Sather has garbage collection, statically-checked strong (contravariant) typing, multiple inheritance, separate implementation and type inheritance, parameterized classes, dynamic dispatch, iteration abstraction, higher-order routines and iters, exception handling, assertions, preconditions, postconditions, and class invariants. Sather code can be compiled into C code and can efficiently link with object files of other languages. pSather, the parallel and distributed extension, presents a shared memory abstraction to the programmer while allowing explicit placement of data and threads.
Sather and the ICSI Sather compiler have a very unrestrictive license aimed at encouraging contribution to the public library without precluding the use of Sather for proprietary projects.
This chapter will provide a basic introduction for new users, pointing to sources of information about the language and the compiler. It also contains a summary of Sather features - for those familiar with another object-oriented language, this section provides an overview of the key features of Sather.
This text has benefitted from corrections, comments and suggestions from several people including Cary D. Renzema, Jerome Feldman, Claudio Fleiner and Arno Jacobsen. Particular thanks to Cary, Arno and Feldman for detailed error reports. Arno also made several suggestions regarding terminology and examples that have been incorporated.
http://www.icsi.berkeley.edu/SatherAt the time of this writing, the only compiler implementing the 1.1 language specification is available from ICSI. It is freely available, includes source for class libraries and the compiler, and compiles into ANSI C. This compiler has been ported to a wide range of UNIX and PC operating systems.
ftp.icsi.berkeley.edu: /pub/satherOther sites also mirror the Sather distribution. The distribution includes installation instructions, '
http://www.icsi.berkeley.edu/~satherICSI also maintains a library of contributed Sather code at this page.
There is a newsgroup devoted to Sather:
comp.lang.satherThere is also a Sather mailing list if you wish to be informed of Sather releases; to subscribe, send email to:
email@example.comIt is not necessary to be on the mailing list if you read the Sather newsgroup.
firstname.lastname@example.org email@example.comThis is also where you want to send bug reports.
Classes define the following features: attributes which make up the internal state of objects, shareds and constants which are shared by all objects of a type, and methods which may be either routines or iterators. Any features are by default public, but may be declared private to allow only the class in which it appears access to it. An attribute or shared may instead be declared readonly to allow only the class in which it appears to modify it. Accessor routines are automatically defined for reading or writing attributes, shareds, and constants. The set of non-private methods in a class defines the interface of the corresponding type. Method definitions consist of statements; for their construction expressions are used. There are special literal expressions for boolean, character, string, integer, and floating point objects.
Certain conditions are described as fatal errors. These conditions should never occur in correct programs and all implementations of Sather must be able to detect them. For efficiency reasons, however, implementations may provide the option of disabling checking for certain conditions.
More generally, when checking options have been turned on by compiler flags, the resulting program cannot crash disastrously or mysteriously. All sources of errors that cause crashes are either eliminated at compile-time or funneled into a few situations (such as accessing beyond array bounds) that are found at run-time precisely at the source of the error.
Sather never converts types implicitly, such as from integer to character, integer to floating point, single to double precision, or subclass to superclass. With neither implicit construction nor conversion, Sather resolves routine overloading (choosing one of several similarly named operations based on argument types) much more clearly than C++. The programmer can easily deduce which routine will be called (page 47).
In Sather, the redefinition of operators is orthogonal to the rest of the language. There is ''syntactic sugar'' (page 116) for standard infix mathematical symbols such as '
Sather provides separate mechanisms for these two concepts. Abstract classes represent interfaces: sets of signatures that subtypes of the abstract class must provide. Other kinds of classes provide implementation. Classes may include implementation from other classes using a special '
Issues surrounding the decision to explicitly separate subtyping and code inclusion in Sather are discussed in the ICSI technical report TR 93-064: ''Engineering a Programming Language: The Type and Class System of Sather,'' also published as . It is available at the Sather WWW page.
Simple looping constructs are more powerful when combined with heavy use of cursor objects (sometimes called 'iterators' in other languages, although Sather uses that term for something else entirely) to iterate through the contents of container objects. Cursor objects can be found in most C++ libraries, and they allow useful iteration abstraction. However, they have a number of problems. They must be explicitly initialized, incremented, and tested in the loop. Cursor objects require maintaining a parallel cursor object hierarchy alongside each container class hierarchy. Since creation is explicit, cursors aren't elegant for describing nested or recursive control structures. They can also prevent a number of important optimizations in inner loops.
An important language improvement in Sather 1.0 over earlier versions was the addition of iterators. Iterators are methods that encapsulate user defined looping control structures just as routines do for algorithms. Code using iterators is more concise, yet more readable than code using the cursor objects needed in C++. It is also safer, because the creation, increment, and termination check are bound together inviolably at one point. Each class may define many sorts of iterators, whereas a traditional approach requires a different yet intimately coupled class for each kind of iteration over the major class. Sather iterators are part of the class interface just like routines.
Iterators act as a lingua-franca for operating on collections of items. Matrices define iterators to yield rows and columns; tree classes have recursive iters to traverse the nodes in pre-order, in-order, and post-order; graph classes have iters to traverse vertices or edges breadth-first and depth-first. Other container classes such as hash tables, queues, etc. all provide iters to yield and sometimes to set elements. Arbitrary iterators may be used together in loops with other code.
The rationale of the Sather iterator construct and comparisons with related constructs in other languages can be found in the ICSI technical report TR 93-045: ''Sather Iters: Object-Oriented Iteration Abstraction,'' also published as . It is available at the Sather WWW page.
Experienced C programmers immediately understand the difference when told about the internal representation the ICSI compiler uses: immutable types are implemented with stack or register allocated C '
Immutable types can have several performance advantages over reference types. Immutable types have no heap management overhead, they don't reserve space to store a type tag, and the absence of aliasing makes more compiler optimizations possible. For a small class like '
Immutable classes aren't strictly necessary; reference classes with immutable semantics work too. For example, the reference class '
There are many other problems. Microsoft's C and C++ compilers defeat the purpose of the invalid flag by using it exclusively to detect floating-point stack overflows, so programmers cannot use it. There is no portable C interface to IEEE exception flags and their behavior with respect to '
Correct IEEE support from various platforms was the single worst porting problem of the Sather 1.0 compiler. In 1.1, we give up and make full IEEE compliance optional. Sather implementations are expected to conform to the spirit, if not the letter, of IEEE 754, although proper exceptions, extended types, underflow handling, and correct handling of positive and negative zero are specifically not required.
The Sather treatment of
pSather differs from concurrent object-oriented languages that try to unify the notions of objects and processes by following the actors model . There can be a grave performance impact for the implicit synchronization this model imposes on threads even when they do not conflict. While allowing for actors, pSather treats object-orientation and parallelism as orthogonal concepts, explicitly exposing the synchronization with new language constructs.
pSather follows the Sather philosophy of shielding programmers from common sources of bugs. One of the great difficulties of parallel programming is avoiding bugs introduced by incorrect synchronization. Such bugs cause completely erroneous values to be silently propagated, threads to be starved out of computational time, or programs to deadlock. They can be especially troublesome because they may only manifest themselves under timing conditions that rarely occur (race conditions) and may be sensitive enough that they don't appear when a program is instrumented for debugging (heisenbugs). pSather makes it easier to write deadlock and starvation free code by providing structured facilities for synchronization. A lock statement automatically performs unlocking when its body exits, even if this occurs under exceptional conditions. It automatically avoids deadlocks when multiple locks are used together. It also guarantees reasonable properties of fairness when several threads are contending for the same lock.
Because high performance appears to require explicit human-directed placement, pSather implements a shared memory abstraction using the most efficient facilities of the target platform available, while allowing the programmer to provide placement directives for control and data (without requiring them). This decouples the performance-related placement from code correctness, making it easy to develop and maintain code enjoying the language benefits available to serial code. Parallel programs can be developed on simulators running on serial machines. A powerful object-oriented approach is to write both serial and parallel machine versions of the fundamental classes in such a way that a user's code remains unchanged when moving between them.
Sather 1.0 was a major language change, introducing bound routines, iterators, proper separation of typing and code inclusion, contravariant typing, strongly typed parameterization, exceptions, stronger optional runtime checks and a new library design . The 1.0 compiler was a completely fresh effort by Stephen Omohundro, David Stoutamire and Robert Greisemer. It was written in 0.5 with the 1.0 features introduced as they became functional. The 1.0 compiler was first released in the summer of 1994, and Stephen left the project shortly afterwards. The pSather 1.0 design was largely due to Jerome Feldman, Stephan Murer and David Stoutamire.
This document describes Sather 1.1, released the summer of 1996. The compiler was originally designed and implemented by S. Omohundro, D. Stoutamire and (later) Robert Griesemer. Boris Vaysman is the current Sather czar and feature implementor. Claudio Fleiner implemented most of the common optimizations , a lot of debugging support, the pSather runtime and back-end support for pSather. Michael Philippsen implmented the front/middle support for pSather. Holger Klawitter implemented type checking of parametrized classes. Arno Jacobsen worked on bound iterators. Illya Varnasky implemented inlining support and Trevor Paring implemented an early version of common subexpression elimination.
A group at the University of Karlsruhe under the direction of Gerhard Goos created a compiler for Sather 0.1. The language their compiler supports, Sather-K, diverged from the ICSI specification when Sather 1.0 was released. Karlsruhe has created a large class library called Karla using Sather-K. More information about Sather-K can be found at:
The name 'Sather' is a pun of sorts - Sather was originally envisioned as a smaller, efficient, cleaned-up alternative to the language Eiffel. However, since its conception the two languages have evolved to be quite distinct.
Steve Omohundro was the original driving force behind Sather, keeping the language specification from being pillaged by the unwashed hordes and serving as point man for the Sather community until he left in 1994. Chu-Cheow Lim bootstrapped the original compiler and was largely responsible for the original 0.x compiler and the first implementation of pSather. David Stoutamire took over as language tsar and compiler writer after Stephen left. That position was, in turn, taken over by Boris Vaysman in late 1995.
Sather has been very much a group effort; many, many people have been involved in the language design discussions including: Subutai Ahmad, Krste Asanovic, Jonathan Bachrach, David Bailey, Joachim Beer, Jeff Bilmes, Chris Bitmead, Peter Blicher, John Boyland, Matthew Brand, Henry Cejtin, Alex Cozzi, Richard Durbin, Jerry Feldman, Carl Feynman, Claudio Fleiner, Ben Gomes, Gerhard Goos, Robert Griesemer, Hermann Häertig, John Hauser, Ari Huttunen, Roberto Ierusalimschy, Arno Jacobsen, Matt Kennel, Holger Klawitter, Phil Kohn, Franz Kurfess, Franco Mazzanti, Stephan Murer, Michael Philippsen, Thomas Rauber, Steve Renals, Noemi de La Rocque Rodriguez, Hans Rohnert, Heinz Schmidt, Carlo Sequin, Andreas Stolcke, Clemens Szyperski, Martin Trapp, Boris Vaysman, and Bob Weiner. Countless others have assisted with practical matters such as porting the compiler and libraries.