We lay out the design of HasCasl, a higher order extension of the algebraic specification language Casl that serves both as a wide-spectrum language for the rigorous specification and development of software, in particular but not exclusively in modern functional programming languages, and as an expressive standard language for higher-order logic. Distinctive features of HasCasl include partial higher order functions, higher order subtyping, shallow polymorphism, and an extensive typeclass mechanism. Moreover, HasCasl provides dedicated specification support for monad-based functional-imperative programming with generic side effects, including a monad-based generic Hoare logic.