Safe Haskell  None 

Language  Haskell2010 
Synopsis
 newtype Sem r a = Sem {}
 type Member e r = MemberNoError e r
 type family Members es r :: Constraint where ...
 send :: Member e r => e (Sem r) a > Sem r a
 embed :: Member (Embed m) r => m a > Sem r a
 run :: Sem '[] a > a
 runM :: Monad m => Sem '[Embed m] a > m 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
 subsume :: Member e r => Sem (e ': r) a > Sem r a
 newtype Embed m (z :: Type > Type) a where
 usingSem :: Monad m => (forall x. Union r (Sem r) x > m x) > Sem r a > m a
 liftSem :: Union r (Sem r) a > Sem r a
 hoistSem :: (forall x. Union r (Sem r) x > Union r' (Sem r') x) > Sem r a > Sem r' a
 type InterpreterFor e r = forall a. Sem (e ': r) a > Sem r a
 (.@) :: 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)
Documentation
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 tradtionally as an Either
, or instead
significantly faster as an IO
Exception
. These
interpretations (and others that you might add) may be used interchangably
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 
type Member e r = MemberNoError e r Source #
A proof that the effect e
is available somewhere inside of the effect
stack r
.
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
embed :: Member (Embed m) r => m a > Sem r a Source #
Embed a monadic action m
in Sem
.
Since: 1.0.0.0
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.
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 two new effects underneath the head of the
list.
Since: 1.2.0.0
subsume :: 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
.
Since: 1.2.0.0
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
usingSem :: Monad m => (forall x. Union r (Sem r) x > m x) > Sem r a > m a Source #
Like runSem
but flipped for better ergonomics sometimes.
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
:: 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.