Liberty Eiffel Programming Environment FAQ

Frequently Asked Questions

General purpose questions about the Liberty Eiffel project

Where does the name "Liberty" come from?

As most Eiffel hackers know, Gustave Eiffel (after whom the language is named), also built the metal structure of the Liberty Statue.

More trivia: Some people may also know that I (Cyril) happen to live very near to Belfort, a town renowned for a sculpture of a lion by Auguste Bartholdi. Bartholdi sculpted the metallic sheets that make the Liberty Statue such an awesome sight.

Is there a Liberty Eiffel mailing list?

Yes! Liberty Eiffel users and developers can share their experiences and ideas by subscribing to the Liberty Eiffel official mailing list by just sending an empty email to this address.

Please, note that this list is not moderated. Please, stay correct on that list.

Is it possible to have the complete source code of Liberty Eiffel?

Since it is a free (as in freedom) Eiffel Compiler, the complete source code of Liberty Eiffel is of course in the distribution. The Eiffel source code for additional libraries is also provided. See also this FAQ question which is related to licensing.

Is it possible to use Liberty Eiffel for large/commercial applications?

It is indeed possible to use Liberty Eiffel for a large application. An Eiffel compiler is a really big project and Liberty Eiffel itself is fully written in Eiffel. Liberty Eiffel is completely free and any private company can use it freely, and distribute (or sell) freely the products made with it. They do not have to pay royalties.

Also note that only classes which are closely related to the compiler itself are under GPL (actually, only the classes of directory SmartEiffel/tools are under GPL). The other classes are not under GPL in order to let people completely free (i.e. all classes of lib).

Always keep in mind that Liberty Eiffel doesn't come with any warranty (please read the COPYING file in the distribution). As explained in the header of non-GPL files, the only important thing is to keep the header unaltered when the corresponding source file is provided in your product.

What is SmallEiffel compared to SmartEiffel?

SmallEiffel is the former name of the SmartEiffel project. At some point in time the development team thought the compiler had become smart enough to warrant a renaming ;) For a list of changes between the various Small-/SmartEiffel versions, have a look there.

How can I help?

The best way to help Liberty Eiffel and its users is probably to pick up some area you're interested in, and develop your own code, libraries, tools or extensions with Liberty Eiffel, and then release them to other users.

A very good way to help us is to follow the Get involved guidelines when you find some problem with Liberty Eiffel.

Why don't you change this and add that?! It would be much better/cooler/whatever!

People must understand that we can't always do everything. We simply can't. Because we don't have the time. Whether we like it or not, we also have other things to do than provide free stuff, modify our compiler and/or libraries to please everybody. We do as much as we can, but we don't do miracles, sorry.

Since Liberty Eiffel is free of charge and open-source, people who do need things we don't provide can always implement them and have them benefit everybody. A good way to do this is to start working on it, and ask other people (i.e. not the Liberty Eiffel team ;) ) to join and help. See the How can I help? question.

Alternatively, someone or some company who does need us to implement something may always consider funding a bit the development of Liberty Eiffel... After all, we've even heard that some people were selling software and making a bit of money with it... ;)))

How do I keep informed about Liberty Eiffel?

The best way is to keep an eye on our web pages, more especially on the Liberty Eiffel main website.

What documentation is provided with Liberty Eiffel?

The documentation provided with Liberty Eiffel is a transcript of what you can find on the original Liberty Eiffel Web site, see the link to this wiki. It is related only to the use and internals of Liberty Eiffel (yes, we know we still have to improve it ;) ).

For information and documentation about the Eiffel programming language, check the links on our Internet resources page. Note that we are not aware of any complete Eiffel language manual freely available on the Web (yet?).

Why don't you post more messages in newsgroups and/or mailing lists?

First, because we strongly believe that too much information kills information. Scientists call this "cognitive overload". :)

Second, because we don't have the time. It takes an awful amount of time to follow discussions, whatever their quality. We try to do that. But it's even more time-consuming to be part of them. So, we often have to choose between posting/mailing, and working directly on Liberty Eiffel. Since our mailboxes tend to overflood, we generally choose the latter :)

Is it difficult to switch from some commercial Eiffel compiler to Liberty Eiffel?

If your original Eiffel software only uses simple types like INTEGER, STRING, ARRAY, BOOLEAN, CHARACTER and DOUBLE, it is usually very simple to modify your code in order to use Liberty Eiffel.

It is a little bit difficult for simple input/output (used with predefined io) because some features have different names. If your original software heavily relies for example on the EiffelBase library, it may be very difficult. For example, one must keep in mind that SmartEiffel.ARRAY inherit SmartEiffel.COLLECTION and that ISE library also have a class COLLECTION. By the way, subclasses of ISE.COLLECTION cannot be used. The ISE.LINKED_LIST can be used in conjunction with SmartEiffel.ARRAY because ISE.LINKED_LIST do not inherit ISE.COLLECTION (no clash).

Questions about Languages and/or Object-Oriented Programming

What is the difference between the Static and the Dynamic Type?

The static type of a variable is determined at compile-time, the dynamic type during run-time. The dynamic type must conform to the static type (it may and often will be the same as the static type).

E.g.: A variable might be declared to refer to an instance of class FRUIT (a_fruit: FRUIT), so the static type is FRUIT, but might be assigned an object of class APPLE (a_fruit := an_apple), which becomes the dynamic type. Obviously, APPLE has to be a descendant of FRUIT in this example.

What is type prediction?

Type prediction is when the compiler attempts to predict an expression's dynamic type at compile time.

Questions about the Eiffel language of the Liberty Eiffel project

Why is Liberty Eiffel case-sensitive?

In fact, like most computer languages, Eiffel does distinguish between upper and lower case. Our decision is simply guided by the goals of legibility, organisation and simplification.

As far as keywords are concerned, it is easier to distinguish them when they are always written in the same way. Thus, we always write loop, rather than Loop or even LoOp. We have to get into good habits from now on in other matters too: this rule is going to be generalised so as to be even stricter in future. From now on, lets all get into the habit of writing Current, Void or True, for example.

Upper case is also used to distinguish class names from other names. Thus, we can know that FOO is a class name and foo is not, without having to look elsewhere in the source code.

A final advantage of the upper/lower case distinction is that it allows more precise error messages. What makes work simpler for humans also simplifies it for compilers.

What is the semantics (meaning) of a manifest string?

It can certainly be important to know what really happens when you define a constant STRING attribute by writing, for example:

Message: STRING is "abc"

that constant attribute definition is in fact a compact way of writing:

Message: STRING is
   once
      create Result.make(3)
      Result.extend('a')
      Result.extend('b')
      Result.extend('c')
   end

You realise, of course, that that function definition is a once function and consequently one string and one string only will be created, whatever happens. When you are investigating performance, you also have to know the difference between:

string := "abc"

and

string := once "abc"

You have probably guessed that, if the instruction without once is in a loop, a new string is created each time that the line in question is executed!

The once expression form above is only valid for manifest STRINGs and UNICODE_STRINGs. Here is an example of a manifest UNICODE_STRING (note the capital U):

unicode_string := once U"abc"

The [compile_to_c] command's -manifest_string_trace option allows you to locate unwanted non-once manifest strings.

Can you explain again the difference between conformance and covariance?

Our paper " Conformance of agents in the Eiffel language has a note p.137 that looks surprising.

Note that the previous rules define conformance rules, this has nothing to do with covariance or contravariance.

The concepts are distinct, even though there is a relation between them.

Covariance is defined in terms of conformance. But assignment is also defined in terms of conformance.

That's exactly, and only, what our paper defines: assignment rules. Those rules were not defined anywhere before. It's important: it's not that we disagreed with any earlier position. We just filled a hole in the specification.

Also note that, even if the agent types notation uses square brackets, it's totally unrelated to the generic classes type notation; therefore generic classes rules cannot apply (except if proved otherwise). An agent type is not a generic type, anymore than a tuple type is a generic type. Those are distinct concepts, only with a similar notation.

Conformance is fundamental in any typed language; on the other hand covariance is "just" an Eiffel extra. Important, sure, but not as fundamental as conformance. Our paper never speaks of covariance, except in the note quoted above, and explained further here. Maybe the paper should have explained the subtlety in so many words.

What is a CATCALL?

In the Eiffel world CATCALL is about type safety problems which are still present in the language. In Eiffel, CATCALL is a short-hand for Changing Availability or Type of CALLs.

Availability is about the exportation status of some method. For example, when a method is redefined, it's exportation status can be changed too.

Type of CALLs is about some possibly covariant redefinition.

What is SCOOP ?

In Eiffel, SCOOP refers to language support for distributed programming. In the Eiffel world, it is an acronym for Simple Concurrent Object-Oriented Programming. SCOOP is not yet included in Liberty Eiffel and is certainly its most important missing feature.

Is there a style guide for Eiffel source code?

Yes. Source code in Liberty tools and libraries should conform to the Coding Style Guide. It seems reasonable to use this as a reference for your own project.

Questions about libraries which comes along with Liberty Eiffel

How should I read a file?

In order to read a file, read a character and then use last_character ONLY if end_of_input has not been reached while reading the character.
Before each read (except the very first), you have to test end_of_input, because all read_* procedures require not end_of_input. But why? This require means that if some previous read failed because end of input has been reached, then it is not valid to try to read again. Example:

file: TEXT_FILE_READ; file_name: STRING
...
-- Assumming here that `file' and `file_name' are not Void:
file.connect_to(file_name)
if file.is_connected then
   from
      file.read_character
   until
      file.end_of_input
   loop
      io.put_character(file.last_character)
      file.read_character
   end
   file.disconnect
end

If you want to read the file line by line, then read_line or read_line_in can be used. But the reading pattern is different. This is because after the last line of the file has been read, then the last line is in last_string and has to be used. Example:

file: TEXT_FILE_READ; file_name: STRING
...
-- Assumming here that `file' and `file_name' are not Void:
file.connect_to(file_name)
if file.is_connected then
   from
   until
      file.end_of_input
   loop
      file.read_line
      if file.end_of_input then
         -- The last line of the file does not end with a new
         -- line character. Remove this test if you don't care.
         std_output.put_string(file.last_string)
      else
         std_output.put_line(tfr.last_string)
      end
   end
   file.disconnect
end

Note: last_string always returns the same STRING object, it's up to you to make a copy if you need to keep the string value.

You can read the examples in tutorial directory from the Liberty Eiffel distribution.

Questions about the tools of Liberty Eiffel

Is it possible to do incremental compilation with Liberty Eiffel?

Because of the Liberty Eiffel type inference mechanism, Liberty Eiffel always produces all needed C files from scratch. As old C files are automatically saved, only modified C files are recompiled. See man/compile for details.

Is there a mechanism to pre-compile libraries?

No, there is no such mechanism in Liberty Eiffel. But if you're concerned about compilation speed, don't worry, pre-computed libraries are not the only way to be fast ! Just try Liberty Eiffel, and you'll see :)

Is it possible to use the Boehm-Demers-Weiser garbage collector with Liberty Eiffel?

Yes.
You just have to pass the compiler option -bdw_gc and that should be it.

How to customise the Garbage Collector for a new architecture?

If your architecture needs special handling to get the GC working (because the stack is not contiguous, because some registers are not written on the stack on `setjmp'...) then you need to implement function `mark_stack_and_registers' for your system in the file SmartEiffel/sys/runtime/c/gc_lib.c.

If you get some message telling you that the stack direction is wrong, then you should change macro definition to use the other generic code (there is one generic code for each stack order).

In order to check the GC, you should be able to run all files of the SmartEiffel/misc/benchmarks/gc directory.

How is Liberty Eiffel compiled?

With Eiffel optimisation options -boost and -no_gc. The garbage collector is indeed useless on the Liberty Eiffel commands: since Liberty Eiffel did not include a GC in its first versions, we were very careful about memory when we developed it.

With C compilation optimisations turned on (it depends on the C compiler used; we generally use gcc). The resulting executables are stripped.

External tools for Liberty Eiffel

Are there editors with support for Liberty Eiffel?

• Emacs has support for syntax highlighting, indenting, flymake mode, code templates and more. For installation see the article Editing Eiffel code with Emacs from Berend de Boer but you may use the distributed version of eiffel.el (in the work/ directory of the source distribution).

• Vim also has similar support for Eiffel and specific additions for Liberty Eiffel

• Atom has an Eiffel package available.

• TextMate has an Eiffel bundle available.

• Gedit offers syntax highlighting.

• SciTE offers syntax highlighting and folding.

• On Windows Notepad++ offers syntax highlighting as well.

• EditPad Pro supports several features for Eiffel.

Is there a GUI or IDE for Liberty Eiffel?

An Eclipse-based IDE is currently under development as part of GSoC 2016.

Is there a debugger for Liberty Eiffel with a GUI?

Currently not. There was a front end for the sedb (SmartEiffel debugger). It is called SmartEiffel Embedded Debugger Output Visualiser (sedbov). It maps the key command from the sedb to a GUI. You can find it at https://opensvn.csie.org/traccgi/sedbov/wiki/WikiStart. It use the script language TCL/TK with the extension expect. For information look at http://www.tcl.tk.

GUI Toolkits

Is it possible to create graphical applications?

The external project eiffel-iup is a plugin to the IUP toolkit, that allow a program to run in different systems without changes - the toolkit provides the application portability. Supported systems include: GTK+, Motif and Windows. The plugin is still not complete, but is stable and in continuous development.

External information about Eiffel

Where can I find more information about Eiffel?

The news group news:comp.lang.eiffel has low post frequency.

I am a beginner. Where can I find tutorials?

You should study and compile the examples in the tutorials directory.

Cetus contains links to tutorial papers (http://www.cetus-links.org/oo_eiffel.html).