Novice Struggles with Haskell Syntax

Posted on July 3, 2014 by Richard Goulter
Tags: ,

I’ve been trying to augment my Hakyll Site with “next post”, “previous post” links in a post. There’s a post in Hakyll Google group asking for the same thing. (Maybe the majority of Hakyll owners just “adapt” from other Hakyll sites…).

I’ve run into trouble trying to do this. Hakyll.Web.Template.Context source was a useful start.
But the trouble I ran into due to lack of experience, or understanding, of some (fundamental) Haskell concepts.

Ugh.

In this excerpt, postContext is out of scope:

    match postsGlob $ do
        route $ setExtension "html"
        compile $ do 
            -- Cyclic dependencies here here:
            identifiers <- getMatches postsGlob
            urls <- urlsOfPosts =<< recentFirst =<< return [Item identifier "" | identifier <- identifiers]

            let postContext =
                    field "nextPost" (nextPostUrl urls) `mappend`
                    postCtxWithTags tags

            pandocCompiler
            >>= saveSnapshot "teaser"
            >>= loadAndApplyTemplate "templates/post.html"    postContext
            >>= loadAndApplyTemplate "templates/default.html" postContext
            >>= relativizeUrls

(Yeah, I know, it’s a heinously long line).
Whereas in this excerpt, it’s in scope:

    match postsGlob $ do
        route $ setExtension "html"
        compile $ do 
            -- Cyclic dependencies here here:
            identifiers <- getMatches postsGlob
            urls <- urlsOfPosts =<< recentFirst =<< return [Item identifier "" | identifier <- identifiers]

            let postContext =
                    field "nextPost" (nextPostUrl urls) `mappend`
                    postCtxWithTags tags

            pandocCompiler
                >>= saveSnapshot "teaser"
                >>= loadAndApplyTemplate "templates/post.html"    postContext
                >>= loadAndApplyTemplate "templates/default.html" postContext
                >>= relativizeUrls

I guess this is why people don’t like Python or other non-C syntaxes where indentation matters.
I was able to figure that out, and considering this excerpt:

    urlOfPost :: Item String -> Compiler String
    urlOfPost =
        fmap (maybe empty $ toUrl) . getRoute . itemIdentifier

But urlsOfPosts (used in above excerpt) has type urlsOfPosts :: [Item String] -> Compiler [String].
While it’s possible to understand urlOfPost by looking at the documentation of each part, it’s not clear to me the implementation of urlsOfPosts.

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