Escher (programming language)
{{Infobox programming language
| name = Escher
| logo =
| paradigm = declarative: functional, logic
| year = mid-1990s
| designer = J.W. Lloyd
| developer =
| latest_release_version =
| latest release date =
| implementations = [http://mentalmodels4life.net/software/ Kee Siong Ng's implementation]
| influenced_by = simple theory of types
| influenced =
| operating_system =
| license =
| website =
| file_ext =
}}
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
- [https://web.archive.org/web/20231008145644/https://www.bristol.ac.uk/engineering/new-structure/?id=1000073 Declarative programming in Escher],{{Dead link|date=October 2024}} JW Lloyd, University of Bristol, Bristol, UK, 1995
- [https://users.cecs.anu.edu.au/~kee/Escher/ An implementation of Escher] (Some dead links can be reached from the [https://web.archive.org/web/20211127165402/http://users.cecs.anu.edu.au/~kee/Escher/ archived page] (or by substituting the new domain in the link in question).)