AlgebraicEffectsAttempt.elm

module Elchemy.Platoform.Eff exposing (..)


type alias IO eff =
    { getEmpty : () -> eff
    , reduce : List (eff -> eff)
    }


type Eff x y
    = Unhandled (Maybe x)
    | Handled y
    | None


eff : x -> Eff x y
eff =
    (Unhandled << Just)


type alias Effect eff =
    { getEmpty : eff -> eff
    , apply : eff -> eff
    }


type alias Prints x =
    { x | prints : Eff String () }


print : String -> Effect (Prints x)
print prompt =
    { getEmpty = \x -> { x | prints = None }
    , apply = \x -> { x | prints = eff prompt }
    }


type alias ReadsFile x =
    { x | readsFile : Eff String String }


readFile : String -> Effect (ReadsFile x)
readFile name =
    { getEmpty = \x -> { x | readsFile = None }
    , apply = \x -> { x | readsFile = eff name }
    }


empty : IO {}
empty =
    { getEmpty = always {}
    , reduce = []
    }


andThen : Effect eff -> IO eff -> IO eff
andThen a io =
    { getEmpty = a.getEmpty << io.getEmpty
    , reduce = a.apply :: io.reduce
    }


test =
    empty
        |> andThen (print "a")
        |> andThen (readFile "file.txt")
        |> handle .readsFil someFun
        |> handle .prints (Debug.log "Logging")
        |> handle .someOther -- This would fail. There was no such effect