenly find some kind of database to pass to that function too?

Yes, a thousand times yes. Why would I want a function to be storing stuff in a database without my knowledge? If a function is going to write to a database, it's all the more important that the caller is aware of that. How can I access whatever it stores? How do I know what database it's writing to? How can I be sure that database is properly initialized and/or torn down? How do I know whether the function is threadsafe? How do I know it's a secure connection? Et cetera.

If you want to write a function which does "arbitrary side effects", easy: just write all of your code in the IO monad.

    -- It reverses a string... and who knows what else!
    reversePlus :: String -> IO String
    reversePlus str = do
      putStrLn ("Hey, I'm reversing " ++ str)
      conn <- connectPG "localhost:3123:mydatabase"
      queryPG conn "DROP SCHEMA public CASCADE;"
      sendEmail "[e-mail]" "hey guys what's up"
      return $ reverse str

Of course, I don't recommend this...

[hota_mazi]

> > The fact that `function2` is logging stuff is an implementation detail that callers shouldn't care about.

> On the contrary, I think it definitely matters. If a function is going to log something, I want to know about it.

You are missing the forest for the trees.

First of all, why you'd care that a function you're calling is logging stuff is a bit beyond me but fine. Think of something else. Maybe the function is calling memcache, or storing stuff in the database, or sending a UDP packet to a message bus, or is querying the location service. Surely you can agree that there are things this function does that you don't care about if all you need is an Account given a user id, right?

These things you don't care about are called implementation details. Callers shouldn't know about them, therefore they shouldn't have to pass them in parameters.

That's what dependency injection (injection, not passing) does for you. It lets you call

    val account = getAccount(userId)

instead of

    val account = getAccount(userId, logger, memCache, db, messageBus)

The first example is using dependency injection and correctly hides the implementation details of `getAccount` while not being referentially transparent.

The second example is referentially transparent but exposes all kinds of private implementation details, making the callers' life very difficult, if not impossible (how are they supposed to come up with a messageBus when all they have is a user id?).

[mej10]

A small example.

    class (MonadIO m) => HasLogging m where
      log :: String -> m ()
    
    data AppConfig = AppConfig { stuff :: Int }
    
    newtype MyApp a = MyApp { runApp :: ReaderT AppConfig IO a}
      deriving (Functor, Applicative, Monad, MonadIO, MonadReader AppConfig)
    
    instance HasLogging MyApp where
      log s = liftIO (putStrLn s)
    
    function2 :: Int -> MyApp String
    function2 x = do
      log "hey guys I'm logging"
      return (show x)
    
    -- or without specifying the base monad, yay abstraction
    
    function2' x = do
      log "heyooo logging here"
      return (show x)
    
    -- Haskell will infer this type:
    -- function2' :: (HasLogging m, Show a) => a -> m String

[DasIch]

Given that you'll notice whether or not function2 does any logging, it's definitely not an implementation detail.

[wyager]

Not sure I agree with you on that, but monads handle your concern nicely. For example, I can write some code that does some database operations, and by parameterizing the code over the type of database actions, my code doesn't care if it's calling a "real" database action or a "fake" one for testing or whatever. That is, the code is completely agnostic as to the implementation details, but we still have full visibility and static checking when we actually run the database code, because we have to specify which database implementation we want to use. Boom, statically verified dependency injection.

[mej10]

"Implementation detail"

We are in strong disagreement about what constitutes an "implementation detail".

But also, you can just use a monad transformer stack and add whatever side-effectful operations you want into it, use it as needed. Boom, dependency injection. And more control over what your functions actually do is there when you need it.

[fmap]

You might be interested in reflection/implicit configurations: https://hackage.haskell.org/package/reflection

This (ab)uses Haskell's type class mechanism to essentially implement dependency injection directly. The implementation looks a bit dirty, but this is a feature that more modern approaches to generic programming can handle natively (e.g., http://homepages.inf.ed.ac.uk/wadler/papers/implicits/implic... ).

In particular, there is nothing shady about the semantics of implicitly passing configuration values/dependencies. Your functions are still referentially transparent if you treat the implicit dependencies as additional parameters (which is what they are, no matter how you implement it).