[charlieflowers]

Ah, I found it: https://wiki.haskell.org/Type_witness

In my own words, a "type witness" is a Haskell idiom (maybe broader?) that allows you to perform dynamic casts.

Here's the first example from that link:

A simple witness type might be defined over the Int, Bool and Char types like so:

data Witness a where IntWitness :: Witness Int BoolWitness :: Witness Bool CharWitness :: Witness Char

dynamicCast IntWitness IntWitness pa = Just pa

dynamicCast BoolWitness BoolWitness pa = Just pa

dynamicCast CharWitness CharWitness pa = Just pa

dynamicCast _ _ _ = Nothing