| Escher | |
|---|---|
| Paradigm | declarative: functional, logic |
| Designed by | J.W. Lloyd |
| First appeared | mid-1990s |
| Typing discipline | static, manifest |
| Major implementations | |
| Kee Siong Ng's implementation | |
| Influenced by | |
| simple theory of types | |
Escher (named for M. C. Escher, "a master of endless loops") 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 that 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,[dead link] JW Lloyd, University of Bristol, Bristol, UK, 1995
- An implementation of Escher (Some dead links can be reached from the archived page (or by substituting the new domain in the link in question).)
