Escher (programming language)

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.

Examples

       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))).

References

• Declarative programming in Escher, JW Lloyd, University of Bristol, Bristol, UK, 1995
• An implementation of Escher