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
Related
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.
I want to define a function that expects an Int, prints an error in the console depending on the number (x) and then updates the State with Nothing.
How can I join those commands in one function?
Here's what I got:
type Env = [(Variable,Int)]
newtype StateError a = StateError { runStateError :: Env -> Maybe (a, Env) }
class Monad m => MonadError m where
throw :: Monad m => a -> m a
instance MonadError StateError where
throw x = StateError (\s -> Nothing)
But I can't figure out how to perform the IO side-effect and then the State update in the same function definition
No
A function in the state monad, such as a -> State s b, is a pure function (no IO) that happens to have an extra function argument s hidden though some handy plumbing.
You can't print to the console from the state monad.
However, Yes!
However! You can use a monad transformer to get both State and some underlying monad such as IO.
I'll provide an example using transformers instead of a custom monad and mtl as it appears you were using. With mtl you can use classes like MonadError to leverage a throw that works well with other libraries that use the mtl classes. On the other hand, if you are the end consumer of this transformer it is less important.
First we'll import the modules that give us MonadIO, StateT, MaybeT, and use newtype deriving so we don't have to type out the monad instance boilerplate:
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
import qualified Control.Monad.Trans.State as S
import Control.Monad.IO.Class
import Control.Monad.Trans.Maybe
import Control.Monad.Trans
Just to be complete we'll spell out the types useful to your abstraction:
type Variable = String
type Env = [(Variable,Int)]
Now we can get to the interesting part - a Monad definition and functions for plumbing. The monad stack is StateT MaybeT IO:
newtype StateError a = StateError { unStateError :: S.StateT Env (MaybeT IO) a }
deriving (Monad, Applicative, Functor)
And we can run it by first unwrapping the newtype, then running the state, and finally the MaybeT:
run :: StateError a -> IO (Maybe (a, Env))
run = runMaybeT . flip S.runStateT [] . unStateError
Usually you'll write an army of functions that provide your monad abstraction. For this question it's just "update the state" and "print to stdout":
modify :: (Env -> Env) -> StateError ()
modify = StateError . S.modify
emit :: Show a => a -> StateError ()
emit = StateError . liftIO . print . show
Armed with our Monad of Power, we can do fancy things like update state and emit IO messages and track failure or success:
updateAndPrint :: Variable -> Int -> StateError ()
updateAndPrint v i =
do emit (v,i)
modify ((v,i):)
Oh, and failure is pretty simple - just fail in our MaybeT monad:
throw :: a -> StateError b
throw _ = fail "" -- same as 'MaybeT (pure Nothing)'
We can use this monad as expected:
> run $ updateAndPrint "var" 1
"(\"var\",1)"
Just (() -- ^ return value of `updateAndPrint`
,[("var",1)]) -- ^ resulting state
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.
First up, a simplified version of the task I want to accomplish: I have several large files (amounting to 30GB) that I want to prune for duplicate entries. To this end, I establish a database of hashes of the data, and open the files one-by-one, hashing each item, and recording it in the database and the output file iff its hash wasn't already in the database.
I know how to do this with iteratees, enumerators, and I wanted to try conduits. I also know how to do it with conduits, but now I want to use conduits & persistent. I'm having problems with the types, and possibly with the entire concept of ResourceT.
Here's some pseudo code to illustrate the problem:
withSqlConn "foo.db" $ runSqlConn $ runResourceT $
sourceFile "in" $= parseBytes $= dbAction $= serialize $$ sinkFile "out"
The problem lies in the dbAction function. I would like to access the database here, naturally. Since the action it does is basically just a filter, I first thought to write it like that:
dbAction = CL.mapMaybeM p
where p :: (MonadIO m, MonadBaseControl IO (SqlPersist m)) => DataType -> m (Maybe DataType)
p = lift $ putStrLn "foo" -- fine
insert $ undefined -- type error!
return undefined
The specific error I get is:
Could not deduce (m ~ b0 m0)
from the context (MonadIO m, MonadBaseControl IO (SqlPersist m))
bound by the type signature for
p :: (MonadIO m, MonadBaseControl IO (SqlPersist m)) =>
DataType -> m (Maybe DataType)
at tools/clean-wac.hs:(33,1)-(34,34)
`m' is a rigid type variable bound by
the type signature for
p :: (MonadIO m, MonadBaseControl IO (SqlPersist m)) =>
DataType -> m (Maybe (DataType))
at tools/clean-wac.hs:33:1
Expected type: m (Key b0 val0)
Actual type: b0 m0 (Key b0 val0)
Note that this might be due to wrong assumptions I made in designing the type signature. If I comment out the type signature and also remove the lift statement, the error message turns into:
No instance for (PersistStore ResourceT (SqlPersist IO))
arising from a use of `p'
Possible fix:
add an instance declaration for
(PersistStore ResourceT (SqlPersist IO))
In the first argument of `CL.mapMaybeM', namely `p'
So this means that we can't access the PersistStore at all via ResourceT?
I cannot write my own Conduit either, without using CL.mapMaybeM:
dbAction = filterP
filterP :: (MonadIO m, MonadBaseControl IO (SqlPersist m)) => Conduit DataType m DataType
filterP = loop
where loop = awaitE >>= either return go
go s = do lift $ insert $ undefined -- again, type error
loop
This resulted in yet another type error I don't fully understand.
Could not deduce (m ~ b0 m0)
from the context (MonadIO m, MonadBaseControl IO (SqlPersist m))
bound by the type signature for
filterP :: (MonadIO m,
MonadBaseControl IO (SqlPersist m)) =>
Conduit DataType m DataType
`m' is a rigid type variable bound by
the type signature for
filterP :: (MonadIO m,
MonadBaseControl IO (SqlPersist m)) =>
Conduit DataType m DataType
Expected type: Conduit DataType m DataType
Actual type: Pipe
DataType DataType DataType () (b0 m0) ()
In the expression: loop
In an equation for `filterP'
So, my question is: is it possible to use persistent like I intended to inside a conduit at all? And if, how? I am aware that since I can use liftIO inside the conduit, I could just go and use, say HDBC, but I wanted to use persistent explicitly in order to understand how it works, and because I like its db-backend agnosticism.
The code below compiles fine for me. Is it possible that the frameworks have moved on inthe meantime and things now just work?
However note the following changes I had to make as the world has changed a bit or I didn't have all your code. I used conduit-1.0.9.3 and persistent-1.3.0 with GHC 7.6.3.
Omitted parseBytes and serialise as I don't have your definitions and defined DataType = ByteString instead.
Introduced a Proxy parameter and an explicit type signature for the undefined value to avoid problems with type family injectivity. These likely don't arise in your real code because it will have a concrete or externally determined type for val.
Used await rather than awaitE and just used () as the type to substitute for the Left case, as awaitE has been retired.
Passed a dummy Connection creation function to withSqlConn - perhaps I should have used some Sqlite specific function?
Here's the code:
{-# LANGUAGE FlexibleContexts, NoMonomorphismRestriction,
TypeFamilies, ScopedTypeVariables #-}
module So133331988 where
import Control.Monad.Trans
import Database.Persist.Sql
import Data.ByteString
import Data.Conduit
import Data.Conduit.Binary
import Data.Proxy
test proxy =
withSqlConn (return (undefined "foo.db")) $ runSqlConn $ runResourceT $
sourceFile "in" $= dbAction proxy $$ sinkFile "out"
dbAction = filterP
type DataType = ByteString
filterP
:: forall m val
. ( MonadIO m, MonadBaseControl IO (SqlPersist m)
, PersistStore m, PersistEntity val
, PersistEntityBackend val ~ PersistMonadBackend m)
=> Proxy val
-> Conduit DataType m DataType
filterP Proxy = loop
where loop = await >>= maybe (return ()) go
go s = do lift $ insert (undefined :: val)
loop
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.