{"id":108640,"date":"2023-08-23T07:00:00","date_gmt":"2023-08-23T14:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=108640"},"modified":"2023-08-23T07:07:09","modified_gmt":"2023-08-23T14:07:09","slug":"20230823-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20230823-00\/?p=108640\/","title":{"rendered":"On writing loops in PPL and continuation-passing style, part 2"},"content":{"rendered":"

Last time, we came up with task-based while<\/code> loop<\/a> that involved creating a custom callable that passed copies of itself to the next iteration.<\/p>\n

This time, we’ll implement the function in terms of a more traditional recursion.<\/p>\n

template<typename Callable>\r\ntask<void> do_while_task(\r\n    std::shared_ptr<Callable> const& f)\r\n{\r\n    return (*f)().then([f](bool loop) {\r\n        return loop ? do_while_task(f) :\r\n                      task_from_result();\r\n    });\r\n}\r\n\r\ntemplate<typename Callable, typename =\r\n    std::enable_if_t<std::is_invocable_v<Callable>>>\r\ntask<void> do_while_task(Callable&& callable)\r\n{\r\n    using Decayed = std::decay_t<Callable>;\r\n    return do_while_task(\r\n        std::make_shared<Decayed>(\r\n            std::forward<Callable>(callable)));\r\n}\r\n<\/pre>\n
The real work happens in the first overload, which takes a ready-made shared_ptr<\/code>. The second overload is a convenience method that lets you pass a callable, and it will wrap it in a shared_ptr<\/code> for you.<\/p>\n
However, if you think about it, in PPL-style programming, the lambda callable itself usually holds a pointer to shared state, so that the various task fragments can share information with each other. Let’s look again at my original example.<\/p>\n
do_while_task([i = 0, widgets]() mutable\r\n{\r\n    if (i >= 3) return task_from_result(false);\r\n    return create_widget().then([index = i++, widgets](auto widget)\r\n    {\r\n        widgets[index] = widget;\r\n        return true;\r\n    });\r\n}).then([] {\r\n    printf(\"Done!\\n\");\r\n});\r\n<\/pre>\n
I cheated here and incremented the i<\/code> variable inside the first lambda, but capturing the unincremented value as index<\/code> for the second lambda. More realistically, the two lambdas need to share state.<\/p>\n
struct lambda_state                                  <\/span>\r\n{                                                    <\/span>\r\n    lambda_state(Widgets* w) : widgets(w) {}         <\/span>\r\n    Widgets* widgets;                                <\/span>\r\n    int i = 0;                                       <\/span>\r\n};                                                   <\/span>\r\n\u00a0                                                    <\/span>\r\nauto state = std::make_shared<lambda_state>(widgets);<\/span>\r\n\r\ndo_while_task([state<\/span>]()\r\n{\r\n    if (state-><\/span>i >= 3) return task_from_result(false);\r\n    return create_widget().then([state<\/span>](auto widget)\r\n    {\r\n        state-><\/span>widgets[state-><\/span>i] = widget;\r\n        state-><\/span>i++;\r\n        return true;\r\n    }\r\n}).then([] {\r\n    printf(\"Done!\\n\");\r\n});\r\n<\/pre>\n
This means that our do_while_task<\/code> creates a shared_ptr<\/code> that itself holds another shared_ptr<\/code>, which seems kind of silly.<\/p>\n
We’ll address this next time.<\/p>\n","protected":false},"excerpt":{"rendered":"
Recursion comes back.<\/p>\n","protected":false},"author":1069,"featured_media":111744,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[25],"class_list":["post-108640","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
Recursion comes back.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/108640","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/users\/1069"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/comments?post=108640"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/108640\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media\/111744"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media?parent=108640"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=108640"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=108640"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}