迭代器模式

正文：https://refactoringguru.cn/design-patterns/iterator

Go代码：https://refactoringguru.cn/design-patterns/iterator/go/example

module iterator_module

    use, intrinsic :: iso_fortran_env, only: int8
    implicit none
    private

    public :: user_type, user_collection_type, user_iterator_type, iterator_type

    !> Abstract types

    !> Collection
    type, abstract :: collection_type
    contains
        procedure(collection_type_create_iterator), deferred :: create_iterator
    end type collection_type

    !> Iterator
    type, abstract :: iterator_type
    contains
        procedure(iterator_t_has_next), deferred :: has_next
        procedure(iterator_t_get_next), deferred :: get_next
    end type iterator_type

    !> User
    type user_type
        character(:), allocatable :: name
        integer(int8) :: age
    end type user_type

    abstract interface

        function collection_type_create_iterator(self) result(iterator)
            import iterator_type, collection_type
            !> TODO:
            class(collection_type), intent(in) :: self
            class(iterator_type), allocatable :: iterator
        end function collection_type_create_iterator

        logical function iterator_t_has_next(self)
            import iterator_type
            class(iterator_type), intent(in) :: self
        end function iterator_t_has_next

        type(user_type) function iterator_t_get_next(self)
            import user_type, iterator_type
            class(iterator_type), intent(inout) :: self
        end function iterator_t_get_next

    end interface

    !> Specific types

    !> User collection
    type, extends(collection_type) :: user_collection_type
        type(user_type), allocatable :: users(:)
    contains
        procedure :: create_iterator => user_collection_t_create_iterator
    end type user_collection_type

    !> User iterator
    type, extends(iterator_type) :: user_iterator_type
        integer :: index
        type(user_type), allocatable :: users(:)
    contains
        procedure :: has_next => user_iterator_t_has_next
        procedure :: get_next => user_iterator_t_get_next
    end type user_iterator_type

contains

    function user_collection_t_create_iterator(self) result(iterator)
        class(user_collection_type), intent(in) :: self
        class(iterator_type), allocatable :: iterator
        ! TODO:
        iterator = user_iterator_type(index=0, users=self%users)
    end function user_collection_t_create_iterator

    logical function user_iterator_t_has_next(self) result(has)
        class(user_iterator_type), intent(in) :: self

        has = merge(.true., .false., self%index < size(self%users))

    end function user_iterator_t_has_next

    type(user_type) function user_iterator_t_get_next(self) result(user)
        class(user_iterator_type), intent(inout) :: self

        self%index = self%index + 1
        user = self%users(self%index)

    end function user_iterator_t_get_next

end module iterator_module

program iterator_main

    use, intrinsic :: iso_fortran_env, only: int8
    use iterator_module, only: user_type, user_collection_type, user_iterator_type, iterator_type

    type(user_type) :: user1, user2, user
    type(user_collection_type) :: user_collection
    ! TODO:
    class(iterator_type), allocatable :: iterator

    user1 = user_type(name="A", age=30_int8)
    user2 = user_type(name="B", age=20_int8)

    user_collection = user_collection_type(users=[user1, user2])

    !> Specific iterator
    allocate (user_iterator_type :: iterator)
    iterator = user_collection%create_iterator()

    do while (iterator%has_next())
        user = iterator%get_next()
        print "(3A,I3)", "User is ", user%name, ", age is ", user%age
    end do

    deallocate (iterator)

end program iterator_main

!> Results shall be:

!  User is A, age is  30
!  User is B, age is  20