Last (git) modification time in Hakyll

While working on this site, I wanted to list metadata for each page describing both the time the page was originally written, and the date on which it was last modified. The first is fairly easy, and is included in Hakyll by default: Given a page at a path such as: writing/2018-08-03-last-git-modification-time-in-haskyll.markdown, the Hakyll compiler will extract the 2018-08-03 portion of the filepath, and fill in the $date$ portion of the template with the formatted date.

The second, listing the modification time, was somewhat more difficult, and took a bit of hacking to get working nicely. To start with, we need some way to extract the modification times. This is performed by the following method:

buildModifications ::  Pattern -> Rules [(Identifier, String)]
buildModifications pattern = do 
    ids <- getMatches pattern
    pairs <- preprocess $ foldM getLastModified [] ids
    return pairs
    where 
        getLastModified l id' = do
            t <- readProcess "git" 
                ["log", "-1", "--format=%ad", "--date=format:%b %d, %Y", (toFilePath id')] ""
            return $ (id', t) : l

Essentially, this function takes a pattern (describing a list of files) and, for each of them, runs the git executable to find when the file was last modified according to git. We don’t really care about the actual modification time, but when we officially “committed” the changes to the site. Finally, we wrap the result in a Rules monad, which implements the MonadMetadata typeclass, smoothly integrating pattern matching and IO (through judicious use of preprocess).

Once we have the modification times, we need to use a context to describe how, and when we should substitute in the times:

modificationCtx :: [(Identifier, String)] -> Context String 
modificationCtx modificationTimes = field "lastModified" $ \item -> do
    let time = find (\x -> (fst x) == (itemIdentifier item)) modificationTimes >>= return . snd 
    return $ fromMaybe "no recent modifications" $ time

This method is fairly straightforward: it takes a given item, looks up the lastModified field in the body, looks up the time (if there is one) of the identifier of the item, and returns a context replacing lastModified with the lookup operation on the item.

For fun, here’s a simple context where we can simply insert the path of the item’s identifier where requested:

githubCtx :: Context String 
githubCtx = field "filepath" $ \item -> do 
    let path = (toFilePath . itemIdentifier) item
    return $ path

To see them both in action, check out the site source.