A face is a name for a value, type or expression in Hoon. In the expression foo=42, foo is the face of the value 42. When that value is pinned to the subject like =+ foo=42, subsequent expressions can reference foo and it will be substituted with 42. Faces are part of Hoon's type system, and are replaced with tree addresses when compiled to Nock.
Further reading
- Hoon school: Syntax lesson: This lesson discusses faces in more detail.