Implementation of iterators

Iteration protocol implemented through two functions

• ìterate(i)
• ìterate(i, s)

Both functions return (element, state) if data available, otherwise nothing.

General strategy to implement iterators

Iterators are implemented as immutable structs. They are parametrized on (at least) input type and output element type.

struct Iter{I,O}
       rows::I
end

iterate(i::Iter) creates starting state s, and immediately calls iterate(i::Iter, s)

function iterate(i::Iter)
	 # prepare state for first iteration
	 state = ...

	 done? && return nothing
	 
	 iterate(i, state)
end

function iterate(i::Iter, s)
	 elt_s, ... = s

	 while true
	       # state transitions
	 
	 end
end

Also implement applicable methods

• Base.IteratorEltype(::Type{Iter})
• Base.eltype(::Type{Iter})
• Base.IteratorSize(::Type{Iter})
• Base.length(i::Iter)
• Base.size(i::Iter)

To support the Tables.jl interface, also implement

• Tables.istable(i::Iter)
• Tables.rowaccess(i::Iter)
• Tables.schema(i::Iter)