Escher (programming language)
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Escher is a declarative programming language that supports both functional programming and logic programming models, developed by J.W. Lloyd in the mid-1990s. It was designed mostly as a research and teaching vehicle. The basic view of programming exhibited by Escher and related languages is the a program is a representation of a theory in some logic framework, and the program's execution (computation) is a deduction from the theory. The logic framework for Escher is Alonzo Church's simple theory of types.
Escher, notably, supports I/O through a monadic type representing the 'outside world', in the style of Haskell. One of the goals of Escher's designers was to support meta-programming, and so the language has comprehensive support for generating and transforming programs.
MODULE Lambda. CONSTRUCT Person/0. FUNCTION Jane, Mary, John: One -> Person.
FUNCTION Mother : Person * Person -> Boolean. Mother(x,y) => x=Jane & y=Mary.
FUNCTION Wife : Person * Person -> Boolean. Wife(x,y) => x=John & y=Jane.
FUNCTION PrimitiveRel : (Person * Person -> Boolean) -> Boolean. PrimitiveRel(r) => r=Mother \/ r=Wife.
FUNCTION Rel : (Person * Person -> Boolean) -> Boolean. Rel(r) => PrimitiveRel(r) \/ (SOME [r1,r2] (r = LAMBDA [u] (SOME [z] (r1(Fst(u),z) & r2(z,Snd(u)))) & PrimitiveRel(r1) & PrimitiveRel(r2))).