Apecs.STM.Prelude

This module re-exports the apecs prelude with STM versions.

Documentation

type System w a = SystemT w STM a

module Apecs.STM

makeWorld :: String -> [Name] -> Q [Dec]

The typical way to create a world record, associated Has instances, and initialization function.

makeWorld "MyWorld" [''Component1, ''Component2, ...]

turns into

data MyWorld = MyWorld Component1 Component2 ... EntityCounter
instance MyWorld `Has` Component1 where ...
instance MyWorld `Has` Component2 where ...
...
instance MyWorld `Has` EntityCounter where ...

initMyWorld :: IO MyWorld
initMyWorld = MyWorld <$> initStore <*> initStore <*> ... <*> initStore

runGC :: System w ()

Explicitly invoke the garbage collector

newEntity_ :: forall (m :: Type -> Type) world component. (MonadIO m, Set world m component, Get world m EntityCounter) => component -> SystemT world m ()

Writes the given components to a new entity without yelding the result. Used mostly for convenience.

newEntity :: forall (m :: Type -> Type) w c. (MonadIO m, Set w m c, Get w m EntityCounter) => c -> SystemT w m Entity

Writes the given components to a new entity, and yields that entity. The return value is often ignored.

global :: Entity

Convenience entity, for use in places where the entity value does not matter, i.e. a global store.

data EntityCounter

Component used by newEntity to track the number of issued entities. Automatically added to any world created with makeWorld

Instances

Component EntityCounter

Defined in Apecs.Util

Associated Types

type Storage EntityCounter

Monoid EntityCounter

Defined in Apecs.Util

Methods

mempty :: EntityCounter

mappend :: EntityCounter -> EntityCounter -> EntityCounter

mconcat :: [EntityCounter] -> EntityCounter

Semigroup EntityCounter

Defined in Apecs.Util

Methods

(<>) :: EntityCounter -> EntityCounter -> EntityCounter

sconcat :: NonEmpty EntityCounter -> EntityCounter

stimes :: Integral b => b -> EntityCounter -> EntityCounter

Show EntityCounter

Defined in Apecs.Util

Methods

showsPrec :: Int -> EntityCounter -> ShowS

show :: EntityCounter -> String

showList :: [EntityCounter] -> ShowS

Eq EntityCounter

Defined in Apecs.Util

Methods

(==) :: EntityCounter -> EntityCounter -> Bool

(/=) :: EntityCounter -> EntityCounter -> Bool

type Storage EntityCounter

Defined in Apecs.Util

type Storage EntityCounter = ReadOnly (Global EntityCounter)

collect :: forall components w (m :: Type -> Type) a. (Get w m components, Members w m components) => (components -> Maybe a) -> SystemT w m [a]

Collect matching components into a list by using the specified test/process function. You can use this to preprocess data before returning. And you can do a test here that depends on data from multiple components. Pass Just to simply collect all the items.

cfoldM_ :: forall w (m :: Type -> Type) c a. (Members w m c, Get w m c) => (a -> c -> SystemT w m a) -> a -> SystemT w m ()

Monadically fold over the game world. Strict in the accumulator.

cfoldM :: forall w (m :: Type -> Type) c a. (Members w m c, Get w m c) => (a -> c -> SystemT w m a) -> a -> SystemT w m a

Monadically fold over the game world. Strict in the accumulator.

cfold :: forall w (m :: Type -> Type) c a. (Members w m c, Get w m c) => (a -> c -> a) -> a -> SystemT w m a

Fold over the game world; for example, cfold max (minBound :: Foo) will find the maximum value of Foo. Strict in the accumulator.

cmapM_ :: forall w (m :: Type -> Type) c. (Get w m c, Members w m c) => (c -> SystemT w m ()) -> SystemT w m ()

Monadically iterates over all entites with a cx

cmapM :: forall w (m :: Type -> Type) cx cy. (Get w m cx, Set w m cy, Members w m cx) => (cx -> SystemT w m cy) -> SystemT w m ()

Monadically iterates over all entites with a cx, and writes their cy.

cmap :: forall w (m :: Type -> Type) cx cy. (Get w m cx, Members w m cx, Set w m cy) => (cx -> cy) -> SystemT w m ()

Maps a function over all entities with a cx, and writes their cy.

($~) :: forall w (m :: Type -> Type) cx cy. (Get w m cx, Set w m cy) => Entity -> (cx -> cy) -> SystemT w m () infixr 2

modify operator

Applies a function, if possible.

modify :: forall w (m :: Type -> Type) cx cy. (Get w m cx, Set w m cy) => Entity -> (cx -> cy) -> SystemT w m ()

Applies a function, if possible.

destroy :: forall w (m :: Type -> Type) c. Destroy w m c => Entity -> Proxy c -> SystemT w m ()

Destroys component c for the given entity.

exists :: forall w (m :: Type -> Type) c. Get w m c => Entity -> Proxy c -> SystemT w m Bool

Returns whether the given entity has component c

($=) :: forall w (m :: Type -> Type) c. Set w m c => Entity -> c -> SystemT w m () infixr 2

set operator

Writes a Component to a given Entity. Will overwrite existing Components.

set :: forall w (m :: Type -> Type) c. Set w m c => Entity -> c -> SystemT w m ()

Writes a Component to a given Entity. Will overwrite existing Components.

get :: forall w (m :: Type -> Type) c. Get w m c => Entity -> SystemT w m c

Read a Component

runWith :: w -> SystemT w m a -> m a

Run a system in a game world

runSystem :: SystemT w m a -> w -> m a

Run a system in a game world

data Not a

Pseudocomponent indicating the absence of a. Mainly used as e.g. cmap $ (a, Not b) -> c to iterate over entities with an a but no b. Can also be used to delete components, like cmap $ a -> (Not :: Not a) to delete every a component.

Constructors

Not

Instances

Has w m c => Has w m (Not c)

Defined in Apecs.Components

Methods

getStore :: SystemT w m (Storage (Not c))

Component c => Component (Not c)

Defined in Apecs.Components

Associated Types

type Storage (Not c)

type Storage (Not c)

Defined in Apecs.Components

type Storage (Not c) = NotStore (Storage c)

data Cache (n :: Nat) s

A cache around another store. Caches store their members in a fixed-size vector, so read/write operations become O(1). Caches can provide huge performance boosts, especially when working with large numbers of components.

The cache size is given as a type-level argument.

Note that iterating over a cache is linear in cache size, so sparsely populated caches might decrease performance. In general, the exact size of the cache does not matter as long as it reasonably approximates the number of components present.

The cache uses entity (-2) internally to represent missing entities. If you manually manipulate Entity values, be careful that you do not use (-2)

The actual cache is not necessarily the given argument, but the next biggest power of two. This is allows most operations to be expressed as bit masks, for a large potential performance boost.

Instances

(MonadIO m, ExplDestroy m s) => ExplDestroy m (Cache n s)

Defined in Apecs.Stores

Methods

explDestroy :: Cache n s -> Int -> m ()

(MonadIO m, ExplGet m s) => ExplGet m (Cache n s)

Defined in Apecs.Stores

Methods

explGet :: Cache n s -> Int -> m (Elem (Cache n s))

explExists :: Cache n s -> Int -> m Bool

(MonadIO m, ExplInit m s, KnownNat n, Cachable s) => ExplInit m (Cache n s)

Defined in Apecs.Stores

Methods

explInit :: m (Cache n s)

(MonadIO m, ExplMembers m s) => ExplMembers m (Cache n s)

Defined in Apecs.Stores

Methods

explMembers :: Cache n s -> m (Vector Int)

(MonadIO m, ExplSet m s) => ExplSet m (Cache n s)

Defined in Apecs.Stores

Methods

explSet :: Cache n s -> Int -> Elem (Cache n s) -> m ()

(KnownNat n, Cachable s) => Cachable (Cache n s)

Defined in Apecs.Stores

type Elem (Cache n s)

Defined in Apecs.Stores

type Elem (Cache n s) = Elem s

newtype Entity

An Entity is just an integer, used to index into a component store. In general, use newEntity, cmap, and component tags instead of manipulating these directly.

For performance reasons, negative values like (-1) are reserved for stores to represent special values, so avoid using these.

Constructors

Entity

Fields

• unEntity :: Int

Instances

Enum Entity

Defined in Apecs.Core

Methods

succ :: Entity -> Entity

pred :: Entity -> Entity

toEnum :: Int -> Entity

fromEnum :: Entity -> Int

enumFrom :: Entity -> [Entity]

enumFromThen :: Entity -> Entity -> [Entity]

enumFromTo :: Entity -> Entity -> [Entity]

enumFromThenTo :: Entity -> Entity -> Entity -> [Entity]

Num Entity

Defined in Apecs.Core

Methods

(+) :: Entity -> Entity -> Entity

(-) :: Entity -> Entity -> Entity

(*) :: Entity -> Entity -> Entity

negate :: Entity -> Entity

abs :: Entity -> Entity

signum :: Entity -> Entity

fromInteger :: Integer -> Entity

Show Entity

Defined in Apecs.Core

Methods

showsPrec :: Int -> Entity -> ShowS

show :: Entity -> String

showList :: [Entity] -> ShowS

Eq Entity

Defined in Apecs.Core

Methods

(==) :: Entity -> Entity -> Bool

(/=) :: Entity -> Entity -> Bool

Ord Entity

Defined in Apecs.Core

Methods

compare :: Entity -> Entity -> Ordering

(<) :: Entity -> Entity -> Bool

(<=) :: Entity -> Entity -> Bool

(>) :: Entity -> Entity -> Bool

(>=) :: Entity -> Entity -> Bool

max :: Entity -> Entity -> Entity

min :: Entity -> Entity -> Entity

type Storage Entity

Defined in Apecs.Components

type Storage Entity = EntityStore

newtype SystemT w (m :: Type -> Type) a

A SystemT is a newtype around `ReaderT w m a`, where w is the game world variable. Systems serve to

• Allow type-based lookup of a component's store through getStore.
• Lift side effects into their host Monad.

Constructors

SystemT

Fields

• unSystem :: ReaderT w m a

Instances

Monad m => MonadReader w (SystemT w m)

Defined in Apecs.Core

Methods

ask :: SystemT w m w

local :: (w -> w) -> SystemT w m a -> SystemT w m a

reader :: (w -> a) -> SystemT w m a

MonadTrans (SystemT w)

Defined in Apecs.Core

Methods

lift :: Monad m => m a -> SystemT w m a

MonadIO m => MonadIO (SystemT w m)

Defined in Apecs.Core

Methods

liftIO :: IO a -> SystemT w m a

Applicative m => Applicative (SystemT w m)

Defined in Apecs.Core

Methods

pure :: a -> SystemT w m a

(<*>) :: SystemT w m (a -> b) -> SystemT w m a -> SystemT w m b

liftA2 :: (a -> b -> c) -> SystemT w m a -> SystemT w m b -> SystemT w m c

(*>) :: SystemT w m a -> SystemT w m b -> SystemT w m b

(<*) :: SystemT w m a -> SystemT w m b -> SystemT w m a

Functor m => Functor (SystemT w m)

Defined in Apecs.Core

Methods

fmap :: (a -> b) -> SystemT w m a -> SystemT w m b

(<$) :: a -> SystemT w m b -> SystemT w m a

Monad m => Monad (SystemT w m)

Defined in Apecs.Core

Methods

(>>=) :: SystemT w m a -> (a -> SystemT w m b) -> SystemT w m b

(>>) :: SystemT w m a -> SystemT w m b -> SystemT w m b

return :: a -> SystemT w m a

MonadCatch m => MonadCatch (SystemT w m)

Defined in Apecs.Core

Methods

catch :: Exception e => SystemT w m a -> (e -> SystemT w m a) -> SystemT w m a

MonadMask m => MonadMask (SystemT w m)

Defined in Apecs.Core

Methods

mask :: ((forall a. SystemT w m a -> SystemT w m a) -> SystemT w m b) -> SystemT w m b

uninterruptibleMask :: ((forall a. SystemT w m a -> SystemT w m a) -> SystemT w m b) -> SystemT w m b

generalBracket :: SystemT w m a -> (a -> ExitCase b -> SystemT w m c) -> (a -> SystemT w m b) -> SystemT w m (b, c)

MonadThrow m => MonadThrow (SystemT w m)

Defined in Apecs.Core

Methods

throwM :: Exception e => e -> SystemT w m a

type family Storage c

Instances

type Storage Entity

Defined in Apecs.Components

type Storage Entity = EntityStore

type Storage EntityCounter

Defined in Apecs.Util

type Storage EntityCounter = ReadOnly (Global EntityCounter)

type Storage ()

Defined in Apecs.Components

type Storage () = ()

type Storage (Filter c)

Defined in Apecs.Components

type Storage (Filter c) = FilterStore (Storage c)

type Storage (Not c)

Defined in Apecs.Components

type Storage (Not c) = NotStore (Storage c)

type Storage (Identity c)

Defined in Apecs.Components

type Storage (Identity c) = Identity (Storage c)

type Storage (Maybe c)

Defined in Apecs.Components

type Storage (Maybe c) = MaybeStore (Storage c)

type Storage (Either ca cb)

Defined in Apecs.Components

type Storage (Either ca cb) = EitherStore (Storage ca) (Storage cb)

type Storage (t_0, t_1)

Defined in Apecs.Components

type Storage (t_0, t_1) = (Storage t_0, Storage t_1)

type Storage (t_0, t_1, t_2)

Defined in Apecs.Components

type Storage (t_0, t_1, t_2) = (Storage t_0, Storage t_1, Storage t_2)

type Storage (t_0, t_1, t_2, t_3)

Defined in Apecs.Components

type Storage (t_0, t_1, t_2, t_3) = (Storage t_0, Storage t_1, Storage t_2, Storage t_3)

type Storage (t_0, t_1, t_2, t_3, t_4)

Defined in Apecs.Components

type Storage (t_0, t_1, t_2, t_3, t_4) = (Storage t_0, Storage t_1, Storage t_2, Storage t_3, Storage t_4)

type Storage (t_0, t_1, t_2, t_3, t_4, t_5)

Defined in Apecs.Components

type Storage (t_0, t_1, t_2, t_3, t_4, t_5) = (Storage t_0, Storage t_1, Storage t_2, Storage t_3, Storage t_4, Storage t_5)

type Storage (t_0, t_1, t_2, t_3, t_4, t_5, t_6)

Defined in Apecs.Components

type Storage (t_0, t_1, t_2, t_3, t_4, t_5, t_6) = (Storage t_0, Storage t_1, Storage t_2, Storage t_3, Storage t_4, Storage t_5, Storage t_6)

type Storage (t_0, t_1, t_2, t_3, t_4, t_5, t_6, t_7)

Defined in Apecs.Components

type Storage (t_0, t_1, t_2, t_3, t_4, t_5, t_6, t_7) = (Storage t_0, Storage t_1, Storage t_2, Storage t_3, Storage t_4, Storage t_5, Storage t_6, Storage t_7)

class Elem (Storage c) ~ c => Component c

A component is defined by specifying how it is stored. The constraint ensures that stores and components are mapped one-to-one.

Associated Types

type Storage c

Instances

Component EntityCounter

Defined in Apecs.Util

Associated Types

type Storage EntityCounter

Component c => Component (Filter c)

Defined in Apecs.Components

Associated Types

type Storage (Filter c)

Component c => Component (Not c)

Defined in Apecs.Components

Associated Types

type Storage (Not c)

class (Monad m, Component c) => Has w (m :: Type -> Type) c where

Has w m c means that world w can produce a Storage c. It is parameterized over m to allow stores to be foreign.

Methods

getStore :: SystemT w m (Storage c)

Instances

Has w m c => Has w m (Filter c)

Defined in Apecs.Components

Methods

getStore :: SystemT w m (Storage (Filter c))

Has w m c => Has w m (Not c)

Defined in Apecs.Components

Methods

getStore :: SystemT w m (Storage (Not c))

explInit :: ExplInit m s => m s

Initialize a new empty store.

type Get w (m :: Type -> Type) c = (Has w m c, ExplGet m (Storage c))

type Set w (m :: Type -> Type) c = (Has w m c, ExplSet m (Storage c))

type Members w (m :: Type -> Type) c = (Has w m c, ExplMembers m (Storage c))

type Destroy w (m :: Type -> Type) c = (Has w m c, ExplDestroy m (Storage c))

asks

Arguments

:: MonadReader r m

=> (r -> a)

The selector function to apply to the environment.

-> m a

Retrieves a function of the current environment.

data Proxy (t :: k)

Proxy is a type that holds no data, but has a phantom parameter of arbitrary type (or even kind). Its use is to provide type information, even though there is no value available of that type (or it may be too costly to create one).

Historically, Proxy :: Proxy a is a safer alternative to the undefined :: a idiom.

>>> Proxy :: Proxy (Void, Int -> Int)
Proxy

Proxy can even hold types of higher kinds,

>>> Proxy :: Proxy Either
Proxy

>>> Proxy :: Proxy Functor
Proxy

>>> Proxy :: Proxy complicatedStructure
Proxy

Constructors

Proxy

Instances

Generic1 (Proxy :: k -> Type)

Defined in GHC.Generics

Associated Types

type Rep1 Proxy :: k -> Type

Methods

from1 :: forall (a :: k0). Proxy a -> Rep1 Proxy a

to1 :: forall (a :: k0). Rep1 Proxy a -> Proxy a

Foldable (Proxy :: TYPE LiftedRep -> Type)

Since: base-4.7.0.0

Defined in Data.Foldable

Methods

fold :: Monoid m => Proxy m -> m

foldMap :: Monoid m => (a -> m) -> Proxy a -> m

foldMap' :: Monoid m => (a -> m) -> Proxy a -> m

foldr :: (a -> b -> b) -> b -> Proxy a -> b

foldr' :: (a -> b -> b) -> b -> Proxy a -> b

foldl :: (b -> a -> b) -> b -> Proxy a -> b

foldl' :: (b -> a -> b) -> b -> Proxy a -> b

foldr1 :: (a -> a -> a) -> Proxy a -> a

foldl1 :: (a -> a -> a) -> Proxy a -> a

toList :: Proxy a -> [a]

null :: Proxy a -> Bool

length :: Proxy a -> Int

elem :: Eq a => a -> Proxy a -> Bool

maximum :: Ord a => Proxy a -> a

minimum :: Ord a => Proxy a -> a

sum :: Num a => Proxy a -> a

product :: Num a => Proxy a -> a

Traversable (Proxy :: Type -> Type)

Since: base-4.7.0.0

Defined in Data.Traversable

Methods

traverse :: Applicative f => (a -> f b) -> Proxy a -> f (Proxy b)

sequenceA :: Applicative f => Proxy (f a) -> f (Proxy a)

mapM :: Monad m => (a -> m b) -> Proxy a -> m (Proxy b)

sequence :: Monad m => Proxy (m a) -> m (Proxy a)

Alternative (Proxy :: Type -> Type)

Since: base-4.9.0.0

Defined in Data.Proxy

Methods

empty :: Proxy a

(<|>) :: Proxy a -> Proxy a -> Proxy a

some :: Proxy a -> Proxy [a]

many :: Proxy a -> Proxy [a]

Applicative (Proxy :: Type -> Type)

Since: base-4.7.0.0

Defined in Data.Proxy

Methods

pure :: a -> Proxy a

(<*>) :: Proxy (a -> b) -> Proxy a -> Proxy b

liftA2 :: (a -> b -> c) -> Proxy a -> Proxy b -> Proxy c

(*>) :: Proxy a -> Proxy b -> Proxy b

(<*) :: Proxy a -> Proxy b -> Proxy a

Functor (Proxy :: Type -> Type)

Since: base-4.7.0.0

Defined in Data.Proxy

Methods

fmap :: (a -> b) -> Proxy a -> Proxy b

(<$) :: a -> Proxy b -> Proxy a

Monad (Proxy :: Type -> Type)

Since: base-4.7.0.0

Defined in Data.Proxy

Methods

(>>=) :: Proxy a -> (a -> Proxy b) -> Proxy b

(>>) :: Proxy a -> Proxy b -> Proxy b

return :: a -> Proxy a

MonadPlus (Proxy :: Type -> Type)

Since: base-4.9.0.0

Defined in Data.Proxy

Methods

mzero :: Proxy a

mplus :: Proxy a -> Proxy a -> Proxy a

Hashable1 (Proxy :: Type -> Type)

Defined in Data.Hashable.Class

Methods

liftHashWithSalt :: (Int -> a -> Int) -> Int -> Proxy a -> Int

Monoid (Proxy s)

Since: base-4.7.0.0

Defined in Data.Proxy

Methods

mempty :: Proxy s

mappend :: Proxy s -> Proxy s -> Proxy s

mconcat :: [Proxy s] -> Proxy s

Semigroup (Proxy s)

Since: base-4.9.0.0

Defined in Data.Proxy

Methods

(<>) :: Proxy s -> Proxy s -> Proxy s

sconcat :: NonEmpty (Proxy s) -> Proxy s

stimes :: Integral b => b -> Proxy s -> Proxy s

Bounded (Proxy t)

Since: base-4.7.0.0

Defined in Data.Proxy

Methods

minBound :: Proxy t

maxBound :: Proxy t

Enum (Proxy s)

Since: base-4.7.0.0

Defined in Data.Proxy

Methods

succ :: Proxy s -> Proxy s

pred :: Proxy s -> Proxy s

toEnum :: Int -> Proxy s

fromEnum :: Proxy s -> Int

enumFrom :: Proxy s -> [Proxy s]

enumFromThen :: Proxy s -> Proxy s -> [Proxy s]

enumFromTo :: Proxy s -> Proxy s -> [Proxy s]

enumFromThenTo :: Proxy s -> Proxy s -> Proxy s -> [Proxy s]

Generic (Proxy t)

Defined in GHC.Generics

Associated Types

type Rep (Proxy t) :: Type -> Type

Methods

from :: Proxy t -> Rep (Proxy t) x

to :: Rep (Proxy t) x -> Proxy t

Ix (Proxy s)

Since: base-4.7.0.0

Defined in Data.Proxy

Methods

range :: (Proxy s, Proxy s) -> [Proxy s]

index :: (Proxy s, Proxy s) -> Proxy s -> Int

unsafeIndex :: (Proxy s, Proxy s) -> Proxy s -> Int

inRange :: (Proxy s, Proxy s) -> Proxy s -> Bool

rangeSize :: (Proxy s, Proxy s) -> Int

unsafeRangeSize :: (Proxy s, Proxy s) -> Int

Read (Proxy t)

Since: base-4.7.0.0

Defined in Data.Proxy

Methods

readsPrec :: Int -> ReadS (Proxy t)

readList :: ReadS [Proxy t]

readPrec :: ReadPrec (Proxy t)

readListPrec :: ReadPrec [Proxy t]

Show (Proxy s)

Since: base-4.7.0.0

Defined in Data.Proxy

Methods

showsPrec :: Int -> Proxy s -> ShowS

show :: Proxy s -> String

showList :: [Proxy s] -> ShowS

Eq (Proxy s)

Since: base-4.7.0.0

Defined in Data.Proxy

Methods

(==) :: Proxy s -> Proxy s -> Bool

(/=) :: Proxy s -> Proxy s -> Bool

Ord (Proxy s)

Since: base-4.7.0.0

Defined in Data.Proxy

Methods

compare :: Proxy s -> Proxy s -> Ordering

(<) :: Proxy s -> Proxy s -> Bool

(<=) :: Proxy s -> Proxy s -> Bool

(>) :: Proxy s -> Proxy s -> Bool

(>=) :: Proxy s -> Proxy s -> Bool

max :: Proxy s -> Proxy s -> Proxy s

min :: Proxy s -> Proxy s -> Proxy s

Hashable (Proxy a)

Defined in Data.Hashable.Class

Methods

hashWithSalt :: Int -> Proxy a -> Int

hash :: Proxy a -> Int

type Rep1 (Proxy :: k -> Type)

Since: base-4.6.0.0

Defined in GHC.Generics

type Rep1 (Proxy :: k -> Type) = D1 ('MetaData "Proxy" "Data.Proxy" "base" 'False) (C1 ('MetaCons "Proxy" 'PrefixI 'False) (U1 :: k -> Type))

type Rep (Proxy t)

Since: base-4.6.0.0

Defined in GHC.Generics

type Rep (Proxy t) = D1 ('MetaData "Proxy" "Data.Proxy" "base" 'False) (C1 ('MetaCons "Proxy" 'PrefixI 'False) (U1 :: Type -> Type))

liftIO :: MonadIO m => IO a -> m a

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, we would have ended up with this error:

• 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 and returns an (m a): liftIO, enabling us to run the program and see the expected results:

> evalStateT printState "hello"
"hello"

> evalStateT printState 3
3

lift :: (MonadTrans t, Monad m) => m a -> t m a

Lift a computation from the argument monad to the constructed monad.

ask :: MonadReader r m => m r

Retrieves the monad environment.