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1+---
2+title: Tags and manipulations
3+what: enhances our blog with tags and explains context manipulations.
4+---
5+
6+## Context manipulations
7+
8+Here, have [a zip file](examples/tagblog.zip) for this tutorial.
9+
10+You probably remember that `Context` objects are just key-value mappings. We can
11+render those with templates, and then the `$key`'s in the template get
12+substituted by the appropriate values. This is a rather flexible system, but
13+there are limitations. Some of these limitations can be solved using
14+_context manipulations_.
15+
16+Like rendering actions, _context manipulations_ are also simply
17+`HakyllAction Context Context` arrows. The `Text.Hakyll.ContextManipulations`
18+contains some functions to easily construct easy variants.
19+
20+One of the most general functions is the `renderValue` function. Let's have a
21+look at it's type.
22+
23+~~~~~{.haskell}
24+renderValue :: String
25+ -> String
26+ -> (String -> String)
27+ -> HakyllAction Context Context
28+~~~~~
29+
30+This is the preferred way of creating context manipulations. The first argument
31+is the `key` to manipulate. The second argument is the `key` where the new value
32+should be placed. If this is the same as the first argument, it will be
33+replaced. The third argument is the function to manipulate the `value` with.
34+
35+As a simple example, let's write a function that puts the `$title` in uppercase.
36+
37+~~~~~{.haskell}
38+import Data.Char (toUpper)
39+
40+titleUpper :: HakyllAction Context Context
41+titleUpper = renderValue "title" "title" $ map toUpper
42+~~~~~
43+
44+Because the destination `key` is the same as the source `key`, we can also use
45+the `changeValue` function here.
46+
47+~~~~~{.haskell}
48+titleUpper = changeValue "title" $ map toUpper
49+~~~~~
50+
51+For further reading, refer to the `Text.Hakyll.ContextManipulations`
52+documentation.
53+
54+## Applying Context Manipulations
55+
56+Because we're dealing with Arrows again, we can use `>>>` to apply our
57+manipulations. For example, we could use or title manipulation like this:
58+
59+~~~~~{.haskell}
60+renderChain ["templates/default.html"]
61+ (createPage "index.markdown" >>> titleUpper)
62+~~~~~
63+
64+## Rendering dates
65+
66+As you remember, in our previous blog, all posts had a file name like
67+`posts/yyyy-mm-dd-title.extension`, as is the Hakyll convention. But they also
68+had a metadata field `date`, containing a human-readable date. This is not very
69+D.R.Y., of course! Hakyll has a specialized `renderValue` function to deal with
70+dates encoded in paths: `renderDate`.
71+
72+~~~~~{.haskell}
73+postManipulation :: HakyllAction Context Context
74+postManipulation = renderDate "date" "%B %e, %Y" "Unknown date"
75+~~~~~
76+
77+That manipulation will:
78+- Read the date from the file name the post was loaded from.
79+- Parse the date and render it in a `%B %e, %Y` format. This is a
80+ `Month day, Year` format.
81+- Put the result in the `date` metadata field.
82+- If the date could not be parsed, it will put `"Unknown date"` in the `date`
83+ metadata field.
84+
85+So, we can throw away our `date: ` lines from our posts, and still use `$date`
86+in our templates.
87+
88+## Abstracting the post list
89+
90+Now, we're going to render tags. This is also done using context manipulations.
91+Hakyll has a specialized module to deal with tags, provided by
92+`Text.Hakyll.Tags`. This module assumes tags are comma separated, and placed in
93+the `tags` metadata field.
94+
95+ ---
96+ title: A third post
97+ author: Publius Ovidius Naso
98+ tags: epic fail, ovidius
99+ ---
100+ Pellentesque tempor blandit elit, vel...
101+
102+But first things first. We need to render a post list for every tag. We already
103+had some code to render a list of all posts. We're just going to abstract this
104+code into a more general function:
105+
106+~~~~{.haskell}
107+renderPostList url title posts = do
108+ let list = createListingWith url ["templates/postitem.html"]
109+ posts [("title", Left title)]
110+ renderChain ["posts.html", "templates/default.html"] list
111+~~~~~
112+
113+Our "render all posts" action can now be written as:
114+
115+~~~~~{.haskell}
116+renderPostList "posts.html" "All posts" renderablePosts
117+~~~~~
118+
119+## Tag links
120+
121+We want to display the tags for our post under the title. But if we use the
122+`$tags` key in a template, we will just have the plain tags - they will not be
123+clickable. We can again solve this with a `ContextManipulation`. We have a
124+function that produces an url for a given tag:
125+
126+~~~~~{.haskell}
127+tagToUrl tag = "$root/tags/" ++ removeSpaces tag ++ ".html"
128+~~~~~
129+
130+`removeSpaces` is an auxiliary function from `Text.Hakyll.File`. Now, there is
131+a specialized `renderValue` function for creating linked tags called
132+`renderTagLinks`. This function simply takes a function that produces an url
133+for a given tag - the function we just wrote. Let's extend our
134+`postManipulation`.
135+
136+~~~~~{.haskell}
137+postManipulation :: HakyllAction Context Context
138+postManipulation = renderDate "date" "%B %e, %Y" "Unknown date"
139+ >>> renderTagLinks tagToUrl
140+~~~~~
141+
142+We apply this manipulation when we load the tags.
143+
144+~~~~~{.haskell}
145+let renderablePosts =
146+ map ((>>> postManipulation) . createPage) postPaths
147+~~~~~
148+
149+So, the `renderTagLinks` function replaces the `$tags` value from
150+`epic fail, random` to `<a href="$root/tags/epic-fail.html">epic fail</a>, ...`.
151+If we click a tag, we get a `404`. That's because we haven't generated the
152+post lists for every tag.
153+
154+## The Tag Map
155+
156+Hakyll provides a function called `readTagMap`. Let's inspect it's type.
157+
158+~~~~~{.haskell}
159+type TagMap = Map String [HakyllAction () Context]
160+readTagMap String [FilePath] -> HakyllAction () TagMap
161+~~~~~
162+
163+You give it a list of paths, and it creates a map that, for every tag, holds
164+a number of posts. We can easily use this to render a post list for every tag.
165+The first argument given is an "identifier", unique to this tag map. Hakyll
166+needs this so it can cache the tags.
167+
168+~~~~~{.haskell}
169+let tagMap = readTagMap "postTags" postPaths
170+~~~~~
171+
172+When we have the `TagMap`, we can need to render a post list for every tag.
173+There is a function in Hakyll designed more or less for this purpose:
174+`withTagMap`. This takes a `TagMap` and an action to execute for every tag and
175+it's associated posts. We pass a small function to it we create ourselves[^1]:
176+
177+[^1]: Exercise for the reader: why do we use `>>> postManipulation` again here?
178+
179+~~~~~{.haskell}
180+let renderListForTag tag posts =
181+ renderPostList (tagToUrl tag)
182+ ("Posts tagged " ++ tag)
183+ (map (>>> postManipulation) posts)
184+withTagMap tagMap renderPostList
185+~~~~~
186+
187+There we go. We now have clickable tags, and a post list for every tag.
188+
189+## A Tag Cloud
190+
191+A tag cloud is a commonly found thing on blogs. Hakyll also provides code to
192+generate a tag cloud. Let's have a look at the `renderTagCloud` function.
193+
194+~~~~~{.haskell}
195+renderTagCloud :: (String -> String)
196+ -> Float
197+ -> Float
198+ -> HakyllAction TagMap String
199+~~~~~
200+
201+The first argument is, once again, a function to create an url for a given tag.
202+Then, we give a minimum and a maximum font size in percent, and we get a tag
203+cloud Arrow back. We can add this to our index:
204+
205+~~~~~{.haskell}
206+let tagCloud = tagMap >>> renderTagCloud tagToUrl 100 200
207+ index = createListing "index.html"
208+ ["templates/postitem.html"]
209+ (take 3 renderablePosts)
210+ [ ("title", Left "Home")
211+ , ("tagcloud", Right tagCloud)
212+ ]
213+renderChain ["index.html", "templates/default.html"] index
214+~~~~~
215+
216+## The gist of it
217+
218+- There's some handy, simple functions in `Text.Hakyll.ContextManipulations`.
219+- Seperate tags by commas and put them in the `$tags` field.
220+- Use `withTagMap` to render a list for every tag.
221+- Hakyll can also create tag clouds.