In computer science and logic, a dependent type is a type whose definition depends on a value. A "string" is a type. A "fixed-length string of length 5" is a dependent type because of the dependence on a value. It is an overlapping feature of type theory and type systems. In intuitionistic type theory, dependent types are used to encode logic's quantifiers like "for all" and "there exists". In functional programming languages like ATS, Agda, Idris and Epigram, dependent types prevent bugs by allowing extremely expressive types.