In Haskell, you often build pure transformations, and then lift them into an effectful context. If I have a function `String -> [String]`, say to parse a line of CSV into its elements, I can lift that to `IO String -> IO [String]` using the IO monad's `fmap`. And then I can compose it with something hypothetical like `readLine :: File -> IO String`, which actually reads the line.

The core logic of a program often doesn't need to care deeply about state or system resources. Pure functional programming is about writing as much as you can in this "functional core", and then lifting the assembled pieces of pipeline into the "imperative shell" (such as the IO monad).

Ok, so you make pure functions and turn them dirty, so to speak. Makes sense.

In our case, almost all core code depends on various parameters, which come from the database.

For example, GB recently left the EU so everything involving GB is now processed under different rules, except old stuff which has to be processed under the old rules. Thus being part of EU or not is a date-dependent database query (it already was, not the first time a country's EU status has changed).

So if I get your explanation correctly, I'd code the core logic as if these parameters were pure, side-effect free, which would make the core logic side-effect free. In the case above, I'd pass a function which maps a (pure) date and string into a (pure) bool, to test for EU membership.

I'd then turn that whole thing dirty via the IO thingy, passing "IO parameters" and receiving "IO results", so I can pass it my EU test function which does a database query.

edit: And I presume my "dirty" database-connecting function can also mutate things, so it can do caching. Don't want to hit that database too often.

> In the case above, I'd pass a function which maps a (pure) date and string into a (pure) bool, to test for EU membership.

i doubt you could make (or really, even want to make) `checkEUMembership` pure, I'm guessing it'd involve a DB lookup of some kind.

in general, you can't always "pull out all the IO" into an only-pure-logic "core"; like if you want to look up one thing and then look up another thing based on the result of the first lookup. and that's okay!

i'm not going to write a whole monad tutorial, but using an `IO Foo` is kind of like using a `Promise<Foo>`¹; you do stuff like this (in JS syntax):

  getX(...).then((x) =>
    getYForX(x).then((y) =>
      foo(x, y)
      // note - nested lambdas/closures, `x` is closed-over
    )
  )

"do-notation" lets you avoid callback hell, similarly to async/await.

---

¹ Unfortunately, JS's Promise#then mixes two things:

• "dirtying" a pure function:

  getNumberFromDB().then((x) => x*2)

which in Haskell would use

  fmap :: (a -> b) -> IO a -> IO

• piping the result into another side-effecting function:

  getNumberFromDB().then((x) => 
    getNameForNumberFromDB(x)
  )

which in Haskell would use the "bind" operator:

  (>>=) :: IO a -> (a -> IO b) -> IO b

> i doubt you could make (or really, even want to make) `checkEUMembership` pure, I'm guessing it'd involve a DB lookup of some kind

Well that was kinda the root of my question. The core logic doesn't really care as such, as long as it could determine EU membership somehow, but actual code would have to use a DB lookup[1].

That of course spirals back to what would that really buy you. You'd write code pretending it's pure while it really isn't. I can see part of the appeal, but I can do that in my current language.

Of course I don't get an error if I do something silly in the middle of some otherwise "pure" module, so there's that.

Anyway, illuminating. I enjoy thinking about these things and challenging my self-taught ways. Thank you all for your contributions, much appreciated!

[1]: An aside but, due to an error on the government side, GB is part of EU today as far as one of their validation checks is concerned. So today only we have to pretend along, for just that one field. This stuff is fun!

> You'd write code pretending it's pure while it really isn't.

in a way, it's the opposite! the point is you can't pretend, you have to make impurity painfully explicit:

  getTradeTax :: CountryId -> CountryId -> IO Float
  --                                       ^ sirens blaring, side-effect alert
  getTradeTax ca cb = do
    aInEu <- lookupEUMemberDB ca
    bInEu <- lookupEUMemberDB cb
    if (aInEu && bInEu)
      then (pure 15.00)           -- made up value
      else getNonEUTradeTax ca cb -- another impure operation

there's the "IO" in the signature, and all the do-notation `<-`, ie syntactic sugar for `>>=`, piping the result into a callback. to use the Promise analogy again, `x <- foo` is kinda like `x = await foo` (but more general, bc Monads are cool)

> I can see part of the appeal, but I can do that in my current language.

true, and i've seen IO-monad-alikes for Python and JS, but most of the benefits come when every library you use has to be explicit about impurity and there's a typechecker enforcing it.

As a concrete example, there's a nice function in Haskell's standard library called `interact :: (String -> String) -> IO ()`: https://hackage.haskell.org/package/base-4.14.1.0/docs/Prelu...

Its argument is a function of type `String -> String` and it returns an `IO ()`, i.e. an i/o action with a trivial result. That action will call the given function on contents of stdin, and writes its result to stdout. Or, equivalently, we can think of `interact f` as transforming a pure string-processing function `f` into a stdio CLI.

Note that laziness (specifically "lazy IO") causes stdin to be read 'on demand', giving us a streaming computation without any extra work. Here's an example implementation of 'wc':

    module Main where
    import System.IO

    main :: IO ()
    main = interact count

    count :: String -> String
    count input = show (length (unwords input))

Bonus: if we want to show off, we could implement 'count' using function composition like this:

    count = show . length . unwords