"""

A function operation that applies an arbitrary function to the input stream

and stores the result as the last value.

"""

mutable struct Func{T,TFunc,TIsValid} <: StreamOperation

const func::TFunc

const is_valid::TIsValid

last_value::T

function Func(func::TFunc, init::T; is_valid::TIsValid=!isnothing) where {T,TFunc,TIsValid}

new{T,TFunc,TIsValid}(func, is_valid, init)

end

function Func{T}(func::TFunc, init::T; is_valid::TIsValid=!isnothing) where {T,TFunc,TIsValid}

new{T,TFunc,TIsValid}(func, is_valid, init)

end

# function Func(func::TFunc) where {TFunc}

# new{Nothing,TFunc}(func, nothing)

# end

end

@inline has_output(op::Func{Nothing}) = false

@inline has_output(op::Func{T}) where {T} = true

# no functor call overload needed, func is directly called, see StreamGraph.jl: _gen_execute_call!

# @inline function (op::Func{Nothing,TFunc})(args...; kwargs...) where {TFunc}

# op.func(args...; kwargs...)

# nothing

# end

# @inline function (op::Func{T,TFunc})(args...; kwargs...) where {T,TFunc}

# op.last_value = op.func(args...; kwargs...)

# nothing

# end

@inline is_valid(op::Func{Nothing}) = true

# @inline is_valid(op::Func{T}) where {T} = !isnothing(op.last_value)

@inline is_valid(op::Func{T,TFunc,TIsValid}) where {T,TFunc,TIsValid} = op.is_valid(op.last_value)

@inline get_state(op::Func{T}) where {T} = op.last_value