The meaning of the word “reify” in programming

I don’t recall hearing of the words “reify” or “reification” in my OOP years, but that may be because I studied aerospace engineering in college, not computer science. Since learning functional programming (FP) I often see those words, so I thought I’d try to understand their meaning.


I ran into the word “reify” when I saw code like this:

trait Foo {

object Foo extends Foo

I’d see code like that then someone would describe that last line as a “reification” process.

What “reify” means

The short answer is that the main definition of reify seems to be:

“Taking an abstract concept and making it concrete.”

For the longer answer, I found the following definitions and examples of reification.

This SO Haskell page offers these definitions:

  • Taking an abstract concept and making it concrete.
  • Reification is a form of instantiation. When you reify a concept, you take something abstract and make it concrete.
  • You can reify Hoare's idea of quicksort into an implementation in the programming language of your choice. In this vein, I spend a lot of time reifying concepts from category theory into Haskell code.

The Haskell wiki offers this definition:

  • To “reify” something is to take something that is abstract and regard it as material. A classic example is the way that the ancients took abstract concepts (e.g. “victory”) and turned them into deities (e.g. Nike, the Greek goddess of victory).

Finally, this SO C# page provides more of the same definitions:

  • Reification is the process of taking an abstract thing and creating a concrete thing.
  • The term reification in C# generics refers to the process by which a generic type definition and one or more generic type arguments (the abstract thing) are combined to create a new generic type (the concrete thing).
  • To phrase it differently, it is the process of taking the definition of List<T> and int and producing a concrete List<int> type.