I am using a library that I can provide with a function a -> IO (), which it will call occasionally.
Because the output of my function depends not only on the a it receives as input, but also on the previous a's, it would be much easier for me to write a function [a] -> IO (), where [a] is infinite.
Can I write a function:
magical :: ([a] -> IO ()) -> (a -> IO ())
That collects the a's it receives from the callback and passes them to my function as a lazy infinite list?
The IORef solution is indeed the simplest one. If you'd like to explore a pure (but more complex) variant, have a look at conduit. There are other implementations of the same concept, see Iteratee I/O, but I found myself conduit to be very easy to use.
A conduit (AKA pipe) is an abstraction of of program that can accept input and/or produce output. As such, it can keep internal state, if needed. In your case, magical would be a sink, that is, a conduit that accepts input of some type, but produces no output. By wiring it into a source, a program that produces output, you complete the pipeline and then ever time the sink asks for an input, the source is run until it produces its output.
In your case you'd have roughly something like
magical :: Sink a IO () -- consumes a stream of `a`s, no result
magical = go (some initial state)
where
go state = do
m'input <- await
case m'input of
Nothing -> return () -- finish
Just input -> do
-- do something with the input
go (some updated state)
This is not exactly what you asked for, but it might be enough for your purposes, I think.
magical :: ([a] -> IO ()) -> IO (a -> IO ())
magical f = do
list <- newIORef []
let g x = do
modifyIORef list (x:)
xs <- readIORef list
f xs -- or (reverse xs), if you need FIFO ordering
return g
So if you have a function fooHistory :: [a] -> IO (), you can use
main = do
...
foo <- magical fooHistory
setHandler foo -- here we have foo :: a -> IO ()
...
As #danidaz wrote above, you probably do not need magical, but can play the same trick directly in your fooHistory, modifying a list reference (IORef [a]).
main = do
...
list <- newIORef []
let fooHistory x = do
modifyIORef list (x:)
xs <- readIORef list
use xs -- or (reverse xs), if you need FIFO ordering
setHandler fooHistory -- here we have fooHistory :: a -> IO ()
...
Control.Concurrent.Chan does almost exactly what I wanted!
import Control.Monad (forever)
import Control.Concurrent (forkIO)
import Control.Concurrent.Chan
setHandler :: (Char -> IO ()) -> IO ()
setHandler f = void . forkIO . forever $ getChar >>= f
process :: String -> IO ()
process ('h':'i':xs) = putStrLn "hi" >> process xs
process ('a':xs) = putStrLn "a" >> process xs
process (x:xs) = process xs
process _ = error "Guaranteed to be infinite"
main :: IO ()
main = do
c <- newChan
setHandler $ writeChan c
list <- getChanContents c
process list
This seems like a flaw in the library design to me. You might consider an upstream patch so that you could provide something more versatile as input.
Related
I've been learning Haskell in the last 2 weeks and decided to try challenges at places such as HackerRank. This has required learning IO. I have read many answers on StackExchange and the general gist is you don't unwrap IO a, you just manipulate that data inside the IO function. That being the case what is the point of all the pure functions, if I'm not allowed to send data from main out to them? Here is some code that reads how many test cases, then for each test case reads N ordered pairs.
main = do
test <- getLine
replicateM (read test) doTest
doTest = do
query<-getLine
rs<-replicateM (read query) readPair
return rs -- just here to make the file compile
readPair :: IO (Int, Int)
readPair = do
input <- getLine
let a = words input in return (read (a!!0) :: Int, read (a!!1) ::Int)
At this point I have a IO [(Int, Int)] inside of rs. I would like to send that data to this function:
validFunction :: [(Int,Int)]->Bool
validFuntion [] = True
validFunction (x:[]) = True
validFunction (x:xs) = (not $ elem (snd x) (fmap snd xs)) && validFunction xs
But I can't seem to figure out how to do that. Any help or suggestions about how to call this function with the data I've read from the user would be appreciated. Or if I'm going about it from the wrong angle, and pointers on what I should be doing would also work.
Edit: From reading lots of other questions on here I now have the general idea that once you're in IO you're stuck there. But what I can't seem to find is the syntax to call a pure function with IO data and get back IO data. I've tried some of the following :
fmap validFunction [rs] :: IO Bool -- tried it with just rs without [] as well
mapM validFunction [rs] :: IO Bool
validFunction rs :: IO Bool
I was able to get this to work:
putStrLn . f . validFunction $ rs
though I'm still not clear on why this lets you pass the IO [(Int, Int)] to validFunction.
First of all, if you use x <- act in do, you essentially have a value. Unless you did something very suspicious, x isn't a IO something, but a something: So it's perfectly fine to use
foo :: Int -> Char
foo = …
bar :: IO Int
bar = …
fooDo :: IO Char
fooDo = do
number <- bar
return (foo number) -- apply foo directly on number
However, IO is an instance of Functor, so we can use fmap to lift foo:
liftedFoo :: IO Int -> IO Char
liftedFoo = fmap foo
So we could have written fooDo like this:
fooDo = fmap foo readLn
Although it's name is now misleading, it still does the same as before. But let's leave this naming voodoo aside, how would you tackle this? Well, your doTest has the correct type:
doTest :: IO [(Int, Int)]
doTest = do
query <- getLine
rs <- replicateM (read query) readPair
return rs
So all that's missing is calling validFunction. We can do that like in fooDo:
doTest :: IO Bool
doTest = do
query <- getLine
rs <- replicateM (read query) readPair
return (validFunction rs)
-- ^^^^^^^^^^^^^^^^^^
-- no IO inside here
-- ^^^^^^
-- back
-- to IO
Or we can fmap over another IO value, like replicateM (read query) readPair:
doTest :: IO Bool
doTest = do
query <- getLine
fmap validFunction (replicateM (read query) readPair)
The latter is harder to read, though. But you write your fooDo doTest as you want to do.
All About Monads explains sequence_:
The sequence_ function (notice the underscore) has the same behavior as sequence but does not return a list of results. It is useful when only the side-effects of the monadic computations are important.
Then, looking at TestSequence.hs:
import Control.Monad
f :: String -> IO ()
f x = print x
run :: [String] -> IO ()
run xs = sequence_ . map f $ xs
I can run it:
λ: run ["foo", "bar"]
"foo"
"bar"
Is sequence_ calling unsafePerformIO on each IO () to get the result, i.e. the ()?
And, is sequence_ discouraged? Or is it, for the IO Monad, simply used "at the end of the world" to run a list of IO actions?
No, it is not calling unsafePerformIO on each IO () action. In fact, its type is not even specific to IO:
sequence_ :: (Monad m, Foldable t) => t (m a) -> m ()
In the old libraries, when it was specific to lists (rather than generic over all Foldables), it was implemented in the following perfectly readable way:
sequence_ [] = return ()
sequence_ (x:xs) = x >> sequence_ xs
It is absolutely not discouraged; sequence_ (and its big brother, mapM_) are extremely useful, to the point that it is even one of my motivating examples for why Monads as an abstraction are useful.
I'm trying to work out if it's possible to write an abstraction for the following situation. Suppose I have a type a with function a -> m Bool e.g. MVar Bool and readMVar. To abstract this concept out I create a newtype wrapper for the type and its function:
newtype MPredicate m a = MPredicate (a,a -> m Bool)
I can define a fairly simple operation like so:
doUnless :: (Monad m) => Predicate m a -> m () -> m ()
doUnless (MPredicate (a,mg)) g = mg a >>= \b -> unless b g
main = do
b <- newMVar False
let mpred = MPredicate (b,readMVar)
doUnless mpred (print "foo")
In this case doUnless would print "foo". Aside: I'm not sure whether a type class might be more appropriate to use instead of a newtype.
Now take the code below, which outputs an incrementing number then waits a second and repeats. It does this until it receives a "turn off" instruction via the MVar.
foobar :: MVar Bool -> IO ()
foobar mvb = foobar' 0
where
foobar' :: Int -> IO ()
foobar' x = readMVar mvb >>= \b -> unless b $ do
let x' = x + 1
print x'
threadDelay 1000000
foobar' x'
goTillEnter :: MVar Bool -> IO ()
goTillEnter mv = do
_ <- getLine
_ <- takeMVar mv
putMVar mv True
main = do
mvb <- newMVar False
forkIO $ foobar mvb
goTillEnter mvb
Is it possible to refactor foobar so that it uses MPredicate and doUnless?
Ignoring the actual implementation of foobar' I can think of a simplistic way of doing something similar:
cycleUnless :: x -> (x -> x) -> MPredicate m a -> m ()
cycleUnless x g mp = let g' x' = doUnless mp (g' $ g x')
in g' $ g x
Aside: I feel like fix could be used to make the above neater, though I still have trouble working out how to use it
But cycleUnless won't work on foobar because the type of foobar' is actually Int -> IO () (from the use of print x').
I'd also like to take this abstraction further, so that it can work threading around a Monad. With stateful Monads it becomes even harder. E.g.
-- EDIT: Updated the below to show an example of how the code is used
{- ^^ some parent function which has the MVar ^^ -}
cycleST :: (forall s. ST s (STArray s Int Int)) -> IO ()
cycleST sta = readMVar mvb >>= \b -> unless b $ do
n <- readMVar someMVar
i <- readMVar someOtherMVar
let sta' = do
arr <- sta
x <- readArray arr n
writeArray arr n (x + i)
return arr
y = runSTArray sta'
print y
cycleST sta'
I have something similar to the above working with RankNTypes. Now there's the additional problem of trying to thread through the existential s, which is not likely to type check if threaded around through an abstraction the likes of cycleUnless.
Additionally, this is simplified to make the question easier to answer. I also use a set of semaphores built from MVar [MVar ()] similar to the skip channel example in the MVar module. If I can solve the above problem I plan to generalize the semaphores as well.
Ultimately this isn't some blocking problem. I have 3 components of the application operating in a cycle off the same MVar Bool but doing fairly different asynchronous tasks. In each one I have written a custom function that performs the appropriate cycle.
I'm trying to learn the "don't write large programs" approach. What I'd like to do is refactor chunks of code into their own mini libraries so that I'm not building a large program but assembling lots of small ones. But so far this particular abstraction is escaping me.
Any thoughts on how I might go about this are very much appreciated!
You want to cleanly combine a stateful action having side effects, a delay, and an independent stopping condition.
The iterative monad transformer from the free package can be useful in these cases.
This monad transformer lets you describe a (possibly nonending) computation as a series of discrete steps. And what's better, it let's you interleave "stepped" computations using mplus. The combined computation stops when any of the individual computations stops.
Some preliminary imports:
import Data.Bool
import Control.Monad
import Control.Monad.Trans
import Control.Monad.Trans.Iter (delay,untilJust,IterT,retract,cutoff)
import Control.Concurrent
Your foobar function could be understood as a "sum" of three things:
A computation that does nothing but reading from the MVar at each step, and finishes when the Mvar is True.
untilTrue :: (MonadIO m) => MVar Bool -> IterT m ()
untilTrue = untilJust . liftM guard . liftIO . readMVar
An infinite computation that takes a delay at each step.
delays :: (MonadIO m) => Int -> IterT m a
delays = forever . delay . liftIO . threadDelay
An infinite computation that prints an increasing series of numbers.
foobar' :: (MonadIO m) => Int -> IterT m a
foobar' x = do
let x' = x + 1
liftIO (print x')
delay (foobar' x')
With this in place, we can write foobar as:
foobar :: (MonadIO m) => MVar Bool -> m ()
foobar v = retract (delays 1000000 `mplus` untilTrue v `mplus` foobar' 0)
The neat thing about this is that you can change or remove the "stopping condition" and the delay very easily.
Some clarifications:
The delay function is not a delay in IO, it just tells the iterative monad transformer to "put the argument in a separate step".
retract brings you back from the iterative monad transformer to the base monad. It's like saying "I don't care about the steps, just run the computation". You can combine retract with cutoff if you want to limit the maximum number of iterations.
untilJustconverts a value m (Maybe a) of the base monad into a IterT m a by retrying in each step until a Just is returned. Of course, this risks non-termination!
MPredicate is rather superfluous here; m Bool can be used instead. The monad-loops package contains plenty of control structures with m Bool conditions. whileM_ in particular is applicable here, although we need to include a State monad for the Int that we're threading around:
import Control.Monad.State
import Control.Monad.Loops
import Control.Applicative
foobar :: MVar Bool -> IO ()
foobar mvb = (`evalStateT` (0 :: Int)) $
whileM_ (not <$> lift (readMVar mvb)) $ do
modify (+1)
lift . print =<< get
lift $ threadDelay 1000000
Alternatively, we can use a monadic version of unless. For some reason monad-loops doesn't export such a function, so let's write it:
unlessM :: Monad m => m Bool -> m () -> m ()
unlessM mb action = do
b <- mb
unless b action
It's somewhat more convenient and more modular in a monadic setting, since we can always go from a pure Bool to m Bool, but not vice versa.
foobar :: MVar Bool -> IO ()
foobar mvb = go 0
where
go :: Int -> IO ()
go x = unlessM (readMVar mvb) $ do
let x' = x + 1
print x'
threadDelay 1000000
go x'
You mentioned fix; sometimes people indeed use it for ad-hoc monadic loops, for example:
printUntil0 :: IO ()
printUntil0 =
putStrLn "hello"
fix $ \loop -> do
n <- fmap read getLine :: IO Int
print n
when (n /= 0) loop
putStrLn "bye"
With some juggling it's possible to use fix with multi-argument functions. In the case of foobar:
foobar :: MVar Bool -> IO ()
foobar mvb = ($(0 :: Int)) $ fix $ \loop x -> do
unlessM (readMVar mvb) $ do
let x' = x + 1
print x'
threadDelay 1000000
loop x'
I'm not sure what's your MPredicate is doing.
First, instead of newtyping a tuple, it's probably better to use a normal algebric data type
data MPredicate a m = MPredicate a (a -> m Bool)
Second, the way you use it, MPredicate is equivalent to m Bool.
Haskell is lazzy, therefore there is no need to pass, a function and it's argument (even though
it's usefull with strict languages). Just pass the result, and the function will be called when needed.
I mean, instead of passing (x, f) around, just pass f x
Of course, if you are not trying to delay the evaluation and really need at some point, the argument or the function as well as the result, a tuple is fine.
Anyway, in the case your MPredicate is only there to delay the function evaluation, MPredicat reduces to m Bool and doUnless to unless.
Your first example is strictly equivalent :
main = do
b <- newMVar False
unless (readMVar b) (print "foo")
Now, if you want to loop a monad until a condition is reach (or equivalent) you should have a look at the monad-loop package. What you are looking it at is probably untilM_ or equivalent.
I have a file number.txt which contains a large number and I read it into an IO String like this:
readNumber = readFile "number.txt" >>= return
In another function I want to create a list of Ints, one Int for each digit…
Lets assume the content of number.txt is:
1234567890
Then I want my function to return [1,2,3,4,5,6,7,8,9,0].
I tried severall versions with map, mapM(_), liftM, and, and, and, but I got several error messages everytime, which I was able to reduce to
Couldn't match expected type `[m0 Char]'
with actual type `IO String'
The last version I have on disk is the following:
module Main where
import Control.Monad
import Data.Char (digitToInt)
main = intify >>= putStrLn . show
readNumber = readFile "number.txt" >>= return
intify = mapM (liftM digitToInt) readNumber
So, as far as I understand the error, I need some function that takes IO [a] and returns [IO a], but I was not able to find such thing with hoogle… Only the other way round seemes to exist
In addition to the other great answers here, it's nice to talk about how to read [IO Char] versus IO [Char]. In particular, you'd call [IO Char] "an (immediate) list of (deferred) IO actions which produce Chars" and IO [Char] "a (deferred) IO action producing a list of Chars".
The important part is the location of "deferred" above---the major difference between a type IO a and a type a is that the former is best thought of as a set of instructions to be executed at runtime which eventually produce an a... while the latter is just that very a.
This phase distinction is key to understanding how IO values work. It's also worth noting that it can be very fluid within a program---functions like fmap or (>>=) allow us to peek behind the phase distinction. As an example, consider the following function
foo :: IO Int -- <-- our final result is an `IO` action
foo = fmap f getChar where -- <-- up here getChar is an `IO Char`, not a real one
f :: Char -> Int
f = Data.Char.ord -- <-- inside here we have a "real" `Char`
Here we build a deferred action (foo) by modifying a deferred action (getChar) by using a function which views a world that only comes into existence after our deferred IO action has run.
So let's tie this knot and get back to the question at hand. Why can't you turn an IO [Char] into an [IO Char] (in any meaningful way)? Well, if you're looking at a piece of code which has access to IO [Char] then the first thing you're going to want to do is sneak inside of that IO action
floob = do chars <- (getChars :: IO [Char])
...
where in the part left as ... we have access to chars :: [Char] because we've "stepped into" the IO action getChars. This means that by this point we've must have already run whatever runtime actions are required to generate that list of characters. We've let the cat out of the monad and we can't get it back in (in any meaningful way) since we can't go back and "unread" each individual character.
(Note: I keep saying "in any meaningful way" because we absolutely can put cats back into monads using return, but this won't let us go back in time and have never let them out in the first place. That ship has sailed.)
So how do we get a type [IO Char]? Well, we have to know (without running any IO) what kind of IO operations we'd like to do. For instance, we could write the following
replicate 10 getChar :: [IO Char]
and immediately do something like
take 5 (replicate 10 getChar)
without ever running an IO action---our list structure is immediately available and not deferred until the runtime has a chance to get to it. But note that we must know exactly the structure of the IO actions we'd like to perform in order to create a type [IO Char]. That said, we could use yet another level of IO to peek at the real world in order to determine the parameters of our action
do len <- (figureOutLengthOfReadWithoutActuallyReading :: IO Int)
return $ replicate len getChar
and this fragment has type IO [IO Char]. To run it we have to step through IO twice, we have to let the runtime perform two IO actions, first to determine the length and then second to actually act on our list of IO Char actions.
sequence :: [IO a] -> IO [a]
The above function, sequence, is a common way to execute some structure containing a, well, sequence of IO actions. We can use that to do our two-phase read
twoPhase :: IO [Char]
twoPhase = do len <- (figureOutLengthOfReadWithoutActuallyReading :: IO Int)
putStrLn ("About to read " ++ show len ++ " characters")
sequence (replicate len getChar)
>>> twoPhase
Determining length of read
About to read 22 characters
let me write 22 charac"let me write 22 charac"
You got some things mixed up:
readNumber = readFile "number.txt" >>= return
the return is unecessary, just leave it out.
Here is a working version:
module Main where
import Data.Char (digitToInt)
main :: IO ()
main = intify >>= print
readNumber :: IO String
readNumber = readFile "number.txt"
intify :: IO [Int]
intify = fmap (map digitToInt) readNumber
Such a function can't exists, because you would be able to evaluate the length of the list without ever invoking any IO.
What is possible is this:
imbue' :: IO [a] -> IO [IO a]
imbue' = fmap $ map return
Which of course generalises to
imbue :: (Functor f, Monad m) => m (f a) -> m (f (m a))
imbue = liftM $ fmap return
You can then do, say,
quun :: IO [Char]
bar :: [IO Char] -> IO Y
main = do
actsList <- imbue quun
y <- bar actsLists
...
Only, the whole thing about using [IO Char] is pointless: it's completely equivalent to the much more straightforward way of working only with lists of "pure values", only using the IO monad "outside"; how to do that is shown in Markus's answer.
Do you really need many different helper functions? Because you may write just
main = do
file <- readFile "number.txt"
let digits = map digitToInt file
print digits
or, if you really need to separate them, try to minimize the amount of IO signatures:
readNumber = readFile "number.txt" --Will be IO String
intify = map digitToInt --Will be String -> [Int], not IO
main = readNumber >>= print . intify
Assume we have an IO action such as
lookupStuff :: InputType -> IO OutputType
which could be something simple such as DNS lookup, or some web-service call against a time-invariant data.
Let's assume that:
The operation never throws any exception and/or never diverges
If it wasn't for the IO monad, the function would be pure, i.e. the result is always the same for equal input parameters
The action is reentrant, i.e. it can be called from multiple threads at the same time safely.
The lookupStuff operation is quite (time-)expensive.
The problem I'm facing is how to properly (and w/o using any unsafe*IO* cheat) implement a reentrant cache, that can be called from multiple threads, and coalesces multiple queries for the same input-parameters into a single request.
I guess I'm after something similiar as GHC's blackhole-concept for pure computations but in the IO "calculation" context.
What is the idiomatic Haskell/GHC solution for the stated problem?
Yeah, basically reimplement the logic. Although it seems similar to what GHC is already doing, that's GHC's choice. Haskell can be implemented on VMs that work very differently, so in that sense it isn't already done for you.
But yeah, just use an MVar (Map InputType OutputType) or even an IORef (Map InputType OutputType) (make sure to modify with atomicModifyIORef), and just store the cache in there. If this imperative solution seems wrong, it's the "if not for the IO, this function would be pure" constraint. If it were just an arbitrary IO action, then the idea that you would have to keep state in order to know what to execute or not seems perfectly natural. The problem is that Haskell does not have a type for "pure IO" (which, if it depends on a database, it is just behaving pure under certain conditions, which is not the same as being a hereditarily pure).
import qualified Data.Map as Map
import Control.Concurrent.MVar
-- takes an IO function and returns a cached version
cache :: (Ord a) => (a -> IO b) -> IO (a -> IO b)
cache f = do
r <- newMVar Map.empty
return $ \x -> do
cacheMap <- takeMVar r
case Map.lookup x cacheMap of
Just y -> do
putMVar r cacheMap
return y
Nothing -> do
y <- f x
putMVar (Map.insert x y cacheMap)
return y
Yeah it's ugly on the inside. But on the outside, look at that! It's just like the type of a pure memoization function, except for it has IO stained all over it.
Here's some code implementing more or less what I was after in my original question:
import Control.Concurrent
import Control.Exception
import Data.Either
import Data.Map (Map)
import qualified Data.Map as Map
import Prelude hiding (catch)
-- |Memoizing wrapper for 'IO' actions
memoizeIO :: Ord a => (a -> IO b) -> IO (a -> IO b)
memoizeIO action = do
cache <- newMVar Map.empty
return $ memolup cache action
where
-- Lookup helper
memolup :: Ord a => MVar (Map a (Async b)) -> (a -> IO b) -> a -> IO b
memolup cache action' args = wait' =<< modifyMVar cache lup
where
lup tab = case Map.lookup args tab of
Just ares' ->
return (tab, ares')
Nothing -> do
ares' <- async $ action' args
return (Map.insert args ares' tab, ares')
The code above builds upon Simon Marlow's Async abstraction as described in Tutorial: Parallel and Concurrent Programming in Haskell:
-- |Opaque type representing asynchronous results.
data Async a = Async ThreadId (MVar (Either SomeException a))
-- |Construct 'Async' result. Can be waited on with 'wait'.
async :: IO a -> IO (Async a)
async io = do
var <- newEmptyMVar
tid <- forkIO ((do r <- io; putMVar var (Right r))
`catch` \e -> putMVar var (Left e))
return $ Async tid var
-- |Extract value from asynchronous result. May block if result is not
-- available yet. Exceptions are returned as 'Left' values.
wait :: Async a -> IO (Either SomeException a)
wait (Async _ m) = readMVar m
-- |Version of 'wait' that raises exception.
wait' :: Async a -> IO a
wait' a = either throw return =<< wait a
-- |Cancels asynchronous computation if not yet completed (non-blocking).
cancel :: Async a -> IO ()
cancel (Async t _) = throwTo t ThreadKilled