existential type

plural existential types

1. computingIn a type system: a type that hides the underlying concrete type(s).

   Existential types are fundamentally no more complicated than universal types (in fact, we will see in §24.3 that existentials can straightforwardly be encoded in terms of universals).

   2002, Benjamin C. Pierce, Types and Programming Languages, MIT Press, →ISBN, page 363

   2 more examples

   Scala 2 supported existential types, a way of abstracting over types. They let you assert that some type exists without specifying exactly what it is, usually because you don't know what it is and you don't need to know it in the current context.

   2021, Dean Wampler, chapter 16, in Programming Scala, 3rd edition, O'Reilly, →ISBN

   This behavior is what gives existential types their name: we are asserting that there exists some concrete type that matches the signature, and we leave it up to the compiler to find what that type is.

   2021, Jon Gjengset, Rust for Rustaceans, No Starch Press, →ISBN, page 34

   Synonyms: existential

2. Used other than figuratively or idiomatically: see existential, type.

   This is an existential type of anxiety that stems from the feeling that we are not at home in this world.

   2001, John F. Schumaker, The Age of Insanity: Modernity and Mental Health, Greenwood Publishing Group, →ISBN, →LCCN, →OCLC, page 122

   1 more example

   He was an existential type dude with no belongings to speak of. He didn't care about tomorrow; he lived in the moment. He was always reading a book; quiet yet explosive.

   2007 November 26, Paul Ferrara, Flash of Eden, AuthorHouse, →ISBN, →LCCN, →OCLC, page 120