Post banner images with Hakyll

This post describes a proof-of-concept Hakyll site which associates a banner image with each post, automatically generates several versions of it, and makes them available as variables for use in templates. Posted by Thomas Sutton on October 25, 2013

I’ve just created a new hakyll-banner-images-demo repository on GitHub which demonstrates how your Hakyll site can:

  1. Associate a banner image with each post.

  2. Generate multiple versions (sizes) of these banner images.

  3. Provide the generated images in variables for use in post templates.

Each post in the site is represented by a directory containing an Markdown file and, optionally, a banner.png image file. This repository contains two posts, one with and one without an image:


The site.hs Haskell program contains only the most basic Hakyll directives to process the Markdown files (indeed, I haven’t even bothered to override the date code; you’ll need to put the date in the Markdown file metadata). In addition, it contains the proof-of-concept code to handle the banner images.

The imageProcessor function is a helper to construct the Hakyll Rules () to process a set of banner images and the Context a allowing them to be used in the associated posts and templates. This function takes a Pattern which matches the banner images and a list describing the different versions to generate of each image.

The first argument to imageProcessor is the pattern identifying the image to be processed. This pattern must end in a full filename. The second argument is a list of versions to create. Each version has a name (the String) and image processing instructions (Nothing to copy the image, Just (x,y) to scale and crop the image to the given dimensions using ImageMagick’s convert command). This generates a Rules () value to process the images (postImages in the code) and a Context a to make them available in posts and templates.

The Rules () value can be “run” just like any create or match statement and the Context a value can be used in the context of post with a path that matches the image pattern (ignoring the filename). This context defines one variable for each of the image versions being generated. In this code, the variables are:

  • banner-small is a 200x100 image.
  • banner-medium is a 600x300 image.
  • banner-full is the original image.

These names are generated from the filename banner.png in the image pattern (this is why the pattern must have a filename) and the name of each version.

With this configuration, the posts included in the repository result in the following files being generated:


As expected, each post has an index.html file and the single banner image has small, medium, and full versions.

The implementation of imageField and the assumption that the pattern ends in a file name are pretty crappy but, overall, I’m quite happy with this approach.

This post was published on October 25, 2013 and last modified on December 4, 2018. It is tagged with: howto, hakyll, images, imagemagick, haskell.