Dynamically typed Haskell

For a while I’ve been intending to make a package that provides a dynamic type and a trivial means to read/write that type to/from CSV/JSON and web requests. I thought that this would be enough to provide a convenient package for doing exploratory work against unknown data.

In Haskell, typically we would model all the types of the data that we want to work with ahead of time. That’s inconvenient if you want to just quickly look through a JSON or CSV file and do some quick calculation: that’s where you would reach for Python. Even type providers1 don’t provide that level of convenience.

I thought, well, it’s pretty easy to define a Dynamic type which is an instance of a bunch of classes like Num, FromCSV and FromJSON, etc. with which one could program like they do in a dynamically typed language.2 But I had other things to work on.

April 1st came around the corner, so I decided that would be a good time to publish such a package then, that way any detractors could be dismissed by “April fools!” Here is the package in all its glory. I have actually already used it several times to play with some public APIs. It’s nice to be able to use regular Haskell once I want to actually write some logic. I can see myself using this fairly often.


  1. Such as those in F#: An F# type provider is a component that provides types, properties, and methods for use in your program.

  2. Oddly, no one seems to have done this. There are plenty of “dynamic type” packages in Haskell, but their authors couldn’t resist making it well-typed in some way.