I have the following code snippet from internet:
calculateLength :: LengthMonad Int
calculateLength = do
-- all the IO operations have to be lifted to the IO monad in the monad stack
liftIO $ putStrLn "Please enter a non-empty string: "
s <- liftIO getLine
if null s
then throwError "The string was empty!"
else return $ length s
and could not understand, why the author use liftIO?
What is the purpose of liftIO?
It is defined as follows:
class (Monad m) => MonadIO m where
-- | Lift a computation from the 'IO' monad.
liftIO :: IO a -> m a
Is it possible to lift IO a -> [a]? It looks like natural transformation.
IO operations like getLine, putStrLn "..." only work inside the IO monad. Using them inside any other monad will trigger a type error.
Still, there are many monads M which are defined in terms of IO (e.g. StateT Int IO, and apparently your LengthMonad as well) and because of that they allow IO actions to be converted into M-actions, and executed as such.
However, we need a conversion for each M:
convertIOintoM1 :: IO a -> M1 a
convertIOintoM2 :: IO a -> M2 a
convertIOintoM3 :: IO a -> M3 a
...
Since this is cumbersome, the library defines a typeclass MonadIO having such conversion function, so that all the functions above can be named liftIO instead.
In practice, liftIO is used each time one wants to run IO actions in another monad, provided such monad allows for it.
Related
I have a Logger type of kind * -> * which can take any type and log the value in a file. I am trying to implement this in a monadic way so that I log and keep working the same. My code looks like
import Control.Applicative
import Control.Monad
import System.IO
import Control.Monad.IO.Class
instance Functor Logger where
fmap = liftM
instance Applicative Logger where
pure = return
(<*>) = ap
newtype Logger a = Logger a deriving (Show)
instance Monad (Logger) where
return = Logger
Logger logStr >>= f = f logStr
instance MonadIO (Logger) where
liftIO a = do
b <- liftIO a
return b
logContent :: (Show a) => a -> Logger a
logContent a = do
b <- liftIO $ logContent2 a
return b
logContent2 :: (Show a) => a -> IO a
logContent2 a = do
fHandle <- openFile "test.log" AppendMode
hPrint fHandle a
hClose fHandle
return (a)
The liftIO function goes on endless loop as it calls itself. I am not able to do b <- a either. Can someone help on getting MonadIO implementation right ?
As noted in the comments, I think you've misunderstood what MonadIO and liftIO do.
These typeclasses and functions come from mtl library. Rather unfortunately, mtl stands for "monad transformer library", but mtl is not a monad transformer library. Rather, mtl is a set of typeclasses that allow you to take a monad that --- and this is important --- already has a particular type of functionality and provide that monad with a consistent interface around that functionality. This ends up being really useful for working with actual monad transformers. That's because mtl allows you to use tell and ask and put to access the Writer, Reader, and State functionality of your monad transformer stack in a consistent way.
Separately from this transformer business, if you already have a custom monad, say that supports arbitrary IO and has State functionality, then you can define a MonadState instance to make the standard state operations (state, get, gets, put, modify) available for your custom monad, and you can define a MonadIO instance to allow an arbitrary IO action to be executed in your custom monad using liftIO. However, none of these typeclasses are capable of adding functionality to a monad that it doesn't already have. In particular, you can't transform an arbitrary monadic action m a into an IO a using a MonadIO instance.
Note that the transformers package contains types that are capable of adding functionality to a monad that it doesn't already have (e.g., adding reader or writer functionality), but there is no transformer to add IO to an arbitrary monad. Such a transformer would be impossible (without unsafe or nonterminating operations).
Also note that the signature for liftIO :: MonadIO m => IO a -> m a puts a MonadIO constraint on m, and this isn't just a trivial constraint. It actually indicates that liftIO only works for monads m that already have IO functionality, so either m is the IO monad, or it's a monad stack with IO at its base. Your Logger example doesn't have IO functionality and so can't have a (sensible) MonadIO instance.
Getting back to your specific problem, it's actually a little bit hard to steer you right here without knowing exactly what you're trying to do. If you just want to add file-based logging to an existing IO computation, then defining a new transformer stack will probably do the trick:
type LogIO = ReaderT Handle IO
logger :: (Show a) => a -> LogIO ()
logger a = do
h <- ask
liftIO $ hPrint h a
runLogIO :: LogIO a -> FilePath -> IO a
runLogIO act fp = withFile fp AppendMode $ \h -> runReaderT act h
and you can write things like:
main :: IO ()
main = runLogIO start "test.log"
start :: LogIO ()
start = do
logger "Starting program"
liftIO . putStrLn $ "Please enter your name:"
n <- liftIO $ getLine
logger n
liftIO . putStrLn $ "Hello, " ++ n
logger "Ending program"
The need to add liftIO calls when using IO actions within the LogIO monad is ugly but largely unavoidable.
This solution would also work for adding file-based logging to pure computations, with the understanding that you have to convert them to IO computations anyway if you want to safely log to a file.
The more general solution is to define your own monad transformer (not merely your own monad), like LoggerT m, together with an associated MonadLogger type class that will add file-based logging to to any IO-capable monad stack. The idea would be that you could then create arbitrary custom monad stacks:
type MyMonad = StateT Int (LoggerT IO)
and then write code that mixes monadic computations from different layers (like mixing state computations and file-based logging):
newSym :: String -> MyMonad String
newSym pfx = do
n <- get
logger (pfx, n)
put (n+1)
return $ pfx ++ show n
Is this what you what you're trying to do? If not, maybe you could describe, either here or in a new question, how you're trying to add logging to some example code.
Sometimes I need to use several nested MonadTrans. For example, I would put a MaybeT inside of a ExceptT, to mimick continue and break in imperative programming:
runExceptT . forM [1..] $ \ _ -> runMaybeT $ do
...
mzero -- this mimics continue
lift $ throwE "..." -- this mimics break
lift . lift $ putStrLn "Hello!"
...
However, as the above code shows, every time I need to do any IO inside of this "artificial loop", I need to put an ugly lift . lift before it. Imagine if I have even more complicated nestings, and a lot of IO operations, this quickly becomes an anonyance. How I make the code cleaner and more concise?
For the particular case of IO, you can use liftIO :: MonadIO m => IO a -> m a to lift an IO action through an arbitrarily deep monad transformer stack:
liftIO $ putStrLn "Hello"
This expression has type MonadIO m => m (). Any transformer stack that contains an instance of MonadIO somewhere inside of it should have a MonadIO instance itself, which is why this works for a stack of any depth.
Could someone give a super simple (few lines) monad transformer example, which is non-trivial (i.e. not using the Identity monad - that I understand).
For example, how would someone create a monad that does IO and can handle failure (Maybe)?
What would be the simplest example that would demonstrate this?
I have skimmed through a few monad transformer tutorials and they all seem to use State Monad or Parsers or something complicated (for a newbee). I would like to see something simpler than that. I think IO+Maybe would be simple, but I don't really know how to do that myself.
How could I use an IO+Maybe monad stack?
What would be on top? What would be on bottom? Why?
In what kind of use case would one want to use the IO+Maybe monad or the Maybe+IO monad? Would that make sense to create such a composite monad at all? If yes, when, and why?
This is available here as a .lhs file.
The MaybeT transformer will allow us to break out of a monad computation much like throwing an exception.
I'll first quickly go over some preliminaries. Skip down to Adding Maybe powers to IO for a worked example.
First some imports:
import Control.Monad
import Control.Monad.Trans
import Control.Monad.Trans.Maybe
Rules of thumb:
In a monad stack IO is always on the bottom.
Other IO-like monads will also, as a rule, always appear on the bottom, e.g. the state transformer monad ST.
MaybeT m is a new monad type which adds the power of the Maybe monad to the monad m - e.g. MaybeT IO.
We'll get into what that power is later. For now, get used to thinking of MaybeT IO as the maybe+IO monad stack.
Just like IO Int is a monad expression returning an Int, MaybeT IO Int is a MaybeT IO expression returning an Int.
Getting used to reading compound type signatures is half the battle to understanding monad transformers.
Every expression in a do block must be from the same monad.
I.e. this works because each statement is in the IO-monad:
greet :: IO () -- type:
greet = do putStr "What is your name? " -- IO ()
n <- getLine -- IO String
putStrLn $ "Hello, " ++ n -- IO ()
This will not work because putStr is not in the MaybeT IO monad:
mgreet :: MaybeT IO ()
mgreet = do putStr "What is your name? " -- IO monad - need MaybeT IO here
...
Fortunately there is a way to fix this.
To transform an IO expression into a MaybeT IO expression use liftIO.
liftIO is polymorphic, but in our case it has the type:
liftIO :: IO a -> MaybeT IO a
mgreet :: MaybeT IO () -- types:
mgreet = do liftIO $ putStr "What is your name? " -- MaybeT IO ()
n <- liftIO getLine -- MaybeT IO String
liftIO $ putStrLn $ "Hello, " ++ n -- MaybeT IO ()
Now all of the statement in mgreet are from the MaybeT IO monad.
Every monad transformer has a "run" function.
The run function "runs" the top-most layer of a monad stack returning
a value from the inside layer.
For MaybeT IO, the run function is:
runMaybeT :: MaybeT IO a -> IO (Maybe a)
Example:
ghci> :t runMaybeT mgreet
mgreet :: IO (Maybe ())
ghci> runMaybeT mgreet
What is your name? user5402
Hello, user5402
Just ()
Also try running:
runMaybeT (forever mgreet)
You'll need to use Ctrl-C to break out of the loop.
So far mgreet doesn't do anything more than what we could do in IO.
Now we'll work on an example which demonstrates the power of mixing
the Maybe monad with IO.
Adding Maybe powers to IO
We'll start with a program which asks some questions:
askfor :: String -> IO String
askfor prompt = do
putStr $ "What is your " ++ prompt ++ "? "
getLine
survey :: IO (String,String)
survey = do n <- askfor "name"
c <- askfor "favorite color"
return (n,c)
Now suppose we want to give the user the ability to end the survey
early by typing END in response to a question. We might handle it
this way:
askfor1 :: String -> IO (Maybe String)
askfor1 prompt = do
putStr $ "What is your " ++ prompt ++ " (type END to quit)? "
r <- getLine
if r == "END"
then return Nothing
else return (Just r)
survey1 :: IO (Maybe (String, String))
survey1 = do
ma <- askfor1 "name"
case ma of
Nothing -> return Nothing
Just n -> do mc <- askfor1 "favorite color"
case mc of
Nothing -> return Nothing
Just c -> return (Just (n,c))
The problem is that survey1 has the familiar staircasing issue which
doesn't scale if we add more questions.
We can use the MaybeT monad transformer to help us here.
askfor2 :: String -> MaybeT IO String
askfor2 prompt = do
liftIO $ putStr $ "What is your " ++ prompt ++ " (type END to quit)? "
r <- liftIO getLine
if r == "END"
then MaybeT (return Nothing) -- has type: MaybeT IO String
else MaybeT (return (Just r)) -- has type: MaybeT IO String
Note how all of the statemens in askfor2 have the same monad type.
We've used a new function:
MaybeT :: IO (Maybe a) -> MaybeT IO a
Here is how the types work out:
Nothing :: Maybe String
return Nothing :: IO (Maybe String)
MaybeT (return Nothing) :: MaybeT IO String
Just "foo" :: Maybe String
return (Just "foo") :: IO (Maybe String)
MaybeT (return (Just "foo")) :: MaybeT IO String
Here return is from the IO-monad.
Now we can write our survey function like this:
survey2 :: IO (Maybe (String,String))
survey2 =
runMaybeT $ do a <- askfor2 "name"
b <- askfor2 "favorite color"
return (a,b)
Try running survey2 and ending the questions early by typing END as a response to either question.
Short-cuts
I know I'll get comments from people if I don't mention the following short-cuts.
The expression:
MaybeT (return (Just r)) -- return is from the IO monad
may also be written simply as:
return r -- return is from the MaybeT IO monad
Also, another way of writing MaybeT (return Nothing) is:
mzero
Furthermore, two consecutive liftIO statements may always combined into a single liftIO, e.g.:
do liftIO $ statement1
liftIO $ statement2
is the same as:
liftIO $ do statement1
statement2
With these changes our askfor2 function may be written:
askfor2 prompt = do
r <- liftIO $ do
putStr $ "What is your " ++ prompt ++ " (type END to quit)?"
getLine
if r == "END"
then mzero -- break out of the monad
else return r -- continue, returning r
In a sense, mzero becomes a way of breaking out of the monad - like throwing an exception.
Another example
Consider this simple password asking loop:
loop1 = do putStr "Password:"
p <- getLine
if p == "SECRET"
then return ()
else loop1
This is a (tail) recursive function and works just fine.
In a conventional language we might write this as a infinite while loop with a break statement:
def loop():
while True:
p = raw_prompt("Password: ")
if p == "SECRET":
break
With MaybeT we can write the loop in the same manner as the Python code:
loop2 :: IO (Maybe ())
loop2 = runMaybeT $
forever $
do liftIO $ putStr "Password: "
p <- liftIO $ getLine
if p == "SECRET"
then mzero -- break out of the loop
else return ()
The last return () continues execution, and since we are in a forever loop, control passes back to the top of the do block. Note that the only value that loop2 can return is Nothing which corresponds to breaking out of the loop.
Depending on the situation you might find it easier to write loop2 rather than the recursive loop1.
Suppose you have to work with IO values that "may fail" in some sense, like foo :: IO (Maybe a), func1 :: a -> IO (Maybe b) and func2 :: b -> IO (Maybe c).
Manually checking for the presence of errors in a chain of binds quickly produces the dreaded "staircase of doom":
do
ma <- foo
case ma of
Nothing -> return Nothing
Just a -> do
mb <- func1 a
case mb of
Nothing -> return Nothing
Just b -> func2 b
How to "automate" this in some way? Perhaps we could devise a newtype around IO (Maybe a) with a bind function that automatically checks if the first argument is a Nothing inside IO, saving us the trouble of checking it ourselves. Something like
newtype MaybeOverIO a = MaybeOverIO { runMaybeOverIO :: IO (Maybe a) }
With the bind function:
betterBind :: MaybeOverIO a -> (a -> MaybeOverIO b) -> MaybeOverIO b
betterBind mia mf = MaybeOverIO $ do
ma <- runMaybeOverIO mia
case ma of
Nothing -> return Nothing
Just a -> runMaybeOverIO (mf a)
This works! And, looking at it more closely, we realize that we aren't using any particular functions exclusive to the IO monad. Generalizing the newtype a little, we could make this work for any underlying monad!
newtype MaybeOverM m a = MaybeOverM { runMaybeOverM :: m (Maybe a) }
And this is, in essence, how the MaybeT transformer works. I have left out a few details, like how to implement return for the transformer, and how to "lift" IO values into MaybeOverM IO values.
Notice that MaybeOverIO has kind * -> * while MaybeOverM has kind (* -> *) -> * -> * (because its first "type argument" is a monad type constructor, that itself requires a "type argument").
Sure, the MaybeT monad transformer is:
newtype MaybeT m a = MaybeT {unMaybeT :: m (Maybe a)}
We can implement its monad instance as so:
instance (Monad m) => Monad (MaybeT m) where
return a = MaybeT (return (Just a))
(MaybeT mmv) >>= f = MaybeT $ do
mv <- mmv
case mv of
Nothing -> return Nothing
Just a -> unMaybeT (f a)
This will allow us to perform IO with the option of failing gracefully in certain circumstances.
For instance, imagine we had a function like this:
getDatabaseResult :: String -> IO (Maybe String)
We can manipulate the monads independently with the result of that function, but if we compose it as so:
MaybeT . getDatabaseResult :: String -> MaybeT IO String
We can forget about that extra monadic layer, and just treat it as a normal monad.
I am attempting to use the IO monad in a Spock application. The following code does not compile:
get "api/entities" $ do
entities <- loadEntities
let e1 : xs = entities
text $ note e1
loadEntities has the type IO [Entity]
The error is Couldn't match type ‘ActionT IO ()’ with ‘t0 -> IO b0’
Is Spock using a monad other than IO? If so, how do I get the result of loadEntities?
Writing answer so people can see a better explanation.
If you look at get it has the type.
get :: MonadIO m => SpockRoute -> ActionT m () -> SpockT m ()
ActionT is a monad transformer and it requires in inner monad to have an instance of MonadIO. If we look at the typeclass MonadIO it has a function
liftIO :: IO a -> m a
This means every that you call execute functions of the type IO a in that stack by using liftIO. ActionT has an instance of MonadIO also and which is the one you are using here to call your IO function. So to call loadEntities you had to
entities <- liftIO loadEntities
If you end of calling a certain function like that a lot you can create a separate module import it qualified and export a more friendly one.
module Lifted (loadEntities) where
import qualified SomeModule as IO
loadEntities :: MonadIO m => m Entities
loadEntities = liftIO IO.loadEntities
This will make it so you don't always have you use liftIO
I've got a bunch of stateful functions inside a State monad. At one point in the program there needs to be some IO actions so I've wrapped IO inside a StateT getting a pair of types like this:
mostfunctions :: State Sometype a
toplevel :: StateT Sometype IO a
To keep things simple I don't want pass the IO context into the main set of functions and I would like to avoid wrapping them in the monad stack type. But in order to call them from the toplevel function I need something akin to a lift, but I'm not trying to lift a value from the inner monad. Rather I want to convert the state in the StateT monad into something equivalent in the State monad. To do this I've got the following:
wrapST :: (State Sometype a) -> StateT Sometype IO a
wrapST f = do s <- get
let (r,s2) = runState f s
put s2
return r
This then get used to interleave things like the following:
toplevel = do liftIO $ Some IO functions
wrapST $ Some state mutations
liftIO $ More IO functions
....
It seems like a fairly obvious block of code so I'm wondering does this function have a standard name, and it is already implemented somewhere in the standard libraries? I've tried to keep the description simple but obviously this extends to pulling one transformer out of a stack, converting the wrapped value to the cousin of the transformer type, skipping the monads below in the stack, and then pushing the results back in at the end.
It may be a good idea to refactor your code to use the type StateT SomeType m a instead of State SomeType a, because the first one is compatible to an arbitrary monad stack. If you'd change it like this, you don't need a function wrapST anymore, since you can call the stateful functions directly.
Okay. Suppose you have a function subOne :: Monad m => State Int Int:
subOne = do a <- get
put $ a - 1
return a
Now, change the types of all functions like this one from State SomeType a to StateT SomeType m a, leaving m as is. This way, your functions can work on any monadic stack. For those functions, that require IO, you can specify, that the monad at the bottom must be IO:
printState :: MonadIO m => StateT Int m ()
printState = do a <- get
liftIO $ print a
Now, it should be possible to use both functions together:
-- You could use me without IO as well!
subOne :: Monad m => StateT Int m ()
subOne = do a <- get
put $ a - 1
printState :: MonadIO m => StateT Int m ()
printState = do a <- get
liftIO $ print a
toZero :: StateT Int IO ()
toZero = do subOne -- A really pure function
printState -- function may perform IO
a <- get
when (a > 0) toZero
PS: I use GHC 7, some of the libs changed midway, so it might be a bit different on GHC 6.
A more direct answer to your question: the function hoist does exactly what you're describing in a slightly more generic way. Example usage:
import Control.Monad.State
import Data.Functor.Identity
import Control.Monad.Morph
foo :: State Int Integer
foo = put 1 >> return 1
bar :: StateT Int IO Integer
bar = hoist (return . runIdentity) foo
hoist is part of the MFunctor class, which is defined like this:
class MFunctor t where
hoist :: Monad m => (forall a. m a -> n a) -> t m b -> t n b
There are instances for most monad tranformers, but not ContT.