Instead of having a function that can accept an arbitrary function as parameter, the compiler enumerates all known uses of the function into a data type and uses that as input instead. The resulting code is no longer general purpose.

It's as if you gave Intel your program and they produced a special optimized processor that runs just the instructions found in your program and nothing else.

> the insight of defunctionalization is to find all actual uses of the higher-order function

> Each distinct use of the filter function yields a new case in the Filter datatype