Genuinely curious, not that familiar with Haskell, just thought you could use something like parameter binding or similar to construct functions like that.
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.
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.
Int -> Int -> IO Bool
(btw Haskell's `IO Bool` would be spelled `IO<Bool>` in C++/Java syntax)
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?
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).
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.
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
)
)
---
¹ Unfortunately, JS's Promise#then mixes two things:
• "dirtying" a pure function:
getNumberFromDB().then((x) => x*2)
fmap :: (a -> b) -> IO a -> IO
getNumberFromDB().then((x) =>
getNameForNumberFromDB(x)
)
(>>=) :: IO a -> (a -> IO b) -> IO bWell 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!
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
> 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.
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))
count = show . length . unwords[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"
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
> Type signature `Int -> Int -> Bool` can be used for a function that does any of the following things
[My emphasis] you meant "can't". That could be one explanation for the confusion that seems to have arisen here.