| Safe Haskell | Safe-Inferred |
|---|---|
| Language | Haskell2010 |
List.Transformer
Description
The ListT type is like a list that lets you interleave effects between
each element of the list.
Synopsis
- newtype ListT m a = ListT {}
- runListT :: Monad m => ListT m a -> m ()
- fold :: Monad m => (x -> a -> x) -> x -> (x -> b) -> ListT m a -> m b
- foldM :: Monad m => (x -> a -> m x) -> m x -> (x -> m b) -> ListT m a -> m b
- select :: (Foldable f, Alternative m) => f a -> m a
- unfold :: Monad m => (b -> m (Maybe (a, b))) -> b -> ListT m a
- take :: Monad m => Int -> ListT m a -> ListT m a
- drop :: Monad m => Int -> ListT m a -> ListT m a
- dropWhile :: Monad m => (a -> Bool) -> ListT m a -> ListT m a
- takeWhile :: Monad m => (a -> Bool) -> ListT m a -> ListT m a
- zip :: Monad m => ListT m a -> ListT m b -> ListT m (a, b)
- data Step m a
- newtype ZipListT m a = ZipListT {
- getZipListT :: ListT m a
- class MonadTrans (t :: (Type -> Type) -> Type -> Type) where
- class Monad m => MonadIO (m :: Type -> Type) where
- class Applicative f => Alternative (f :: Type -> Type) where
- class MFunctor (t :: (Type -> Type) -> k -> Type) where
Introduction
The type's definition is very short:
newtypeListTm a = ListT { next :: m (Stepm a) }
Every ListT begins with an outermost effect (the 'm', commonly IO). The return value of that effect is either:
dataStepm a = Cons a (ListTm a) | Nil
- Cons: a new list element followed by the rest of the list
- Nil : an empty list
Example: stdin, stdout
You most commonly use the ListT when you wish to generate each element of
the list using IO. For example, you can read lines from standard input:
import List.Transformer
import qualified System.IO
stdin :: ListT IO String
stdin = ListT (do
eof <- System.IO.isEOF
if eof
then return Nil
else do
string <- getLine
return (Cons string stdin) )You can also loop over a ListT to consume elements one-at-a-time. You
"pay as you go" for effects, only running what you actually need:
stdout :: ListT IO String -> IO ()
stdout strings = do
s <- next strings
case s of
Nil -> return ()
Cons string strings' -> do
putStrLn string
stdout strings'Combining stdin and stdout forwards lines one-by-one from standard input
to standard output:
main :: IO () main = stdout stdin
These lines stream in constant space, never retaining more than one line in memory:
$ runghc aboveExample.hs Test<Enter> Test 123<Enter> 123 ABC<Enter> ABC <Ctrl-D> $
Core operations
The most important operations that you should familiarize yourself with are:
empty :: ListT IO a
pure, return :: a -> ListT IO a
liftIO :: IO a -> ListT IO a
(<|>) :: ListT IO a -> ListT IO a -> ListT IO a
- (
>>=), which powersdonotation andMonadComprehensions
(>>=) :: ListT IO a -> (a -> ListT IO b) -> ListT IO b
select :: [a] -> ListT IO a
Monadic combination
Sometimes we can simplify the code by taking advantage of the fact that the
Monad instance for ListT behaves like a list comprehension:
stdout :: ListT IO String -> IO ()
stdout strings = runListT (do
string <- strings
liftIO (putStrLn string) )You can read the above code as saying: "for each string in strings,
call putStrLn on string."
You can even use list comprehension syntax if you enable the
MonadComprehensions language extension:
stdout strings = runListT [ r | str <- strings, r <- liftIO (putStrLn str) ]
There are a few ways we could consider defining a ListT analogue to the mapM
function from Prelude, but none are given in this library because they need
require only (>>=) and some trivial lifting.
mapM :: (a -> IO b) -> [a] -> IO [b] ( \f xs -> xs >>= f ) :: (a -> ListT IO b) -> ListT IO a -> ListT IO b ( \f xs -> select xs >>= lift . f ) :: (a -> IO b) -> [a] -> ListT IO b ( \f xs -> xs >>= lift . f ) :: (a -> IO b) -> ListT IO a -> ListT IO b
A critical difference between mapM and ListT's monad is that ListT will
stream in constant space, whereas mapM buffers the entire output list before
returning a single element.
Exercise: Interaction
To test your understanding, guess what this code does and then test your guess by running the code:
import List.Transformer (ListT,runListT,liftIO, (<|>),select) import Data.Foldable (asum) import Data.List (repeat) strings ::ListTIO String strings = doselect(repeat())asum[ pure "" , pure "Say something:" , do x <-liftIOgetLine return ("You said: "<|>x) ] main :: IO () main =runListT(do string <- pure "Hello, there!"<|>stringsliftIO(putStrLn string) )
ListT
This is like a list except that you can interleave effects between each list element. For example:
stdin :: ListT IO String
stdin = ListT (do
eof <- System.IO.isEOF
if eof
then return Nil
else do
line <- getLine
return (Cons line stdin) )The mnemonic is "List Transformer" because this type takes a base Monad,
'm', and returns a new transformed Monad that adds support for
list comprehensions
Instances
Consuming
This library is designed to stream results in constant space and does not expose an obvious way to collect all the results into memory. As a rule of thumb if you think you need to collect all the results in memory try to instead see if you can consume the results as they are being generated (such as in all the above examples). If you can stream the data from start to finish then your code will use significantly less memory and your program will become more responsive.
fold :: Monad m => (x -> a -> x) -> x -> (x -> b) -> ListT m a -> m b Source #
Use this to fold a ListT into a single value. This is designed to be
used with the foldl library:
import Control.Foldl (purely) import List.Transformer (fold) purely fold :: Monad m => Fold a b -> ListT m a -> m b
... but you can also use the fold function directly:
fold (+) 0 id :: Num a => ListT m a -> m a
>>>fold (<>) "" id (select ["a", "b", "c", "d", "e"])"abcde"
Constructing
empty is the empty list with no effects.
Use pure/return to construct a singleton list with no effects. Use liftIO
to turn an effect into a singleton list whose sole element is the effect's result.
Suppose you want to build a ListT with three elements and no effects.
You could write:
pure 1 <|> pure 2 <|> pure 3 :: ListT IO Int
... although you would probably prefer to use select instead:
select [1, 2, 3] :: ListT IO Int
select :: (Foldable f, Alternative m) => f a -> m a Source #
Convert any collection that implements Foldable to another collection that
implements Alternative
For this library, the most common specialized type for select will be:
select :: [a] -> ListT IO a
unfold :: Monad m => (b -> m (Maybe (a, b))) -> b -> ListT m a Source #
unfold step seed generates a ListT from a step function and an
initial seed.
Removing elements
take :: Monad m => Int -> ListT m a -> ListT m a Source #
take n xs takes n elements from the head of xs.
>>>let list xs = do x <- select xs; liftIO (print (show x)); return x>>>let sum = fold (+) 0 id>>>sum (take 2 (list [5,4,3,2,1]))"5" "4" 9
drop :: Monad m => Int -> ListT m a -> ListT m a Source #
drop n xs drops n elements from the head of xs, but still runs their
effects.
>>>let list xs = do x <- select xs; liftIO (print (show x)); return x>>>let sum = fold (+) 0 id>>>sum (drop 2 (list [5,4,3,2,1]))"5" "4" "3" "2" "1" 6
dropWhile :: Monad m => (a -> Bool) -> ListT m a -> ListT m a Source #
dropWhile pred xs drops elements from the head of xs if they
satisfy the predicate, but still runs their effects.
>>>let list xs = do x <- select xs; liftIO (print (show x)); return x>>>let sum = fold (+) 0 id>>>sum (dropWhile even (list [2,4,5,7,8]))"2" "4" "5" "7" "8" 20
takeWhile :: Monad m => (a -> Bool) -> ListT m a -> ListT m a Source #
takeWhile pred xs takes elements from xs until the predicate pred fails
>>>let list xs = do x <- select xs; liftIO (print (show x)); return x>>>let sum = fold (+) 0 id>>>sum (takeWhile even (list [2,4,5,7,8]))"2" "4" "5" 6
To filter elements from a list based on a predicate, use guard.
For example, the following function is analogous to filter:
filter :: Monad m => (a -> m Bool) -> ListT m a -> ListT m a
filter pred as = do
a <- as
b <- lift (pred a)
guard b
return aConcatenation
Use (<|>) to concatenate two lists.
(<|>) :: ListT IO a -> ListT IO a -> ListT IO a
Use asum to flatten a list of lists.
asum :: [ListT IO a] -> ListT IO a
Use join to flatten a ListT of ListTs.
join :: ListT IO (ListT IO a) -> ListT IO a
Pairwise combination
The (<>) operation joins every combination of an element from one list with
an element from the other.
>>>runListT ( (select ["a", "b"] <> select ["1", "2", "3"]) >>= (liftIO . print) )"a1" "a2" "a3" "b1" "b2" "b3"
This is the same combinatorial effect that (>>=) produces.
>>>runListT (do x <- select ["a", "b"]; y <- select ["1", "2", "3"]; liftIO (print (x <> y)))"a1" "a2" "a3" "b1" "b2" "b3"
zip :: Monad m => ListT m a -> ListT m b -> ListT m (a, b) Source #
zip xs ys zips two ListT together, running the effects of each before
possibly recursing. Notice in the example below, 4 is output even though
it has no corresponding element in the second list.
>>>let list xs = do x <- select xs; liftIO (print (show x)); return x>>>runListT (zip (list [1,2,3,4,5]) (list [6,7,8]))"1" "6" "2" "7" "3" "8" "4"
Repetition
Unbounded repetition can be induced using select (repeat ())cycle:
cycle1 :: Monad m => a -> ListT m a
cycle1 a = do
select (Data.List.repeat ())
return acycle2 :: Monad m => [a] -> ListT m a
cycle2 as = do
select (Data.List.repeat ())
select ascycle3 :: Monad m => m a -> ListT m a
cycle3 m = do
select (Data.List.repeat ())
lift mcycle4 :: Monad m => [m a] -> ListT m a
cycle4 ms = do
select (Data.List.repeat ())
m <- select ms
lift mcycle5 :: Monad m => ListT m a -> ListT m a
cycle5 x = do
select (Data.List.repeat ())
xcycle6 :: Monad m => [ListT m a] -> ListT m a
cycle6 lists = do
select (Data.List.repeat ())
x <- select lists
xIn a similar manner, we can use replicate as the initial selection
to achieve bounded repetition:
replicate1 :: Monad m => Int -> a -> ListT m a
replicate1 n a = do
select (Data.List.replicate n ())
return areplicate2 :: Monad m => Int -> [a] -> ListT m a
replicate2 n as = do
select (Data.List.replicate n ())
select asreplicate3 :: Monad m => Int -> m a -> ListT m a
replicate3 n m = do
select (Data.List.replicate n ())
lift mreplicate4 :: Monad m => Int -> [m a] -> ListT m a
replicate4 n ms = do
select (Data.List.replicate n ())
m <- select ms
lift mreplicate5 :: Monad m => Int -> ListT m a -> ListT m a
replicate5 n x = do
select (Data.List.replicate n ())
xreplicate6 :: Monad m => Int -> [ListT m a] -> ListT m a
replicate6 n lists = do
select (Data.List.replicate n ())
x <- select lists
xStep
Pattern match on this type when you loop explicitly over a ListT using
next. For example:
stdout :: ListT IO String -> IO ()
stdout l = do
s <- next l
case s of
Nil -> return ()
Cons x l' -> do
putStrLn x
stdout l'Instances
| MFunctor Step Source # | |
| Foldable m => Foldable (Step m) Source # | |
Defined in List.Transformer Methods fold :: Monoid m0 => Step m m0 -> m0 # foldMap :: Monoid m0 => (a -> m0) -> Step m a -> m0 # foldMap' :: Monoid m0 => (a -> m0) -> Step m a -> m0 # foldr :: (a -> b -> b) -> b -> Step m a -> b # foldr' :: (a -> b -> b) -> b -> Step m a -> b # foldl :: (b -> a -> b) -> b -> Step m a -> b # foldl' :: (b -> a -> b) -> b -> Step m a -> b # foldr1 :: (a -> a -> a) -> Step m a -> a # foldl1 :: (a -> a -> a) -> Step m a -> a # elem :: Eq a => a -> Step m a -> Bool # maximum :: Ord a => Step m a -> a # minimum :: Ord a => Step m a -> a # | |
| (Monad m, Traversable m) => Traversable (Step m) Source # | |
| Monad m => Functor (Step m) Source # | |
Alternative instances
Similar to ZipList in base: a newtype wrapper over ListT that
overrides its normal Applicative instance (combine every combination)
with one that "zips" outputs together one at a time.
>>>let xs = do x <- select [1,2,3,4]; liftIO (print x)>>>let ys = do y <- select [5,6]; liftIO (print y)>>>runListT (xs *> ys)1 5 6 2 5 6 3 5 6 4 5 6>>>runListT (getZipListT (ZipListT xs *> ZipListT ys))1 5 2 6 3
Note that the final "3" is printed even though it isn't paired with anything.
While this can be used to do zipping, it is usually more convenient to
just use zip. This is more useful if you are working with a function
that expects "an Applicative instance", written to be polymorphic over
all Applicatives.
Constructors
| ZipListT | |
Fields
| |
Instances
Re-exports
class MonadTrans (t :: (Type -> Type) -> Type -> Type) where #
The class of monad transformers. Instances should satisfy the
following laws, which state that lift is a monad transformation:
Methods
lift :: Monad m => m a -> t m a #
Lift a computation from the argument monad to the constructed monad.
Instances
| MonadTrans ListT Source # | |
Defined in List.Transformer | |
| MonadTrans ListT | |
Defined in Control.Monad.Trans.List | |
| MonadTrans MaybeT | |
Defined in Control.Monad.Trans.Maybe | |
| MonadTrans (ErrorT e) | |
Defined in Control.Monad.Trans.Error | |
| MonadTrans (ExceptT e) | |
Defined in Control.Monad.Trans.Except | |
| MonadTrans (IdentityT :: (Type -> Type) -> Type -> Type) | |
Defined in Control.Monad.Trans.Identity | |
| MonadTrans (ReaderT r) | |
Defined in Control.Monad.Trans.Reader | |
| MonadTrans (StateT s) | |
Defined in Control.Monad.Trans.State.Lazy | |
| MonadTrans (StateT s) | |
Defined in Control.Monad.Trans.State.Strict | |
| Monoid w => MonadTrans (WriterT w) | |
Defined in Control.Monad.Trans.Writer.Lazy | |
| Monoid w => MonadTrans (WriterT w) | |
Defined in Control.Monad.Trans.Writer.Strict | |
| MonadTrans (ContT r) | |
Defined in Control.Monad.Trans.Cont | |
| Monoid w => MonadTrans (RWST r w s) | |
Defined in Control.Monad.Trans.RWS.Lazy | |
| Monoid w => MonadTrans (RWST r w s) | |
Defined in Control.Monad.Trans.RWS.Strict | |
class Monad m => MonadIO (m :: Type -> Type) where #
Monads in which IO computations may be embedded.
Any monad built by applying a sequence of monad transformers to the
IO monad will be an instance of this class.
Instances should satisfy the following laws, which state that liftIO
is a transformer of monads:
Methods
Lift a computation from the IO monad.
This allows us to run IO computations in any monadic stack, so long as it supports these kinds of operations
(i.e. IO is the base monad for the stack).
Example
import Control.Monad.Trans.State -- from the "transformers" library printState :: Show s => StateT s IO () printState = do state <- get liftIO $ print state
Had we omitted liftIO
• Couldn't match type ‘IO’ with ‘StateT s IO’ Expected type: StateT s IO () Actual type: IO ()
The important part here is the mismatch between StateT s IO () and IO ()
Luckily, we know of a function that takes an IO a(m a): liftIO
> evalStateT printState "hello" "hello" > evalStateT printState 3 3
Instances
class Applicative f => Alternative (f :: Type -> Type) where #
A monoid on applicative functors.
If defined, some and many should be the least solutions
of the equations:
Methods
The identity of <|>
(<|>) :: f a -> f a -> f a infixl 3 #
An associative binary operation
One or more.
Zero or more.
Instances
class MFunctor (t :: (Type -> Type) -> k -> Type) where #
A functor in the category of monads, using hoist as the analog of fmap:
hoist (f . g) = hoist f . hoist g hoist id = id
Methods
hoist :: forall m n (b :: k). Monad m => (forall a. m a -> n a) -> t m b -> t n b #
Lift a monad morphism from m to n into a monad morphism from
(t m) to (t n)
The first argument to hoist must be a monad morphism, even though the
type system does not enforce this
Instances
| MFunctor ListT Source # | |
| MFunctor Step Source # | |
| MFunctor Lift | |
| MFunctor MaybeT | |
| MFunctor (Backwards :: (Type -> Type) -> Type -> Type) | |
| MFunctor (ExceptT e :: (Type -> Type) -> Type -> Type) | |
| MFunctor (IdentityT :: (Type -> Type) -> Type -> Type) | |
| MFunctor (ReaderT r :: (Type -> Type) -> Type -> TYPE LiftedRep) | |
| MFunctor (StateT s :: (Type -> Type) -> Type -> TYPE LiftedRep) | |
| MFunctor (StateT s :: (Type -> Type) -> Type -> TYPE LiftedRep) | |
| MFunctor (WriterT w :: (Type -> Type) -> Type -> Type) | |
| MFunctor (WriterT w :: (Type -> Type) -> Type -> Type) | |
| MFunctor (Product f :: (Type -> Type) -> Type -> Type) | |
| Functor f => MFunctor (Compose f :: (Type -> Type) -> Type -> Type) | |
| MFunctor (RWST r w s :: (Type -> Type) -> Type -> TYPE LiftedRep) | |
| MFunctor (RWST r w s :: (Type -> Type) -> Type -> TYPE LiftedRep) | |