From Seo Wiki - Search Engine Optimization and Programming Languages
|Designed by||Steve Omohundro|
|Developer||Free Software Foundation, University of Karlsruhe, University of Waikato|
|Typing discipline||static, strong|
|Major implementations||GNU Sather, Sather-K, Sather-W|
|Influenced by||Eiffel, CLU, Common Lisp, Scheme|
Sather is an object-oriented programming language. It originated circa 1990 at the International Computer Science Institute at the University of California, Berkeley, developed by an international team led by Steve Omohundro. It supports garbage collection and generics by subtypes.
Originally, it was based on Eiffel, but it has diverged, and now includes several functional programming features. It is probably best to view it as an object-oriented language, with many ideas borrowed from Eiffel. Even the name is inspired by Eiffel; the Sather Tower is a recognizable landmark at Berkeley. Sather also takes inspiration from other programming languages and paradigms: iterators, design by contract, abstract classes, multiple inheritance, anonymous functions, operator overloading, contravariant type system. Some of these features are normally only found in functional programming languages.
The original Berkeley implementation is now maintained by many people, not all at Berkeley, and has been adopted by the Free Software Foundation therefore becoming GNU Sather. There are at least two other implementations: Sather-K from the University of Karlsruhe, and Sather-W from the University of Waikato.
The former ICSI sather compiler (now GNU Sather) is implemented as a compiler to C, i.e., the compiler does not output object or machine code, but takes Sather source code and generates C source code as an intermediate language. Optimizing is by the C compiler, Sather code often performs better than the corresponding C++ code, and the generated C code can always be optimized by hand.
class HELLO_WORLD is main is #OUT+"Hello World\n"; end; end;
A few remarks:
- Class names are ALL CAPS; this is not only a convention but is enforced by the compiler.
- The method called
mainis the entry point for execution. It may belong to any class, but if this is different from
MAIN, it must be specified as a compiler option.
#is the constructor symbol, calling method
createof the corresponding class; here it is used for instantiating the
OUTclass, which is actually stdout.
+operator has been overloaded here to stand for stream append.
- Operators such as
+are syntactic sugar for conventionally named method calls:
a + bstands for
a.plus(b). The usual arithmetic precedence conventions are used to resolve the calling order of methods in complex formulae.
- The program layout allows for pre- and post-conditions (not shown here), showing Sather's Eiffel lineage.
Example of iterators
class MAIN is main is loop i := 1.upto!(10); #OUT + i + "\n"; end; end; end;
This program prints numbers from 1 to 10.
end construct is the preferred means of defining loops (although
until are also available). Within the construct, one or more iterators may be used. Iterator names always end with an exclamation mark (this convention is enforced by the compiler).
upto! is a method of the integer class
INT accepting one
once argument, meaning its value will not change as the iterator yields.
upto! could be implemented in the
INT class like this:
upto!(once m:INT):SAME is i: INT := self; -- initialise i to the value of self, -- that is the integer of which this method is called loop if i>m then quit -- leave the loop when i goes beyond m end; yield i; -- else use i as return value and stay in the loop i := i + 1; -- and increment end; end;
Type information for variables is denoted by a postfix syntax
variable:CLASS. The type can often be inferred and thus the typing information is optional, like in
SAME is a convenience pseudo-class referring to the current class.