Content Type Bliss

Recently I came across Timo von Holtz’s servant-JuicyPixels package. It describes servant-compatible content-types for JuicyPixel’s DynamicImage data type, and clocks under 100 airy lines.

Timo and I realized there is a pretty neat demonstration of the advantage of abstracting away content-type serialization and deserialization: the world’s most concise image-conversion web service. Essentially the same application is available as the image conversion example in Timo’s package.

(If you want to know more about how content-types work in servant, the content-type section of the tutorial has more information.)

The Application

Our goal is to provide a service that converts images between formats based on the Content-Type and Accept headers of the request:

$ curl localhost:8001 -H "Content-Type: image/png"  \
                      -H "Accept: image/jpeg"  \
                      --data-binary "@haskell-logo.png" \
                      > haskell-logo.jpeg

To get there, we need to do a couple of things. We need to of course run the application:

And describe the API:

As you can see, we state that we accept and can return a variety of image formats.

The application is then:

And for the clincher, the handler:

And that’s it!


This is just the limit of the relative gain of abstracting content-types - there is nothing to the application besides them!

Essentially the same idea could of course be applied to other areas. Document conversion with Pandoc, video and audio formats, etc.