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defmodule Meeseeks.Accumulator do
@moduledoc """
Accumulator structs package some means of storing nodes selected during
the selection process along with a method for checking if the selection
should be terminated early and a method for returning the stored nodes.
Users will not typically need to define, or even know about, accumulators,
but for users trying to do things like return selected nodes as Document
nodes (or even tuple-tree nodes) instead of as Results, a custom accumulator
will provide the solution.
"""
alias Meeseeks.{Accumulator, Document}
@type t :: struct
@doc """
Invoked to add a selected node to the accumulator.
"""
@callback add(accumulator :: t, document :: Document.t(), id :: Document.node_id()) :: t
@doc """
Invoked to determine if the accumulator is satisfied.
"""
@callback complete?(accumulator :: t) :: boolean
@doc """
Invoked to return the accumulation.
"""
@callback return(accumulator :: t) :: any
# add
@doc """
Provided a document and a node id, returns an updated accumulator.
"""
@spec add(t, Document.t(), Document.node_id()) :: t
def add(%{__struct__: struct} = accumulator, document, id) do
struct.add(accumulator, document, id)
end
# complete?
@doc """
Checks if an accumulator has reached an arbitrary state of completion and
no longer wishes to be added to.
"""
@spec complete?(t) :: boolean
def complete?(%{__struct__: struct} = accumulator) do
struct.complete?(accumulator)
end
# return
@doc """
Returns the values accumulated by the accumulator.
"""
@spec return(t) :: any
def return(%{__struct__: struct} = accumulator) do
struct.return(accumulator)
end
# __using__
@doc false
defmacro __using__(_) do
quote do
@behaviour Accumulator
@impl Accumulator
def add(_, _, _), do: raise("add/3 not implemented")
@impl Accumulator
def complete?(_), do: false
@impl Accumulator
def return(_), do: raise("return/1 not implemented")
defoverridable add: 3, complete?: 1, return: 1
end
end
end
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