One solution is to use the tools provided by reflection package to dynamically generate the FromJSON instance at runtime, after having constructed the parse function.
The reflection repository already contains an example of a dynamically generated Monoid instance. I have adapted it for the FromJSON case; the code can be found in this gist.
Some tips for defining these kinds of local instances:
- You never ever have to declare a Reifies instance, this is done by the internal machinery of the reflection package, through some dark magic I don't understand, even if it's explained here.
- Instead, you use Reifies as a precondition for the instance you are actually declaring (FromJSON in this case). You are expressing something like "whenever this phantom type s reifies a typeclass dictionary at the type level, I can reflect it back and use the recovered dictionary to implement the methods of my instance".
- The actual value passed to the reflect function is ignored, it only serves to tell the function what type reifies the value we want to reflect. Assuming you have a Reifies s a constraint, you can always pass something like Proxy :: Proxy s to reflect and obtain an a.
- At some point, you will need a small auxiliary function to convince the compiler that the phantom type in your datatype is the same as the phantom type in the Proxy supplied by reify. Otherwise the Reifies constraint won't be satisfied, and your instance won't kick in!
- Notice that, thanks to parametricity, the phantom type in the Proxy supplied by reify can never escape the callback. So be sure to strip from your result any newtype that still carries the phantom type!
See also this answer on Stack Overflow.