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diff --git a/examples/hakyll/tutorials/part09.markdown b/examples/hakyll/tutorials/part09.markdown deleted file mode 100644 index 4cc1d43..0000000 --- a/examples/hakyll/tutorials/part09.markdown +++ /dev/null @@ -1,104 +0,0 @@ ---- -title: CategoryBlog -what: explains how to use categories instead of tags ---- - -## Categories - -Most people familiar with "tags" will also know the concept "categories". - - - -In fact, tags are harder to implement because they have to be represented as a -many-to-many relation, and categories are a simple 1-to-many relation. - - - -This is also the reason you can "simulate" categories using tags. In this -tutorial we will adapt the blog to use categories instead of tags. Here is -[a zip file]($root/examples/categoryblog.zip) containing the files used in this -tutorial. - -## About category support - -Categories are simpler, but they are usually used in custom ways. That's why -Hakyll provides less "standard" functions to deal with them. But this gives us -another chance to learn some of the things we can do with Hakyll. - -## Reading Categories - -Tags are located in the `tags` metadata field. Since one post can only belong -in one category, a different approach was chosen here. The category of a post -is determined by the subfolder it is in. Here you see the directory layout for -our posts using categories: - - posts - |-- coding - | |-- 2009-11-05-a-first-post.markdown - | |-- 2009-11-28-a-third-post.markdown - | `-- 2009-12-04-this-blog-aint-dead.markdown - `-- random - |-- 2009-11-10-another-post.markdown - `-- 2009-12-23-almost-christmas.markdown - -Because we find all our posts in different subdirectories, sorting them is a -little harder: we still want them sorted by date, so it boils down to sorting -them by "base name". I hope it does not surprise you Hakyll provides a function -for that: - -~~~~~{.haskell} -postPaths <- liftM (reverse . sortByBaseName) - (getRecursiveContents "posts") -~~~~~ - -We reverse them again, because we want the most recent posts first. Now, we can -use the `readCategoryMap` function instead of `readTagMap`, which has the same -signature, but assigns categories based on the folders the posts are in. - -~~~~~{.haskell} -categoryMap <- readCategoryMap "categoryMap" renderablePosts -~~~~~ - -The rest of the `hakyll.hs` is very similar to the one in the previous -tutorial, except we want to render a category list instead of a tag cloud. - -## Rendering a category list - -Because rendering a category list is quite easy, and it would be hard to -write a "general" function for this, hakyll does not provide such a function -- -but it is not hard to write. First, we write an auxiliary function that produces -a list item for one category: - -~~~~~{.haskell} -categoryListItem category posts = - "<li>" ++ link category (categoryToUrl category) - ++ " - " ++ show (length posts) ++ " items.</li>" -~~~~~ - -This is nothing more that some basic string concatenation to create a `li` HTML -element. The function that applies this on every element in the `TagMap` is more -interesting: - -~~~~~{.haskell} -categoryList :: HakyllAction TagMap String -categoryList = arr $ uncurry categoryListItem <=< toList -~~~~~ - -This function might seem a little harder to understand if you are not familiar -with the `<=<` operator -- but it's just right-to-left monad composition in the -list monad. `uncurry categoryListItem <=< toList` is a pure function we want to -execute on the `TagMap`. But this is not possible in Hakyll[^1]. We need to make -an arrow of this function. The `arr` function solves this problem easily. - -[^1]: This is a feature, not a bug. It helps dependency handling. - -We then add this to our index page, and we are done. Feel free to hack around -with the source code. If you still have questions, feel free to ask them at the -[google discussion group](http://groups.google.com/group/hakyll). - -## The gist of it - -- Hakyll supports categories as well as tags. -- Tags are actually a generalization of categories. -- Use `readCategoryMap` to read categories. -- You need to write some custom functions to render category lists etc. |