Brtdku

I am trying to use hakyll and hakyll-images to implement an example from the hakyll-images Readme which performs an image scaling as I will need to do. The types do not unify for the given example and I am seeking advice on how to proceed.

The failing example from the hakyll-images Readme is below.

import Hakyll

import Hakyll.Images        ( loadImage

                            , scaleImageCompiler

                            )

main = hakyll $ do

    -- Scale images to fit within a 600x400 box

    -- Aspect ratio will be preserved

    match "images/*" $ do

        route idRoute

        compile $ loadImage

            >>= scaleImageCompiler 600 400

Attempting to compile gives an error:

site.hs:12:9: error:

    • No instance for (Writable

                         hakyll-images-0.3.1:Hakyll.Images.Common.Image)

        arising from a use of ‘compile’

    • In a stmt of a 'do' block:

        compile $ loadImage >>= scaleImageCompiler 600 400

      In the second argument of ‘($)’, namely

        ‘do route idRoute

            compile $ loadImage >>= scaleImageCompiler 600 400’

      In a stmt of a 'do' block:

        match "images/*"

          $ do route idRoute

               compile $ loadImage >>= scaleImageCompiler 600 400

   |

12 |         compile $ loadImage >>= scaleImageCompiler 600 400

   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The error is because type Image, defined by loadImage, is required by compile to be an instance of typeclass Writable. The types of the functions used from hakyll and hakyll-images, copied from the hackage documentation, are shown below.

    route :: Routes -> Rules ()

    idRoute :: Routes

    compile :: (Binary a, Typeable a, Writable a) => Compiler (Item a) -> Rules ()

    loadImage :: Compiler (Item Image)

    scaleImageCompiler :: Width -> Height -> Item Image -> Compiler (Item Image)

Image is defined in hakyll-images as type Image = Image_ ByteString.
I am not sure what Image_ is; its definition is not linked in that documentation for the Hakyll.Images module.

In any case, it appears that the example from hakyll-images's Readme does not compile due to Image not being an instance of Writable. I'm wondering if perhaps the hakyll-images package became out-of-sync with hakyll at some point leading to the example no longer compiling.

Does this assessment seem correct?
What do you suggest for how might I approach a solution?

I am considering:

• Updating hakyll-images by somehow adding a Writable instance for Image.


• Using some other set or combination of functions to perform the aspect-ratio-preserving image scalings.


• Ditching hakyll-images and finding some other way to scale the images.

This behavior is a bug that made its way into the hakyll-images 0.3.1 release. It was fixed subsequently in hakyll-images 0.4 and above. Simply update to the latest version to get rid of this problem.

This was a gross oversight and tests have been added such that this will not happen again.

If you wanted to implement the instances yourself, you can take a look at how it is done here.