[magicalhippo]

Why couldn't it be a function that queries a database, like answering "on invoice x, does line y exist"?

Genuinely curious, not that familiar with Haskell, just thought you could use something like parameter binding or similar to construct functions like that.

[ToJans]

Haskell functions are pure, which means they can only access/use what is in their parameters, so unless you pass in some extra context (typically using a monad or an effect), you do not have access to the "outside world".

[magicalhippo]

You can bind parameters though in Haskell, no?

I'm used to Boost.Bind and similar, so was thinking a scenario where you bind the database connection parameter and pass the resulting function to something else.

As the sibling pointed out though, I now get that the result would be "tainted" so to speak.

[johnday]

A database connection would operate either in IO (the generic "i am now talking to an unreliable outside world") monad, or some more specific monad.

In this case you can think of a monad a bit like a computational context. If one is not present, you simply cannot[^1] instruct a Haskell program to perform those operations in a type safe way even if you give it a valid database connection identifier.

[1] Well, you can, but if you do you're explicitly taking away all the safeguards that Haskell introduces, and it would never pass code review.

[uryga]

if it has side-effects, it has to be something like

  Int -> Int -> IO Bool

otherwise the type checker won't let you perform any side-effecting operations

(btw Haskell's `IO Bool` would be spelled `IO<Bool>` in C++/Java syntax)

[magicalhippo]

And there's no way to get rid of the IO then, presumably (I mean otherwise I could have just used that as a wrapper).

edit: So at work, just about every value depends directly or indirectly on stuff that comes from files or from the database. So would they all have to be wrapped by IO?

[Twisol]

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).

[magicalhippo]

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.

[uryga]

> 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

[magicalhippo]

> 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!

[chriswarbo]

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

[zvrba]

But is _just reading_ from a database a "side-effect"? It is non-deterministic, but it has no side-effects. So IO jumbles together the two notions.

[uryga]

pedantically, talking to the database will involve sending a request or some kind of IPC, all of which are usually considered side effects.

[handwavy analogy alert]

people often call it "side-effects" as a shorthand. in reality an expression being of type `IO Foo` mostly just tells the compiler that order of execution matters (which isn't the case with pure functions):

  do print "a"
     print "b"
 
  -- obviously not the same as
  
  do print "b"
     print "a"

and also that it can't eliminate common expressions:

  do a <- readBytes file 100
     b <- readBytes file 100
     doStuff a b

  -- obviously not the same as

  do x <- readBytes file 100
     doStuff x x

it's a way of enforcing ordering in a lazy language where evaluation order isn't really defined.