Safe Haskell  None 

Language  Haskell2010 
 Core Types
 Running Sem
 Type synonyms for user convenience
 Interoperating With Other Monads
 Lifting
 Trivial Interpretation
 Creating New Effects
 Combinators for Interpreting FirstOrder Effects
 Combinators for Interpreting HigherOrder Effects
 Combinators for Interpreting Directly to IO
 Kind Synonyms
 Composing IObased Interpreters
 Tactics
Synopsis
 data Sem r a
 type Member e r = MemberNoError e r
 type MemberWithError e r = (MemberNoError e r, WhenStuck (LocateEffect e r) (AmbiguousSend e r))
 type family Members es r :: Constraint where ...
 run :: Sem '[] a > a
 runM :: Monad m => Sem '[Embed m] a > m a
 runFinal :: Monad m => Sem '[Final m] a > m a
 type InterpreterFor e r = forall a. Sem (e ': r) a > Sem r a
 type InterpretersFor es r = forall a. Sem (Append es r) a > Sem r a
 newtype Embed m (z :: Type > Type) a where
 embed :: Member (Embed m) r => m a > Sem r a
 embedToFinal :: (Member (Final m) r, Functor m) => Sem (Embed m ': r) a > Sem r a
 data Final m z a
 embedFinal :: (Member (Final m) r, Functor m) => m a > Sem r a
 raise :: forall e r a. Sem r a > Sem (e ': r) a
 raiseUnder :: forall e2 e1 r a. Sem (e1 ': r) a > Sem (e1 ': (e2 ': r)) a
 raiseUnder2 :: forall e2 e3 e1 r a. Sem (e1 ': r) a > Sem (e1 ': (e2 ': (e3 ': r))) a
 raiseUnder3 :: forall e2 e3 e4 e1 r a. Sem (e1 ': r) a > Sem (e1 ': (e2 ': (e3 ': (e4 ': r)))) a
 raise2Under :: forall e3 e1 e2 r a. Sem (e1 ': (e2 ': r)) a > Sem (e1 ': (e2 ': (e3 ': r))) a
 raise3Under :: forall e4 e1 e2 e3 r a. Sem (e1 ': (e2 ': (e3 ': r))) a > Sem (e1 ': (e2 ': (e3 ': (e4 ': r)))) a
 raise_ :: forall r r' a. Raise r r' => Sem r a > Sem r' a
 subsume_ :: forall r r' a. Subsume r r' => Sem r a > Sem r' a
 subsume :: forall e r a. Member e r => Sem (e ': r) a > Sem r a
 makeSem :: Name > Q [Dec]
 makeSem_ :: Name > Q [Dec]
 interpret :: FirstOrder e "interpret" => (forall x rInitial. e (Sem rInitial) x > Sem r x) > Sem (e ': r) a > Sem r a
 intercept :: (Member e r, FirstOrder e "intercept") => (forall x rInitial. e (Sem rInitial) x > Sem r x) > Sem r a > Sem r a
 reinterpret :: forall e1 e2 r a. FirstOrder e1 "reinterpret" => (forall rInitial x. e1 (Sem rInitial) x > Sem (e2 ': r) x) > Sem (e1 ': r) a > Sem (e2 ': r) a
 reinterpret2 :: forall e1 e2 e3 r a. FirstOrder e1 "reinterpret2" => (forall rInitial x. e1 (Sem rInitial) x > Sem (e2 ': (e3 ': r)) x) > Sem (e1 ': r) a > Sem (e2 ': (e3 ': r)) a
 reinterpret3 :: forall e1 e2 e3 e4 r a. FirstOrder e1 "reinterpret3" => (forall rInitial x. e1 (Sem rInitial) x > Sem (e2 ': (e3 ': (e4 ': r))) x) > Sem (e1 ': r) a > Sem (e2 ': (e3 ': (e4 ': r))) a
 rewrite :: forall e1 e2 r a. (forall rInitial x. e1 (Sem rInitial) x > e2 (Sem rInitial) x) > Sem (e1 ': r) a > Sem (e2 ': r) a
 transform :: forall e1 e2 r a. Member e2 r => (forall rInitial x. e1 (Sem rInitial) x > e2 (Sem rInitial) x) > Sem (e1 ': r) a > Sem r a
 interpretH :: (forall x rInitial. e (Sem rInitial) x > Tactical e (Sem rInitial) r x) > Sem (e ': r) a > Sem r a
 interceptH :: Member e r => (forall x rInitial. e (Sem rInitial) x > Tactical e (Sem rInitial) r x) > Sem r a > Sem r a
 reinterpretH :: forall e1 e2 r a. (forall rInitial x. e1 (Sem rInitial) x > Tactical e1 (Sem rInitial) (e2 ': r) x) > Sem (e1 ': r) a > Sem (e2 ': r) a
 reinterpret2H :: forall e1 e2 e3 r a. (forall rInitial x. e1 (Sem rInitial) x > Tactical e1 (Sem rInitial) (e2 ': (e3 ': r)) x) > Sem (e1 ': r) a > Sem (e2 ': (e3 ': r)) a
 reinterpret3H :: forall e1 e2 e3 e4 r a. (forall rInitial x. e1 (Sem rInitial) x > Tactical e1 (Sem rInitial) (e2 ': (e3 ': (e4 ': r))) x) > Sem (e1 ': r) a > Sem (e2 ': (e3 ': (e4 ': r))) a
 withLowerToIO :: Member (Embed IO) r => ((forall x. Sem r x > IO x) > IO () > IO a) > Sem r a
 type Effect = (Type > Type) > Type > Type
 type EffectRow = [Effect]
 (.@) :: Monad m => (forall x. Sem r x > m x) > (forall y. (forall x. Sem r x > m x) > Sem (e ': r) y > Sem r y) > Sem (e ': r) z > m z
 (.@@) :: Monad m => (forall x. Sem r x > m x) > (forall y. (forall x. Sem r x > m x) > Sem (e ': r) y > Sem r (f y)) > Sem (e ': r) z > m (f z)
 type Tactical e m r x = forall f. Functor f => Sem (WithTactics e f m r) (f x)
 type WithTactics e f m r = Tactics f m (e ': r) ': r
 getInitialStateT :: forall f m r e. Sem (WithTactics e f m r) (f ())
 pureT :: a > Tactical e m r a
 runTSimple :: m a > Tactical e m r a
 bindTSimple :: forall m f r e a b. (a > m b) > f a > Sem (WithTactics e f m r) (f b)
 runT :: m a > Sem (WithTactics e f m r) (Sem (e ': r) (f a))
 bindT :: (a > m b) > Sem (WithTactics e f m r) (f a > Sem (e ': r) (f b))
 getInspectorT :: forall e f m r. Sem (WithTactics e f m r) (Inspector f)
 newtype Inspector f = Inspector {}
Core Types
The Sem
monad handles computations of arbitrary extensible effects.
A value of type Sem r
describes a program with the capabilities of
r
. For best results, r
should always be kept polymorphic, but you can
add capabilities via the Member
constraint.
The value of the Sem
monad is that it allows you to write programs
against a set of effects without a predefined meaning, and provide that
meaning later. For example, unlike with mtl, you can decide to interpret an
Error
effect traditionally as an Either
, or instead
as (a significantly faster) IO
Exception
. These
interpretations (and others that you might add) may be used interchangeably
without needing to write any newtypes or Monad
instances. The only
change needed to swap interpretations is to change a call from
runError
to errorToIOFinal
.
The effect stack r
can contain arbitrary other monads inside of it. These
monads are lifted into effects via the Embed
effect. Monadic values can be
lifted into a Sem
via embed
.
Higherorder actions of another monad can be lifted into higherorder actions
of Sem
via the Final
effect, which is more powerful
than Embed
, but also less flexible to interpret.
A Sem
can be interpreted as a pure value (via run
) or as any
traditional Monad
(via runM
or runFinal
).
Each effect E
comes equipped with some interpreters of the form:
runE ::Sem
(E ': r) a >Sem
r a
which is responsible for removing the effect E
from the effect stack. It
is the order in which you call the interpreters that determines the
monomorphic representation of the r
parameter.
Order of interpreters can be important  it determines behaviour of effects that manipulate state or change control flow. For example, when interpreting this action:
>>>
:{
example :: Members '[State String, Error String] r => Sem r String example = do put "start" let throwing, catching :: Members '[State String, Error String] r => Sem r String throwing = do modify (++"throw") throw "error" get catching = do modify (++"catch") get catch @String throwing (\ _ > catching) :}
when handling Error
first, state is preserved after error
occurs:
>>>
:{
example & runError & fmap (either id id) & evalState "" & runM & (print =<<) :} "startthrowcatch"
while handling State
first discards state in such cases:
>>>
:{
example & evalState "" & runError & fmap (either id id) & runM & (print =<<) :} "startcatch"
A good rule of thumb is to handle effects which should have "global" behaviour over other effects later in the chain.
After all of your effects are handled, you'll be left with either
a
, a Sem
'[] a
, or a Sem
'[ Embed
m ] a
value, which can be consumed respectively by Sem
'[ Final
m ] arun
, runM
, and
runFinal
.
Examples
As an example of keeping r
polymorphic, we can consider the type
Member
(State
String) r =>Sem
r ()
to be a program with access to
get
::Sem
r Stringput
:: String >Sem
r ()
methods.
By also adding a
Member
(Error
Bool) r
constraint on r
, we gain access to the
throw
:: Bool >Sem
r acatch
::Sem
r a > (Bool >Sem
r a) >Sem
r a
functions as well.
In this sense, a
constraint is
analogous to mtl's Member
(State
s) r
and should
be thought of as such. However, unlike mtl, a MonadState
s mSem
monad may have
an arbitrary number of the same effect.
For example, we can write a Sem
program which can output either
Int
s or Bool
s:
foo :: (Member
(Output
Int) r ,Member
(Output
Bool) r ) =>Sem
r () foo = dooutput
@Int 5output
True
Notice that we must use XTypeApplications
to specify that we'd like to
use the (Output
Int
) effect.
Since: 0.1.2.0
Instances
Monad (Sem f) Source #  
Functor (Sem f) Source #  
Member Fixpoint r => MonadFix (Sem r) Source #  
Defined in Polysemy.Internal  
Member (Fail :: (Type > Type) > Type > Type) r => MonadFail (Sem r) Source #  Since: 1.1.0.0 
Defined in Polysemy.Internal  
Applicative (Sem f) Source #  
Member (Embed IO) r => MonadIO (Sem r) Source #  This instance will only lift 
Defined in Polysemy.Internal  
Member NonDet r => Alternative (Sem r) Source #  
Member NonDet r => MonadPlus (Sem r) Source #  Since: 0.2.1.0 
Citizen (Sem r a > b) (Sem r a > b) Source #  
Defined in Polysemy.Law  
Citizen (Sem r a) (Sem r a) Source #  
Defined in Polysemy.Law 
type Member e r = MemberNoError e r Source #
A proof that the effect e
is available somewhere inside of the effect
stack r
.
type MemberWithError e r = (MemberNoError e r, WhenStuck (LocateEffect e r) (AmbiguousSend e r)) Source #
Like Member
, but will produce an error message if the types are
ambiguous. This is the constraint used for actions generated by
makeSem
.
Be careful with this. Due to quirks of TypeError
,
the custom error messages emitted by this can potentially override other,
more helpful error messages.
See the discussion in
Issue #227.
Since: 1.2.3.0
type family Members es r :: Constraint where ... Source #
Makes constraints of functions that use multiple effects shorter by
translating single list of effects into multiple Member
constraints:
foo ::Members
'[Output
Int ,Output
Bool ,State
String ] r =>Sem
r ()
translates into:
foo :: (Member
(Output
Int) r ,Member
(Output
Bool) r ,Member
(State
String) r ) =>Sem
r ()
Since: 0.1.2.0
Running Sem
runFinal :: Monad m => Sem '[Final m] a > m a Source #
Lower a Sem
containing only a single lifted, final Monad
into that
monad.
If you also need to process an
effect, use this together with
Embed
membedToFinal
.
Since: 1.2.0.0
Type synonyms for user convenience
type InterpreterFor e r = forall a. Sem (e ': r) a > Sem r a Source #
Type synonym for interpreters that consume an effect without changing the return value. Offered for user convenience.
r
Is kept polymorphic so it's possible to place constraints upon it:
teletypeToIO ::Member
(Embed IO) r =>InterpreterFor
Teletype r
type InterpretersFor es r = forall a. Sem (Append es r) a > Sem r a Source #
Variant of InterpreterFor
that takes a list of effects.
@since 1.5.0.0
Interoperating With Other Monads
Embed
newtype Embed m (z :: Type > Type) a where Source #
An effect which allows a regular Monad
m
into the Sem
ecosystem. Monadic actions in m
can be lifted into Sem
via
embed
.
For example, you can use this effect to lift IO
actions directly into
Sem
:
embed
(putStrLn "hello") ::Member
(Embed
IO) r =>Sem
r ()
That being said, you lose out on a significant amount of the benefits of
Sem
by using embed
directly in application code; doing
so will tie your application code directly to the underlying monad, and
prevent you from interpreting it differently. For best results, only use
Embed
in your effect interpreters.
Consider using trace
and traceToIO
as
a substitute for using putStrLn
directly.
Since: 1.0.0.0
embed :: Member (Embed m) r => m a > Sem r a Source #
Embed a monadic action m
in Sem
.
Since: 1.0.0.0
Final
For advanced uses of Final
, including creating your own interpreters
that make use of it, see Polysemy.Final
An effect for embedding higherorder actions in the final target monad of the effect stack.
This is very useful for writing interpreters that interpret higherorder effects in terms of the final monad.
Final
is more powerful than Embed
, but is also less flexible
to interpret (compare runEmbedded
with finalToFinal
).
If you only need the power of embed
, then you should use Embed
instead.
Beware: Final
actions are interpreted as actions of the final monad,
and the effectful state visible to
withWeavingToFinal
/ withStrategicToFinal
/ interpretFinal
is that of all interpreters run in order to produce the final monad.
This means that any interpreter built using Final
will not
respect local/global state semantics based on the order of
interpreters run. You should signal interpreters that make use of
Final
by adding a 
suffix to the names of these.Final
State semantics of effects that are not interpreted in terms of the final monad will always appear local to effects that are interpreted in terms of the final monad.
State semantics between effects that are interpreted in terms of the final monad depend on the final monad. For example, if the final monad is a monad transformer stack, then state semantics will depend on the order monad transformers are stacked.
Since: 1.2.0.0
Instances
type DefiningModule Final Source #  
Defined in Polysemy.Final 
embedFinal :: (Member (Final m) r, Functor m) => m a > Sem r a Source #
withWeavingToFinal
admits an implementation of embed
.
Just like embed
, you are discouraged from using this in application code.
Since: 1.2.0.0
Lifting
raiseUnder :: forall e2 e1 r a. Sem (e1 ': r) a > Sem (e1 ': (e2 ': r)) a Source #
Like raise
, but introduces a new effect underneath the head of the
list. See raiseUnder2
or raiseUnder3
for introducing more effects. If
you need to introduce even more of them, check out subsume_
.
raiseUnder
can be used in order to turn transformative interpreters
into reinterpreters. This is especially useful if you're writing an
interpreter which introduces an intermediary effect, and then want to use
an existing interpreter on that effect.
For example, given:
fooToBar ::Member
Bar r =>Sem
(Foo ': r) a >Sem
r a runBar ::Sem
(Bar ': r) a >Sem
r a
You can write:
runFoo ::Sem
(Foo ': r) a >Sem
r a runFoo = runBar  Consume Bar . fooToBar  Interpret Foo in terms of the new Bar .raiseUnder
 Introduces Bar under Foo
Since: 1.2.0.0
raiseUnder2 :: forall e2 e3 e1 r a. Sem (e1 ': r) a > Sem (e1 ': (e2 ': (e3 ': r))) a Source #
Like raise
, but introduces two new effects underneath the head of the
list.
Since: 1.2.0.0
raiseUnder3 :: forall e2 e3 e4 e1 r a. Sem (e1 ': r) a > Sem (e1 ': (e2 ': (e3 ': (e4 ': r)))) a Source #
Like raise
, but introduces three new effects underneath the head of the
list.
Since: 1.2.0.0
raise2Under :: forall e3 e1 e2 r a. Sem (e1 ': (e2 ': r)) a > Sem (e1 ': (e2 ': (e3 ': r))) a Source #
Like raise
, but introduces an effect two levels underneath the head of
the list.
Since: 1.4.0.0
raise3Under :: forall e4 e1 e2 e3 r a. Sem (e1 ': (e2 ': (e3 ': r))) a > Sem (e1 ': (e2 ': (e3 ': (e4 ': r)))) a Source #
Like raise
, but introduces an effect three levels underneath the head
of the list.
Since: 1.4.0.0
raise_ :: forall r r' a. Raise r r' => Sem r a > Sem r' a Source #
Introduce an arbitrary number of effects on top of the effect stack. This
function is highly polymorphic, so it may be good idea to use its more
concrete versions (like raise
) or type annotations to avoid vague errors
in ambiguous contexts.
Since: 1.4.0.0
subsume_ :: forall r r' a. Subsume r r' => Sem r a > Sem r' a Source #
Allows reordering and adding known effects on top of the effect stack, as
long as the polymorphic "tail" of new stack is a raise
d version of the
original one. This function is highly polymorphic, so it may be a good idea
to use its more concrete version (subsume
), fitting functions from the
raise
family or type annotations to avoid vague errors in ambiguous
contexts.
Since: 1.4.0.0
Trivial Interpretation
subsume :: forall e r a. Member e r => Sem (e ': r) a > Sem r a Source #
Interprets an effect in terms of another identical effect.
This is useful for defining interpreters that use reinterpretH
without immediately consuming the newly introduced effect.
Using such an interpreter recursively may result in duplicate effects,
which may then be eliminated using subsume
.
For a version that can introduce an arbitrary number of new effects and
reorder existing ones, see subsume_
.
Since: 1.2.0.0
Creating New Effects
Effects should be defined as a GADT (enable XGADTs
), with kind (*
> *) > * > *
. Every primitive action in the effect should be its
own constructor of the type. For example, we can model an effect which
interacts with a tty console as follows:
data Console m a where WriteLine :: String > Console m () ReadLine :: Console m String
Notice that the a
parameter gets instantiated at the desired return
type of the actions. Writing a line returns a ()
, but reading one
returns String
.
By enabling XTemplateHaskell
, we can use the makeSem
function
to generate smart constructors for the actions. These smart constructors
can be invoked directly inside of the Sem
monad.
makeSem ''Console
results in the following definitions:
writeLine ::Member
Console r => String >Sem
r () readLine ::Member
Console r =>Sem
r String
Effects which don't make use of the m
parameter are known as
"firstorder effects."
HigherOrder Effects
Every effect has access to the m
parameter, which corresponds to the
Sem
monad it's used in. Using this parameter, we're capable of
writing effects which themselves contain subcomputations.
For example, the definition of Error
is
dataError
e m a whereThrow
:: e >Error
e m aCatch
:: m a > (e > m a) >Error
e m a
where Catch
is an action that can run an exception
handler if its first argument calls throw
.
makeSem ''Error
throw
::Member
(Error
e) r => e >Sem
r acatch
::Member
(Error
e) r =>Sem
r a > (e >Sem
r a) >Sem
r a
As you see, in the smart constructors, the m
parameter has become
.Sem
r
makeSem :: Name > Q [Dec] Source #
If T
is a GADT representing an effect algebra, as described in the
module documentation for Polysemy, $(
automatically
generates a smart constructor for every data constructor of makeSem
''T)T
. This also
works for data family instances. Names of smart constructors are created by
changing first letter to lowercase or removing prefix :
in case of
operators. Fixity declaration is preserved for both normal names and
operators.
Since: 0.1.2.0
makeSem_ :: Name > Q [Dec] Source #
Like makeSem
, but does not provide type signatures and fixities. This
can be used to attach Haddock comments to individual arguments for each
generated function.
data Output o m a where Output :: o > Output o m () makeSem_ ''Output   Output the value @o@. output :: forall o r . Member (Output o) r => o  ^ Value to output. > Sem r ()  ^ No result.
Because of limitations in Template Haskell, signatures have to follow some rules to work properly:
makeSem_
must be used before the explicit type signatures signatures have to specify argument of
Sem
representing union of effects asr
(e.g.
)Sem
r ()  all arguments in effect's type constructor have to follow naming scheme from data constructor's declaration:
data Foo e m a where FooC1 :: Foo x m () FooC2 :: Foo (Maybe x) m ()
should have x
in type signature of fooC1
:
fooC1 :: forall x r. Member (Foo x) r => Sem r ()
and Maybe x
in signature of fooC2
:
fooC2 :: forall x r. Member (Foo (Maybe x)) r => Sem r ()
 all effect's type variables and
r
have to be explicitly quantified usingforall
(order is not important)
These restrictions may be removed in the future, depending on changes to the compiler.
Change in (TODO(Sandy): version): in case of GADTs, signatures now only use names from data constructor's type and not from type constructor declaration.
Since: 0.1.2.0
Combinators for Interpreting FirstOrder Effects
:: FirstOrder e "interpret"  
=> (forall x rInitial. e (Sem rInitial) x > Sem r x)  A natural transformation from the handled effect to other effects
already in 
> Sem (e ': r) a  
> Sem r a 
The simplest way to produce an effect handler. Interprets an effect e
by
transforming it into other effects inside of r
.
:: (Member e r, FirstOrder e "intercept")  
=> (forall x rInitial. e (Sem rInitial) x > Sem r x)  A natural transformation from the handled effect to other effects
already in 
> Sem r a  
> Sem r a 
Like interpret
, but instead of handling the effect, allows responding to
the effect while leaving it unhandled. This allows you, for example, to
intercept other effects and insert logic around them.
:: forall e1 e2 r a. FirstOrder e1 "reinterpret"  
=> (forall rInitial x. e1 (Sem rInitial) x > Sem (e2 ': r) x)  A natural transformation from the handled effect to the new effect. 
> Sem (e1 ': r) a  
> Sem (e2 ': r) a 
Like interpret
, but instead of removing the effect e
, reencodes it in
some new effect f
. This function will fuse when followed by
runState
, meaning it's free to reinterpret
in terms of
the State
effect and immediately run it.
:: forall e1 e2 e3 r a. FirstOrder e1 "reinterpret2"  
=> (forall rInitial x. e1 (Sem rInitial) x > Sem (e2 ': (e3 ': r)) x)  A natural transformation from the handled effect to the new effects. 
> Sem (e1 ': r) a  
> Sem (e2 ': (e3 ': r)) a 
Like reinterpret
, but introduces two intermediary effects.
:: forall e1 e2 e3 e4 r a. FirstOrder e1 "reinterpret3"  
=> (forall rInitial x. e1 (Sem rInitial) x > Sem (e2 ': (e3 ': (e4 ': r))) x)  A natural transformation from the handled effect to the new effects. 
> Sem (e1 ': r) a  
> Sem (e2 ': (e3 ': (e4 ': r))) a 
Like reinterpret
, but introduces three intermediary effects.
rewrite :: forall e1 e2 r a. (forall rInitial x. e1 (Sem rInitial) x > e2 (Sem rInitial) x) > Sem (e1 ': r) a > Sem (e2 ': r) a Source #
Rewrite an effect e1
directly into e2
, and put it on the top of the
effect stack.
Since: 1.2.3.0
transform :: forall e1 e2 r a. Member e2 r => (forall rInitial x. e1 (Sem rInitial) x > e2 (Sem rInitial) x) > Sem (e1 ': r) a > Sem r a Source #
Transform an effect e1
into an effect e2
that is already somewhere
inside of the stack.
Since: 1.2.3.0
Combinators for Interpreting HigherOrder Effects
:: Member e r  
=> (forall x rInitial. e (Sem rInitial) x > Tactical e (Sem rInitial) r x)  A natural transformation from the handled effect to other effects
already in 
> Sem r a  Unlike 
> Sem r a 
:: forall e1 e2 r a. (forall rInitial x. e1 (Sem rInitial) x > Tactical e1 (Sem rInitial) (e2 ': r) x)  A natural transformation from the handled effect to the new effect. 
> Sem (e1 ': r) a  
> Sem (e2 ': r) a 
Like reinterpret
, but for higherorder effects.
See the notes on Tactical
for how to use this function.
:: forall e1 e2 e3 r a. (forall rInitial x. e1 (Sem rInitial) x > Tactical e1 (Sem rInitial) (e2 ': (e3 ': r)) x)  A natural transformation from the handled effect to the new effects. 
> Sem (e1 ': r) a  
> Sem (e2 ': (e3 ': r)) a 
Like reinterpret2
, but for higherorder effects.
See the notes on Tactical
for how to use this function.
:: forall e1 e2 e3 e4 r a. (forall rInitial x. e1 (Sem rInitial) x > Tactical e1 (Sem rInitial) (e2 ': (e3 ': (e4 ': r))) x)  A natural transformation from the handled effect to the new effects. 
> Sem (e1 ': r) a  
> Sem (e2 ': (e3 ': (e4 ': r))) a 
Like reinterpret3
, but for higherorder effects.
See the notes on Tactical
for how to use this function.
Combinators for Interpreting Directly to IO
:: Member (Embed IO) r  
=> ((forall x. Sem r x > IO x) > IO () > IO a)  A lambda that takes the lowering function, and a finalizing 
> Sem r a 
Run an effect stack all the way down to IO
by running it in a new
thread, and temporarily turning the current thread into an event poll.
This function creates a thread, and so should be compiled with threaded
.
Since: 0.5.0.0
Kind Synonyms
Composing IObased Interpreters
:: Monad m  
=> (forall x. Sem r x > m x)  The lowering function, likely 
> (forall y. (forall x. Sem r x > m x) > Sem (e ': r) y > Sem r y)  
> Sem (e ': r) z  
> m z 
Some interpreters need to be able to lower down to the base monad (often
IO
) in order to function properly  some good examples of this are
lowerError
and lowerResource
.
However, these interpreters don't compose particularly nicely; for example,
to run lowerResource
, you must write:
runM . lowerError runM
Notice that runM
is duplicated in two places here. The situation gets
exponentially worse the more intepreters you have that need to run in this
pattern.
Instead, .@
performs the composition we'd like. The above can be written as
(runM .@ lowerError)
The parentheses here are important; without them you'll run into operator precedence errors.
Warning: This combinator will duplicate work that is intended to be
just for initialization. This can result in rather surprising behavior. For
a version of .@
that won't duplicate work, see the .@!
operator in
polysemyzoo.
Interpreters using Final
may be composed normally, and
avoid the work duplication issue. For that reason, you're encouraged to use

interpreters instead of Final
lower
interpreters whenever
possible.
:: Monad m  
=> (forall x. Sem r x > m x)  The lowering function, likely 
> (forall y. (forall x. Sem r x > m x) > Sem (e ': r) y > Sem r (f y))  
> Sem (e ': r) z  
> m (f z) 
Like .@
, but for interpreters which change the resulting type  eg.
lowerError
.
Tactics
Higherorder effects need to explicitly thread other effects' state through themselves. Tactics are a domainspecific language for describing exactly how this threading should take place.
The first computation to be run should use runT
, and subsequent
computations in the same environment should use bindT
. Any
firstorder constructors which appear in a higherorder context may use
pureT
to satisfy the typechecker.
type Tactical e m r x = forall f. Functor f => Sem (WithTactics e f m r) (f x) Source #
Tactical
is an environment in which you're capable of explicitly
threading higherorder effect states. This is provided by the (internal)
effect Tactics
, which is capable of rewriting monadic actions so they run
in the correct stateful environment.
Inside a Tactical
, you're capable of running pureT
, runT
and bindT
which are the main tools for rewriting monadic stateful environments.
For example, consider trying to write an interpreter for
Resource
, whose effect is defined as:
dataResource
m a whereBracket
:: m a > (a > m ()) > (a > m b) >Resource
m b
Here we have an m a
which clearly needs to be run first, and then
subsequently call the a > m ()
and a > m b
arguments. In a Tactical
environment, we can write the threading code thusly:
Bracket
alloc dealloc use > do alloc' <runT
alloc dealloc' <bindT
dealloc use' <bindT
use
where
alloc' ::Sem
(Resource
': r) (f a1) dealloc' :: f a1 >Sem
(Resource
': r) (f ()) use' :: f a1 >Sem
(Resource
': r) (f x)
The f
type here is existential and corresponds to "whatever
state the other effects want to keep track of." f
is always
a Functor
.
alloc'
, dealloc'
and use'
are now in a form that can be
easily consumed by your interpreter. At this point, simply bind
them in the desired order and continue on your merry way.
We can see from the types of dealloc'
and use'
that since they both
consume a f a1
, they must run in the same stateful environment. This
means, for illustration, any put
s run inside the use
block will not be visible inside of the dealloc
block.
Power users may explicitly use getInitialStateT
and bindT
to construct
whatever data flow they'd like; although this is usually unnecessary.
type WithTactics e f m r = Tactics f m (e ': r) ': r Source #
getInitialStateT :: forall f m r e. Sem (WithTactics e f m r) (f ()) Source #
:: m a  The monadic action to lift. This is usually a parameter in your effect. 
> Tactical e m r a 
Run a monadic action in a Tactical
environment. The stateful environment
used will be the same one that the effect is initally run in.
Use bindTSimple
if you'd prefer to explicitly manage your stateful
environment.
This is a less flexible but significantly simpler variant of runT
.
Instead of returning a Sem
action corresponding to the provided action,
runTSimple
runs the action immediately.
Since: 1.5.0.0
:: forall m f r e a b. (a > m b)  The monadic continuation to lift. This is usually a parameter in your effect. Continuations executed via 
> f a  
> Sem (WithTactics e f m r) (f b) 
Lift a kleisli action into the stateful environment.
You can use bindTSimple
to execute an effect parameter of the form
a > m b
by providing the result of a runTSimple
or another
bindTSimple
.
This is a less flexible but significantly simpler variant of bindT
.
Instead of returning a Sem
kleisli action corresponding to the
provided kleisli action, bindTSimple
runs the kleisli action immediately.
Since: 1.5.0.0
:: m a  The monadic action to lift. This is usually a parameter in your effect. 
> Sem (WithTactics e f m r) (Sem (e ': r) (f a)) 
:: (a > m b)  The monadic continuation to lift. This is usually a parameter in your effect. Continuations lifted via 
> Sem (WithTactics e f m r) (f a > Sem (e ': r) (f b)) 
getInspectorT :: forall e f m r. Sem (WithTactics e f m r) (Inspector f) Source #
Get a natural transformation capable of potentially inspecting values
inside of f
. Binding the result of getInspectorT
produces a function that
can sometimes peek inside values returned by bindT
.
This is often useful for running callback functions that are not managed by polysemy code.
Example
We can use the result of getInspectorT
to "undo" pureT
(or any of the other
Tactical
functions):
ins <getInspectorT
fa <pureT
"hello" fb <pureT
True let a =inspect
ins fa  Just "hello" b =inspect
ins fb  Just True
A container for inspect
. See the documentation for getInspectorT
.
Inspector  
