Safe Haskell | None |
---|---|

Language | Haskell2010 |

Operators meant as replacements for traditional `Sem`

type and `Member`

/
`Members`

constraints, that allow you to specify types of your actions and
interpreters in more concise way, without mentioning unnecessary details:

foo ::`Member`

(`Lift`

`IO`

) r =>`String`

->`Int`

->`Sem`

r ()

can be written simply as:

foo ::`String`

->`Int`

->`IO`

`~@>`

()

Working example with operators:

import Data.Function import Polysemy import Polysemy.Operators import Polysemy.Random data ConsoleIO m a where WriteStrLn ::`String`

-> ConsoleIO m () ReadStrLn :: ConsoleIO m`String`

ShowStrLn ::`Show`

a => a -> ConsoleIO m ()`makeSem`

''ConsoleIO -- runConsoleIO :: Member (Lift IO) r => Sem (ConsoleIO : r) a -> Sem r a runConsoleIO :: ConsoleIO : r`@>`

a ->`IO`

`~@`

r`@>`

a runConsoleIO =`interpret`

\case WriteStrLn s ->`sendM`

`$`

`putStrLn`

s ReadStrLn ->`sendM`

`getLine`

ShowStrLn v ->`sendM`

`$`

`IO`

() main = program`&`

runConsoleIO`&`

`runRandomIO`

`&`

`runM`

-- program :: Members '[Random, ConsoleIO] r => Sem r () program :: '[`Random`

, ConsoleIO]`>@>`

() program = do writeStrLn "It works! Write something:" val <- readStrLn writeStrLn`$`

"Here it is!: "`++`

val num <-`random`

@`Int`

writeStrLn`$`

"Some random number:" showStrLn num

Please keep in mind that constraints created through these operators are limited to the action they are being used on, for example:

foo :: (forall x. r`@>`

x ->`IO`

x) ->`IO`

(forall a. Foo : r`@>`

a ->`IO`

`~@`

r`@>`

a)

The first argument in the signature above won't have access to the
`(`

constraint in the result - in such cases, use a normal
constraint instead:`IO`

~@)

foo ::`Member`

(`Lift`

`IO`

) r => (forall x. r`@>`

x ->`IO`

x) ->`IO`

(forall a. Foo : r`@>`

a -> r`@>`

a)

See the documentation of specific operators for more details.

## Synopsis

- type (@>) = Sem
- type (@-) e = Sem '[e]
- type (@~) m = Sem '[Lift m]
- type (>@) es s = Members es (SemList s) => s
- type (-@) e s = Member e (SemList s) => s
- type (~@) m s = Member (Lift m) (SemList s) => s
- type (>@>) es a = forall r. Members es r => Sem r a
- type (-@>) e a = forall r. Member e r => Sem r a
- type (~@>) m a = forall r. Member (Lift m) r => Sem r a

`Sem`

operators

Infix equivalents of `Sem`

with versions for specifiying list of effects
(`@>`

), single effect (`@-`

) and single monad (`@~`

) as effects of union.
Use (`>@>`

), (`-@>`

) or (`~@>`

) instead if you are not making any
transformations on union and just want to use some members instead.

**Examples:**

`Sem`

with list of multiple effects:

foo ::`Sem`

(`State`

`Int`

: r) ()

can be written as:

foo ::`State`

`Int`

: r`@>`

()

`Sem`

with list of one effect:

foo ::`Sem`

'[`State`

`Int`

] ()

can be written as both (with the latter preferred):

foo :: '[`State`

`Int`

]`@>`

()

and:

foo ::`State`

`Int`

`@-`

()

where effect without list gets put into one automatically.

`Sem`

with **exactly** one, lifted monad:

foo ::`Sem`

'[`Lift`

`IO`

] ()

can be written simply as:

foo ::`IO`

`@~`

()

and will be automatically lifted and put into list.

`Member`

operators

Infix equivalents of `Member`

(s) constraint used directly in *return* type,
specifiying list of members (`>@`

), single member (`-@`

) or single monad
(`~@`

), meant to be paired with some of the `Sem`

operators ((`@>`

), (`@-`

)
and (`@~`

)). Use (`>@>`

), (`-@>`

) or (`~@>`

) instead if you are not making
any transformations on union and just want to use some members instead.

**Examples:**

List of multiple members:

foo ::`Members`

'[`State`

`Int`

,`Input`

`String`

] r =>`Sem`

(`Output`

[`String`

] : r) () ->`Sem`

r ()

can be written as:

foo ::`Output`

[`String`

] : r`@>`

() -> '[`State`

`Int`

,`Input`

`String`

]`>@`

r`@>`

()

One member:

foo ::`Member`

(`State`

`Int`

) r =>`Sem`

(`Output`

[`String`

] : r) () ->`Sem`

r ()

can be written as both (with the latter preferred):

foo ::`Output`

[`String`

] : r`@>`

() -> '[`State`

`Int`

]`>@`

r`@>`

()

and:

foo ::`Output`

[`String`

] : r`@>`

() ->`State`

`Int`

`-@`

r`@>`

()

**Exactly** one, lifted monad as a member:

foo ::`Member`

(`Lift`

`IO`

) r =>`Sem`

(`Output`

[`String`

] : r) () ->`Sem`

r ()

can be written simply as:

foo ::`Output`

[`String`

] : r`@>`

() ->`IO`

`~@`

r`@>`

()

# Combined operators

Joined versions of one of (`>@`

), (`-@`

), (`~@`

) and (`@>`

) with implicit,
hidden list of effects in union --- suited for actions that only use one
`Sem`

in their type.

**Examples:**

List of members over some `Sem`

:

foo ::`Members`

'[`State`

`String`

,`Input`

`Int`

] r =>`String`

->`Int`

->`Sem`

r ()

can be written as:

foo ::`String`

->`Int`

-> '[`State`

`String`

,`Input`

`Int`

]`>@>`

()

Single member:

foo ::`Member`

(`Input`

`Int`

) r =>`String`

->`Int`

->`Sem`

r ()

can be written as both (with the latter preferred):

foo ::`String`

->`Int`

-> '[`Input`

`Int`

]`>@>`

()

and:

foo ::`String`

->`Int`

->`Input`

`Int`

`-@>`

()

**Exactly** one, lifted monad as a member:

foo ::`Member`

(`Lift`

`IO`

) r =>`Sem`

r ()

can be written simply as:

foo ::`IO`

`~@>`

()