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diff --git a/web/tutorials/05-arrows.markdown b/web/tutorials/05-arrows.markdown deleted file mode 100644 index 3f2e244..0000000 --- a/web/tutorials/05-arrows.markdown +++ /dev/null @@ -1,223 +0,0 @@ ---- -title: Arrow Magic: Metadata Dependent Page Generation -author: Florian Hars ---- - -## Supporting a "published: false" attribute on pages - -Many content management systems or blog platforms support -some kind of workflow that display articles differently or -not at all depending on which state the article is in, for -example whether it has a "published" attribute or not. -Hakyll has no built-in support for anything like this, but since -its compilers are just arrows, it is easy to implement arbitrary -metadata dependent behaviour for rendering pages. - -Let's start by adding support for a "published" attributes to the -`simpleblog` example. We want to consider a blog post published if it -has a `published` metadata element that does not have the value -`false`. A function to test for this is simple: - -~~~~~{.haskell} -isPublished :: Page a -> Bool -isPublished p = - let published = getField "published" p in - published /= "" && published /= "false" -~~~~~ - -The next step is to write a function that tags a page with its -published status, which can be either unpublished or published, using -the standard `Either` datatype and then transform this function -into a `Compiler`. The latter can be done with the standard `arr` -function from `Control.Arrow`, which lifts a function into an arrow: - -~~~~~{.haskell} -isPagePublished :: Compiler (Page a) (Either (Page a) (Page a)) -isPagePublished = arr (\p -> if isPublished p then Right p else Left p) -~~~~~ - -For the next processing steps we now need a compiler that takes an -`Either (Page a) (Page a)` instead of the usual `Page a` as an -input. But the former can be built up from the latter using some -standard combinators from the `Control.Arrow` library. The simplest -one is `|||`, which takes two compilers (arrows) with the same output -type and returns a new compiler that takes an `Either` of the input -types of the Compilers as an input. Maybe we just want to render our -unpublished posts with a big warning that they are provisional, so we -just want to render the unpublished `Left` pages with another template -than the published `Right` pages: - -~~~~~{.haskell} - match "posts/*" $ do - route $ setExtension ".html" - compile $ pageCompiler - >>> isPagePublished - >>> (applyTemplateCompiler "templates/embargo-post.html" - ||| applyTemplateCompiler "templates/post.html") - >>> applyTemplateCompiler "templates/default.html" - >>> relativizeUrlsCompiler -~~~~~ - -With the conditional rendering in place, the next step is to hide -the unpublished posts from the homepage and the list of posts. -Both lists are generated from the results of a requireAllA call. -The last argument of requireAllA is a Compiler, and requireAllA -passes a pair consisting of the currently rendered page and a list -of all the required pages. All we have to do to suppress the pages -is to write a Compiler that takes such a pair as input, leaves the -first element of the pair unchanged and filters out all the unpublished -pages from list in the second element of the pair and then pass the -output from this compiler to the existing compilers handling the -list generation for the `index` and `posts` pages. - -Again, we can use a function from `Control.Arrow` to build this -compiler from simpler ones, in this case it is `***`, which combines -two arrows to one arrow from pairs to pairs. For our purposes, we -combine the identity arrow, which leaves its input unchanged, and an -ordinary `filter` on a list lifted into the compiler arrow: - -~~~~~{.haskell} -filterPublished :: Compiler (Page a, [Page b]) (Page a, [Page b]) -filterPublished = id *** arr (filter isPublished) -~~~~~ - -All that remains to do is to chain this compiler in front of the existing -compilers passed to requireAllA in the code for `posts.html` - -~~~~~{.haskell} - >>> requireAllA "posts/*" (filterPublished >>> addPostList) -~~~~~ - -and for `index.html`: - -~~~~~{.haskell} - >>> requireAllA "posts/*" - (filterPublished - >>> (id *** arr (take 3 . reverse . sortByBaseName)) - >>> addPostList) -~~~~~ - -You may have noticed that the code for the index page uses the same -`id *** something` construct to extract some elements from the list -of all posts. - -## Don't generate unpublished pages at all - -The above code will treat unpublished posts differently and hide them -from all lists of posts, but they will still be generated, and someone -who knows their URLs will still be able to access them. That may be -what you need, but sometimes you might want to suppress them -completely. The simplest way to do so is to leave the rendering -pipeline for `"posts/*"` unchanged and just add the `isPagePublished` -compiler at the end. This will not compile, since hakyll knows how to -write a `Page String`, but not how to write an `Either (Page String) -(Page String)`. But that can be amended by a simple type class -declaration: - -~~~~~{.haskell} -instance Writable b => Writable (Either a b) where - write p (Right b) = write p b - write _ _ = return () -~~~~~ - -Now hakyll will happily generate published pages and ignore -unpublished ones. This solution is of course slightly wasteful, as at -will apply all the templates to an unpublished page before finally -discarding it. You can avoid this by using the `+++` function, which -does for the sum datatype `Either` what `***` does for the product -type pair: - -~~~~~{.haskell} - match "posts/*" $ do - route $ setExtension ".html" - compile $ pageCompiler - >>> isPagePublished - >>> (id +++ (applyTemplateCompiler "templates/post.html" - >>> applyTemplateCompiler "templates/default.html" - >>> relativizeUrlsCompiler)) -~~~~~ - -The other problem with this solution is more severe: hakyll will no -longer generate the index and posts pages due to a rare problem in -haskell land: a runtime type error. Hakyll tries to be smart and reuse -the parsed pages from the `match "posts/*"` when processing the -`requireAllA "posts/*"` calls by caching them. But the compilers there -still expect a list of pages instead of a list of eithers, so we have -to replace `filterPublised` with something that works on the -latter. Luckily (or, probably, by design), `Data.Either` provides just -the function we need, so the new filtering compiler is actually -shorter that the original, even though it has a more intimidating -type: - -~~~~~{.haskell} -filterPublishedE :: Compiler (Page a, [Either (Page b) (Page b)]) (Page a, [Page b]) -filterPublishedE = id *** arr rights -~~~~~ - -## Timed releases - -Exploiting the fact that compilers are arrows, we can do more mixing -and matching of compilers to further refine how hakyll deals with page -attributes like `published`. Maybe you want `cron` to update your blog -while you are on vacation, so you want posts to be considered -published if the `published` field is either `true` or a time in the -past. - -If you happen to live in the UK in winter or enjoy to do time zone -calculation in your head, your new function to test if a page is -published and the compiler derived from it might then look like this - -~~~~~{.haskell} -isPublishedYet :: Page a -> UTCTime -> Bool -isPublishedYet page time = - let published = getField "published" page in - published == "true" || after published - where - after published = - let publishAt = parseTime defaultTimeLocale "%Y-%m-%d %H:%M" published in - fromMaybe False (fmap (\embargo -> embargo < time) publishAt) - -isPagePublishedYet :: Compiler (Page a, UTCTime) (Either (Page a) (Page a)) -isPagePublishedYet = arr (\(p,t) -> if isPublishedYet p t then Right p else Left p) -~~~~~ - -This compiler has a pair of a page and a time as its input, and we can -use yet another function from `Control.Arrow` to construct a compiler -that generates the input for it, the function `&&&`. It takes two -compilers (arrows) with the same input type and constructs a compiler -from that type to a pair of the output types of the two compilers. -For the first argument we take the `pageCompiler` which we already -call at the beginning of the page compilation. The second argument -should be a compiler with the same input type as `pageCompiler` that -returns the current time. But the current time lives in the IO monad -and does not at all depend on the resource the current page is -generated from, so we have to cheat a little bit by calling -`unsafeCompiler` with a function that ignores its argument and returns -an `IO UTCTime`, which `unsafeCompiler` will unwrap for us: - -~~~~~{.haskell} - match "posts/*" $ do - route $ setExtension ".html" - compile $ (pageCompiler &&& (unsafeCompiler (\_ -> getCurrentTime))) - >>> isPagePublishedYet - >>> (id +++ ( ... as above ...)) -~~~~~ - -This is all we have to change if we don't generate unpublished pages -at all. If we just hide them from the lists, the call to `|||` -discards the information that a page is not (yet) published that was -encoded in the `Either`. In that case we could use the `setField` -function from `Hakyll.Web.Page.Metadata` to rewrite the `published` -field of the left and right pages in `isPagePublished(Yet)` to -canonical values that the original `isPublished` function called from -`filterPublished` understands: - -~~~~~{.haskell} -isPagePublishedYet = arr (\(p,t) -> if isPublishedYet p t then pub p else unpub p) - where - pub p = Right $ setField "published" "true" p - unpub p = Left $ setField "published" "false" p -~~~~~ - -The final version of this code can be found in the timedblog example, -together with the required `import` statements. |