Liberty Eiffel Programming Environment FAQ
Frequently Asked Questions
General purpose questions about the LibertyEiffel project
Where does the name "Liberty" come from?
As most Eiffel hackers know, Gustave Eiffel (for whom the language is named), also built the metallic 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 LibertyEiffel mailing list?
Yes! LibertyEiffel users and developers can share their experiences and ideas by subscribing to the LibertyEiffel 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 Eiffel source code of LibertyEiffel?
Since it is a free (as in freedom) Eiffel Compiler, the complete source code of LibertyEiffel is of course in the distribution. The source code for additional libraries is also provided. See also this FAQ question which is related to licensing.
Is it possible to use LibertyEiffel for large/commercial applications?
It is indeed possible to use LibertyEiffel for a large application. An Eiffel compiler is a really big project and LibertyEiffel itself is fully written in Eiffel. LibertyEiffel 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 LibertyEiffel 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 kept 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. We changed because we thought the compiler had become smart enough ;)
For a list of changes between the last version of SmallEiffel and the first one of SmartEiffel, have a look there.
How can I help?
The best way to help LibertyEiffel and its users is probably to pick up some area you're interested in, and develop your own code, libraries, tools or extensions with LibertyEiffel, 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 LibertyEiffel.
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 LibertyEiffel 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 LibertyEiffel 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 LibertyEiffel... 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 LibertyEiffel?
The best way is to keep an eye on our web pages, more especially on the LibertyEiffel main website.
What documentation is provided with LibertyEiffel?
The documentation provided with LibertyEiffel is a transcript of what you can find on the original LibertyEiffel Web site, see the link to this wiki. It is related only to the use and internals of LibertyEiffel (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 LibertyEiffel. Since our mailboxes tend to overflood, we generally choose the latter :)
Is it difficult to switch from some commercial Eiffel compiler to LibertyEiffel?
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 LibertyEiffel.
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 LibertyEiffel project
Why is LibertyEiffel 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 LibertyEiffel 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 LibertyEiffel
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 LibertyEiffel distribution.
Questions about the tools of LibertyEiffel
Is it possible to do incremental compilation with LibertyEiffel?
Because of the LibertyEiffel type inference mechanism, LibertyEiffel 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 LibertyEiffel?
Yes.
You just have to pass the compiler option -bdw_gc and this should it be.
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 LibertyEiffel
Are there editors with syntax highlighting?
The emacs editor has syntax highlighting.
For installing see the article Editing Eiffel code with Emacs from Berend de Boer (http://www.berenddeboer.net/eiffel/eiffel_and_emacs.html) but you may use the distributed version of eiffel.el (in the misc/ directory).
The VIM editor also has syntax highlighting. Just use the provided . vim files (in the misc/ directory).
Are there a GUI for SmartEiffel?
No. But the emacs editor is a kind of GUI. It contains a command for compile and displaying errors.
Is there a debugger for Liberty Eiffel with a GUI
Yes and no. There is a front end for the sedb (LibertyEiffel debugger). It is called LibertyEiffel 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.
External information about Eiffel
Where can i find more information about Eiffel?
The EiffelZone contains a lot of libraries (http://www.eiffelzone.com).
Cetus lists more informations about Eiffel (http://www.cetus-links.org/oo_eiffel.html).
The news group news:comp.lang.eiffel has low post frequency.
I am a beginner. Where can i find tutorials?
You should compile the examples in the directory tutorials.
Cetus contains links to tutorial papers (http://www.cetus-links.org/oo_eiffel.html).
Questions about the LibertyEiffel Wiki
Why do you have a Table of Contents page in your Wiki?
The reason for that is that we want to be able to print a paper version of all the information available in the wiki. The Table Of Contents page indicates the order of pages to be printed. Note that the new command to produce the great book is not yet written.
Why are all pages name in English?
The question may look strange for this English wiki. But note that we also have a French version.
As you may have noticed, all pages names are in English, even in the French wiki. For example, while reading the French Table des matières, the name of the page is still Table of Contents. Well, it is just to simplify the wiki itself. Note that it is straightforward to change for example an English link to the corresponding French link and conversely.
How to add figures created with tgif?
At the moment, all the figures in the Big Book are created by the program tgif, because it lets you produce .png files for the on-line version (the wiki) as well as .eps files for producing LaTeX documents (for paper versions).
Since figures are required to be editable by everyone, the source files for the tgif figures, the .obj files, are put on the server. It is the rule that for every file Foo.png on the server there must also be the source tgif file whose name must be Foo.obj.
To put a file on the server, use the Upload file button on the left-hand menu, which brings up the page of the same name, the upload file page.
To go the opposite way, when you want to download the tgif source file for a figure, you have to click on the Special pages button in the left-hand menu, in order to get to the page which contains the Image list link. Then click on the description of the .obj file that you want so as to be able to do the usual Save Link Target on the .obj file.
Is it essential to use tgif for figures?
For the moment, all the figures have been created with tgif, mainly for historical reasons, but also because this program works well, runs under Linux and can transform its schemas both into .png format and also, which is absolutely essential, into .eps format. Furthermore, tgif's source format, the .obj format, is an easily legible and portable text format.
We do not have anything against any other program, so long as that program is also able to produce the .png format for the wiki as well as a format that is compatible with LaTeX. The format of the source files must also be legible and portable. Finally, the program in question must also run under Linux and be free software.
As you will have realised, we really do want to keep life simple and so we prefer to restrict the number of programs used to produce figures. If you really want us to accept another program, contact us by using the most appropriate page of our Wiki. (which is what?)