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----
-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-published status, which can be either unpublished or published, using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-unpublished posts with a big warning that they are provisional, so we
-just want to render the unpublished `Left` pages with another template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.