Eiffel (programming language)
Eiffel is an object-oriented programming language designed by Bertrand Meyer (an object-orientation proponent and author of Object-Oriented Software Construction) and Eiffel Software. Meyer conceived the language in 1985 with the goal of increasing the reliability of commercial software development; the first version becoming available in 1986. In 2005, Eiffel became an ISO-standardized language.
The design of the language is closely connected with the Eiffel programming method. Both are based on a set of principles, including design by contract, command–query separation, the uniform-access principle, the single-choice principle, the open–closed principle, and option–operand separation.
Many concepts initially introduced by Eiffel later found their way into Java, C#, and other languages. New language design ideas, particularly through the Ecma/ISO standardization process, continue to be incorporated into the Eiffel language.
Eiffel emphasizes declarative statements over procedural code and attempts to eliminate the need for bookkeeping instructions.
Eiffel shuns coding tricks or coding techniques intended as optimization hints to the compiler. The aim is not only to make the code more readable, but also to allow programmers to concentrate on the important aspects of a program without getting bogged down in implementation details. Eiffel's simplicity is intended to promote simple, extensible, reusable, and reliable answers to computing problems. Compilers for computer programs written in Eiffel provide extensive optimization techniques, such as automatic in-lining, that relieve the programmer of part of the optimization burden.
Eiffel was originally developed by Eiffel Software, a company founded by Bertrand Meyer. Object-Oriented Software Construction contains a detailed treatment of the concepts and theory of the object technology that led to Eiffel's design.
The design goal behind the Eiffel language, libraries, and programming methods is to enable programmers to create reliable, reusable software modules. Eiffel supports multiple inheritance, genericity, polymorphism, encapsulation, type-safe conversions, and parameter covariance. Eiffel's most important contribution to software engineering is design by contract (DbC), in which assertions, preconditions, postconditions, and class invariants are employed to help ensure program correctness without sacrificing efficiency.
Eiffel's design is based on object-oriented programming theory, with only minor influence of other paradigms or concern for support of legacy code. Eiffel formally supports abstract data types. Under Eiffel's design, a software text should be able to reproduce its design documentation from the text itself, using a formalized implementation of the "Abstract Data Type".
EiffelStudio is an integrated development environment available under either an open source or a commercial license. It offers an object-oriented environment for software engineering. EiffelEnvision is a plug-in for Microsoft Visual Studio that allows users to edit, compile, and debug Eiffel projects from within the Microsoft Visual Studio IDE. Five other open source implementations are available: "The Eiffel Compiler" tecomp; Gobo Eiffel; SmartEiffel, the GNU implementation, based on an older version of the language; LibertyEiffel, based on the SmartEiffel compiler; and Visual Eiffel.
Several other programming languages incorporate elements first introduced in Eiffel. Sather, for example, was originally based on Eiffel but has since diverged, and now includes several functional programming features. The interactive-teaching language Blue, forerunner of BlueJ, is also Eiffel-based. The Apple Media Tool includes an Eiffel-based Apple Media Language.
The Eiffel language definition is an international standard of the ISO. The standard was developed by ECMA International, which first approved the standard on 21 June 2005 as Standard ECMA-367, Eiffel: Analysis, Design and Programming Language. In June 2006, ECMA and ISO adopted the second version. In November 2006, ISO first published that version. The standard can be found and used free of charge on the ECMA site. The ISO version is identical in all respects except formatting.
Eiffel Software, "The Eiffel Compiler" tecomp and Eiffel-library-developer Gobo have committed to implementing the standard; Eiffel Software's EiffelStudio 6.1 and "The Eiffel Compiler" tecomp implement some of the major new mechanisms—in particular, inline agents, assigner commands, bracket notation, non-conforming inheritance, and attached types. The SmartEiffel team has turned away from this standard to create its own version of the language, which they believe to be closer to the original style of Eiffel. Object Tools has not disclosed whether future versions of its Eiffel compiler will comply with the standard. LibertyEiffel implements a dialect somewhere in between the SmartEiffel language and the standard.
The standard cites the following, predecessor Eiffel-language specifications:
The current version of the standard from June 2006 contains some inconsistencies (e.g. covariant redefinitions). The ECMA committee has not yet announced any timeline and direction on how to resolve the inconsistencies.
An Eiffel "system" or "program" is a collection of classes. Above the level of classes, Eiffel defines cluster, which is essentially a group of classes, and possibly of subclusters (nested clusters). Clusters are not a syntactic language construct, but rather a standard organizational convention. Typically an Eiffel program will be organized with each class in a separate file, and each cluster in a directory containing class files. In this organization, subclusters are subdirectories. For example, under standard organizational and casing conventions,
x.e might be the name of a file that defines a class called X.
A class contains features, which are similar to "routines", "members", "attributes" or "methods" in other object-oriented programming languages. A class also defines its invariants, and contains other properties, such as a "notes" section for documentation and metadata. Eiffel's standard data types, such as
ARRAY, are all themselves classes.
Every system must have a class designated as "root", with one of its creation procedures designated as "root procedure". Executing a system consists of creating an instance of the root class and executing its root procedure. Generally, doing so creates new objects, calls new features, and so on.
Eiffel has five basic executable instructions: assignment, object creation, routine call, condition, and iteration. Eiffel's control structures are strict in enforcing structured programming: every block has exactly one entry and exactly one exit.
Unlike many object-oriented languages, but like Smalltalk, Eiffel does not permit any assignment into attributes of objects, except within the features of an object, which is the practical application of the principle of information hiding or data abstraction, requiring formal interfaces for data mutation. To put it in the language of other object-oriented programming languages, all Eiffel attributes are "protected", and "setters" are needed for client objects to modify values. An upshot of this is that "setters" can, and normally do, implement the invariants for which Eiffel provides syntax.
While Eiffel does not allow direct access to the features of a class by a client of the class, it does allow for the definition of an "assigner command", such as:
While a slight bow to the overall developer community to allow something looking like direct access (e.g. thereby breaking the Information Hiding Principle), the practice is dangerous as it hides or obfuscates the reality of a "setter" being used. In practice, it is better to redirect the call to a setter rather than implying a direct access to a feature like
some_attribute as in the example code above.
Finally, scoping can be selectively and precisely controlled to any class in the Eiffel project universe, such as:
Here, the compiler will allow only the classes listed between the curly braces to access the features within the feature group (e.g. DECIMAL, DCM_MA_DECIMAL_PARSER, DCM_MA_DECIMAL_HANDLER).
A programming language's look and feel is often conveyed using a "Hello, world!" program. Such a program written in Eiffel might be:
This program contains the class
HELLO_WORLD. The constructor (create routine) for the class, named
make, invokes the
world!" message to the output.
The concept of Design by Contract is central to Eiffel. The contracts assert what must be true before a routine is executed (precondition) and what must hold to be true after the routine finishes (post-condition). Class Invariant contracts define what assertions must hold true both before and after any feature of a class is accessed (both routines and attributes). Moreover, contracts codify into executable code developer and designers assumptions about the operating environment of the features of a class or the class as a whole by means of the invariant.
The Eiffel compiler is designed to include the feature and class contracts in various levels. EiffelStudio, for example, executes all feature and class contracts during execution in the "Workbench mode." When an executable is created, the compiler is instructed by way of the project settings file (e.g. ECF file) to either include or exclude any set of contracts. Thus, an executable file can be compiled to either include or exclude any level of contract, thereby bringing along continuous levels of unit and integration testing. Moreover, contracts can be continually and methodically exercised by way of the Auto-Test feature found in EiffelStudio.
The Design by Contract mechanisms are tightly integrated with the language and guide redefinition of features in inheritance:
In addition, the language supports a "check instruction" (a kind of "assert"), loop invariants, and loop variants (which guarantee loop termination).
Void-safety, like static typing, is another facility for improving software quality. Void-safe software is protected from run time errors caused by calls to void references, and therefore will be more reliable than software in which calls to void targets can occur. The analogy to static typing is a useful one. In fact, void-safe capability could be seen as an extension to the type system, or a step beyond static typing, because the mechanism for ensuring void-safety is integrated into the type system.
The guard against void target calls can be seen by way of the notion of attachment and (by extension) detachment (e.g. detachable keyword). The void-safe facility can be seen in a short re-work of the example code used above:
The code example above shows how the compiler can statically address the reliability of whether
some_attribute will be attached or detached at the point it is used. Notably, the
attached keyword allows for an "attachment local" (e.g.
l_attribute), which is scoped to only the block of code enclosed by the if-statement construct. Thus, within this small block of code, the local variable (e.g.
l_attribute) can be statically guaranteed to be non-void (i.e. void-safe).
The primary characteristic of a class is that it defines a set of features: as a class represents a set of run-time objects, or "instances", a feature is an operation on these objects. There are two kinds of features: queries and commands. A query provides information about an instance. A command modifies an instance.
The command-query distinction is important to the Eiffel method. In particular:
Eiffel does not allow argument overloading. Each feature name within a class always maps to a specific feature within the class. One name, within one class, means one thing. This design choice helps the readability of classes, by avoiding a cause of ambiguity about which routine will be invoked by a call. It also simplifies the language mechanism; in particular, this is what makes Eiffel's multiple inheritance mechanism possible.
Names can, of course, be reused in different classes. For example, the feature plus (along with its infix alias "+") is defined in several classes: INTEGER, REAL, STRING, etc.
A generic class is a class that varies by type (e.g. LIST [PHONE], a list of phone numbers; ACCOUNT [G->ACCOUNT_TYPE], allowing for ACCOUNT [SAVINGS] and ACCOUNT [CHECKING], etc.). Classes can be generic, to express that they are parameterized by types. Generic parameters appear in square brackets:
G is known as a "formal generic parameter". (Eiffel reserves "argument" for routines, and uses "parameter" only for generic classes.) With such a declaration G represents within the class an arbitrary type; so a function can return a value of type G, and a routine can take an argument of that type:
LIST [INTEGER] and
LIST [WORD] are "generic derivations" of this class. Permitted combinations (with
il: LIST [INTEGER],
wl: LIST [WORD]) are:
WORD are the "actual generic parameters" in these generic derivations.
It is also possible to have 'constrained' formal parameters, for which the actual parameter must inherit from a given class, the "constraint". For example, in
HASH_TABLE [INTEGER, STRING] is valid only if
STRING inherits from
HASHABLE (as it indeed does in typical Eiffel libraries). Within the class, having
KEY constrained by
HASHABLE means that for
x: KEY it is possible to apply to
x all the features of
HASHABLE, as in
To inherit from one or more others, a class will include an
inherit clause at the beginning:
The class may redefine (override) some or all of the inherited features. This must be explicitly announced at the beginning of the class through a
redefine subclause of the inheritance clause, as in
Classes may be defined with
deferred class rather than with
class to indicate that the class may not be directly instantiated. Non-instantiatable classes are called abstract classes in some other object-oriented programming languages. In Eiffel parlance, only an "effective" class can be instantiated (it may be a descendant of a deferred class). A feature can also be deferred by using the
deferred keyword in place of a
do clause. If a class has any deferred features it must be declared as deferred; however, a class with no deferred features may nonetheless itself be deferred.
Deferred classes play some of the same role as interfaces in languages such as Java, though many object-oriented programming theorists believe interfaces are themselves largely an answer to Java's lack of multiple inheritance (which Eiffel has).
A class that inherits from one or more others gets all its features, by default under their original names. It may, however, change their names through
rename clauses. This is required in the case of multiple inheritance if there are name clashes between inherited features; without renaming, the resulting class would violate the no-overloading principle noted above and hence would be invalid.
Tuples types may be viewed as a simple form of class, providing only attributes and the corresponding "setter" procedure. A typical tuple type reads
and could be used to describe a simple notion of birth record if a class is not needed. An instance of such a tuple is simply a sequence of values with the given types, given in brackets, such as
Components of such a tuple can be accessed as if the tuple tags were attributes of a class, for example if
t has been assigned the above tuple then
t.weight has value 3.5.
Thanks to the notion of assigner command (see below), dot notation can also be used to assign components of such a tuple, as in
The tuple tags are optional, so that it is also possible to write a tuple type as
TUPLE [STRING, REAL, DATE]. (In some compilers this is the only form of tuple, as tags were introduced with the ECMA standard.)
The precise specification of e.g.
TUPLE [A, B, C] is that it describes sequences of at least three elements, the first three being of types
C respectively. As a result,
TUPLE [A, B, C] conforms to (may be assigned to)
TUPLE [A, B], to
TUPLE [A] and to
TUPLE (without parameters), the topmost tuple type to which all tuple types conform.
Eiffel's "agent" mechanism wraps operations into objects. This mechanism can be used for iteration, event-driven programming, and other contexts in which it is useful to pass operations around the program structure. Other programming languages, especially ones that emphasize functional programming, allow a similar pattern using continuations, closures, or generators; Eiffel's agents emphasize the language's object-oriented paradigm, and use a syntax and semantics similar to code blocks in Smalltalk and Ruby.
For example, to execute the
my_action block for each element of
my_list, one would write:
my_action only on elements satisfying
my_condition, a limitation/filter can be added:
In these examples,
my_condition are routines. Prefixing them with
agent yields an object that represents the corresponding routine with all its properties, in particular the ability to be called with the appropriate arguments. So if
a represents that object (for example because
a is the argument to
do_all), the instruction
will call the original routine with the argument
x, as if we had directly called the original routine:
my_action (x). Arguments to
call are passed as a tuple, here
It is possible to keep some arguments to an agent open and make others closed. The open arguments are passed as arguments to
call: they are provided at the time of agent use. The closed arguments are provided at the time of agent definition. For example, if
action2 has two arguments, the iteration
The distinction between open and closed operands (operands = arguments + target) corresponds to the distinction between bound and free variables in lambda calculus. An agent expression such as
action2 (?, y) with some operands closed and some open corresponds to a version of the original operation curried on the closed operands.
The agent mechanism also allows defining an agent without reference to an existing routine (such as
action2), through inline agents as in
The inline agent passed here can have all the trappings of a normal routine, including precondition, postcondition, rescue clause (not used here), and a full signature. This avoids defining routines when all that's needed is a computation to be wrapped in an agent. This is useful in particular for contracts, as in an invariant clause that expresses that all elements of a list are positive:
The current agent mechanism leaves a possibility of run-time type error (if a routine with n arguments is passed to an agent expecting m arguments with m < n). This can be avoided by a run-time check through the precondition
call. Several proposals for a purely static correction of this problem are available, including a language change proposal by Ribet et al.
A routine's result can be cached using the
once keyword in place of
do. Non-first calls to a routine require no additional computation or resource allocation, but simply return a previously computed result. A common pattern for "once functions" is to provide shared objects; the first call will create the object, subsequent ones will return the reference to that object. The typical scheme is:
The returned object—
Result in the example—can itself be mutable, but its reference remains the same.
Often "once routines" perform a required initialization: multiple calls to a library can include a call to the initialization procedure, but only the first such call will perform the required actions. Using this pattern initialization can be decentralized, avoiding the need for a special initialization module. "Once routines" are similar in purpose and effect to the singleton pattern in many programming languages, and to the Borg pattern used in Python.
By default, a "once routine" is called once per thread. The semantics can be adjusted to once per process or once per object by qualifying it with a "once key", e.g.
Eiffel provides a mechanism to allow conversions between various types. The mechanisms coexists with inheritance and complements it. To avoid any confusion between the two mechanisms, the design enforces the following principle:(Conversion principle) A type may not both conform and convert to another.
NEWSPAPER may conform to
INTEGER converts to
REAL (and does not inherit from it).
The conversion mechanism simply generalizes the ad hoc conversion rules (such as indeed between
REAL) that exist in most programming languages, making them applicable to any type as long as the above principle is observed. For example, a
DATE class may be declared to convert to
STRING; this makes it possible to create a string from a date simply through
as a shortcut for using an explicit object creation with a conversion procedure:
To make the first form possible as a synonym for the second, it suffices to list the creation procedure (constructor)
make_from_date in a
convert clause at the beginning of the class.
As another example, if there is such a conversion procedure listed from
TUPLE [day: INTEGER; month: STRING; year: INTEGER], then one can directly assign a tuple to a date, causing the appropriate conversion, as in
Exception handling in Eiffel is based on the principles of design by contract. For example, an exception occurs when a routine's caller fails to satisfy a precondition, or when a routine cannot ensure a promised postcondition. In Eiffel, exception handling is not used for control flow or to correct data-input mistakes.
An Eiffel exception handler is defined using the rescue keyword. Within the rescue section, the retry keyword executes the routine again. For example, the following routine tracks the number of attempts at executing the routine, and only retries a certain number of times:
This example is arguably flawed for anything but the simplest programs, however, because connection failure is to be expected. For most programs a routine name like attempt_connecting_to_server would be better, and the postcondition would not promise a connection, leaving it up to the caller to take appropriate steps if the connection was not opened.
A number of networking and threading libraries are available, such as EiffelNet and EiffelThreads. A concurrency model for Eiffel, based on the concepts of design by contract, is SCOOP, or Simple Concurrent Object-Oriented Programming, not yet part of the official language definition but available in EiffelStudio. CAMEO is an (unimplemented) variation of SCOOP for Eiffel. Concurrency also interacts with exceptions. Asynchronous exceptions can be troublesome (where a routine raises an exception after its caller has itself finished).
Eiffel's view of computation is completely object-oriented in the sense that every operation is relative to an object, the "target". So for example an addition such as
Of course, the former is the conventional syntax and usually preferred. Operator syntax makes it possible to use either form by declaring the feature (for example in
INTEGER, but this applies to other basic classes and can be used in any other for which such an operator is appropriate):
The range of operators that can be used as "alias" is quite broad; they include predefined operators such as "+" but also "free operators" made of non-alphanumeric symbols. This makes it possible to design special infix and prefix notations, for example in mathematics and physics applications.
Every class may in addition have one function aliased to "", the "bracket" operator, allowing the notation
a [i, ...] as a synonym for
a.f (i, ...) where
f is the chosen function. This is particularly useful for container structures such as arrays, hash tables, lists etc. For example, access to an element of a hash table with string keys can be written
"Assigner commands" are a companion mechanism designed in the same spirit of allowing well-established, convenient notation reinterpreted in the framework of object-oriented programming. Assigner commands allow assignment-like syntax to call "setter" procedures. An assignment proper can never be of the form
a.x := v as this violates information hiding; you have to go for a setter command (procedure). For example, the hash table class can have the function and the procedure
Then to insert an element you have to use an explicit call to the setter command:
(in the same way that
phone_book ["JILL SMITH"] is a synonym for
number := phone_book.item ("JILL SMITH")), provided the declaration of
item now starts (replacement for ) with
put as the assigner command associated with
item and, combined with the bracket alias, makes  legal and equivalent to . (It could also be written, without taking advantage of the bracket, as
phone_book.item ("JILL SMITH") := New_person.
note: The argument list of a's assigner is constrained to be: (a's return type;all of a's argument list...)
Eiffel is not case-sensitive. The tokens
MAKE all denote the same identifier. See, however, the "style rules" below.
Comments are introduced by
-- (two consecutive dashes) and extend to the end of the line.
The semicolon, as instruction separator, is optional. Most of the time the semicolon is just omitted, except to separate multiple instructions on a line. This results in less clutter on the program page.
There is no nesting of feature and class declarations. As a result, the structure of an Eiffel class is simple: some class-level clauses (inheritance, invariant) and a succession of feature declarations, all at the same level.
It is customary to group features into separate "feature clauses" for more readability, with a standard set of basic feature tags appearing in a standard order, for example:-- ... Declarations of initialization commands (creation procedures/constructors) ...-- ... Declarations of non-boolean queries on the object state, e.g. item ...-- ... Declarations of boolean queries on the object state, e.g. is_empty ...-- ... Declarations of commands that change the structure, e.g. put ...
In contrast to most curly bracket programming languages, Eiffel makes a clear distinction between expressions and instructions. This is in line with the Command-Query Separation principle of the Eiffel method.
Much of the documentation of Eiffel uses distinctive style conventions, designed to enforce a consistent look-and-feel. Some of these conventions apply to the code format itself, and others to the standard typographic rendering of Eiffel code in formats and publications where these conventions are possible.
While the language is case-insensitive, the style standards prescribe the use of all-capitals for class names (
LIST), all-lower-case for feature names (
make), and initial capitals for constants (
Avogadro). The recommended style also suggests underscore to separate components of a multi-word identifier, as in
The specification of Eiffel includes guidelines for displaying software texts in typeset formats: keywords in bold, user-defined identifiers and constants are shown in
italics, comments, operators, and punctuation marks in
Roman, with program text in
blue as in the present article to distinguish it from explanatory text. For example, the "Hello, world!" program given above would be rendered as below in Eiffel documentation:
Eiffel is a purely object-oriented language but provides an open architecture for interfacing with "external" software in any other programming language.
It is possible for example to program machine- and operating-system level operations in C. Eiffel provides a straightforward interface to C routines, including support for "inline C" (writing the body of an Eiffel routine in C, typically for short machine-level operations).
Although there is no direct connection between Eiffel and C, many Eiffel compilers (Visual Eiffel is one exception) output C source code as an intermediate language, to submit to a C compiler, for optimizing and portability. As such, they are examples of transcompilers. The Eiffel Compiler tecomp can execute Eiffel code directly (like an interpreter) without going via an intermediate C code or emit C code which will be passed to a C compiler in order to obtain optimized native code. On .NET, the EiffelStudio compiler directly generates CIL (Common Intermediate Language) code. The SmartEiffel compiler can also output Java bytecode.