Difference between revisions of "Eiffel Breaking Changes"
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* Storables won't be retrievable by old systems | * Storables won't be retrievable by old systems | ||
| − | == | + | ==I/O and external operations== |
| + | ===Before=== | ||
| + | Any I/O operations could raise an exception upon failure. | ||
| + | ===After=== | ||
| + | No I/O operations will raise an exception upon failure. | ||
| + | ===Rationale=== | ||
| + | Writing exception free code for manipulating external resources is difficult at the moment, forcing you to use rescue clauses too many times in code that may have a very complex flow. Performing a check after the operation to see if it was successful is exception free but allows for a better and more accurate error reporting. | ||
| + | ===Issues=== | ||
| + | Changing the code would break too much code. The idea is to keep the existing as is, but add a new set of classes from a new I/O library outside of EiffelBase which will be exception free. It has mostly an impact on IO_MEDIUM descendants. | ||
==Remove explicit creation procedure from <e>TUPLE</e>== | ==Remove explicit creation procedure from <e>TUPLE</e>== | ||
Revision as of 04:34, 23 January 2016
Warning: Article under development
Contents
- 1 Introduction
- 2 Library changes
- 3 Language Change
Introduction
As of this writing, January 21st 2016, the Eiffel language has slowly evolved over the years and added numerous new facilities. Initially, the changes were done by Eiffel Software while maintaining backward compatibility. Then, a major work was done between 2001 and 2005 to standardize the language which became an ECMA standard in 2005 (ECMA-367) and an ISO standard in 2006 (ISO/IEC 25436:2006). That work established some constructs initially implemented in EiffelStudio and also introduced some new ones. Most of the changes introduced by the standardization have been implemented but there are quite a few remaining. In some cases, it is due to the ration cost/benefit of the implementation, in some others it is a breaking changes. In addition, since the standardization was completed, Eiffel Software kept adding new language features too.
This page is going to list all the changes that needs to be done by Eiffel Software to conform to the standard which cover both the language but also the library facilities required to support the language specification.
Library changes
Strings
Before
STRING maps to STRING_8 And we have STRING_32, IMMUTABLE_STRING_8 and IMMUTABLE_STRING_32
After
STRING should be the equivalent of what IMMUTABLE_STRING_32 was before. STRING_8 should be the equivalent of what IMMUTABLE_STRING_8 was before. MUTABLE_STRING_8 should be the equivalent of what STRING_8 was before. MUTABLE_STRING_32 should be the equivalent of what STRING_32 was before.
Rational
When STRING_32 was added, we did not want to break existing code if it became the default. For example, many Eiffel libraries having a binding with C, could directly use an Eiffel STRING object as argument to a C routines. Of course this is very dangerous and we have changed this over the year to introduce safer mechanism (such as C_STRING).
Now the world is Unicode, not using a Unicode string by default does not make sense. On top of that, string instances are rarely modified and we have seen over the year many bugs because users have a tendency to forget this important fact. Which caused a lot of code to duplicate strings just to be sure, making the code less efficient. This is why we want to map STRING to IMMUTABLE_STRING_32.
The end benefit is that we would have cleaner string manipulations and more importantly much more efficient string operations (search, string sharing, traversals, ...)
Issues
A lot of code would not compile anymore by this change. Pretty much all libraries would have to be rewritten.
Void-safe and invariant-safe copy
Before
Whenever a user redefined copy, it was the assumption that argument's fields would be all set to their default value (case of copy called indirectly via twin) or to some tangible value (normal case of doing x.copy (y)).
After
Whenever a user redefines copy, all the fields are properly set to the fields of the original object which is either the object on which we call twin or the source being copied from.
Rationale
This makes the code fully void-safe as we will never try to access fields that are declared attached but actually Void.
This removes the need to disable invariant checking in the implementation of twin.
Issues
Because copy can be called directly or indirectly, we used to perform the following test on a field that was supposedly attached at runtime:
if field = Void then -- Called from `twin` ... else -- Direct call to `copy' ... end
This kind of code won't work anymore. Instead one has to do
if Current = other then -- Called from `twin` ... else -- Direct call to `copy' ... end
but this is not ideal since we cannot distinguish between a.twin from a.copy (a). Since calling the later is very rare, the above solution is a good one overall.
Restrict export to copy
Before
You could duplicate objects as you wish.
After
You can only duplicate objects if they are marked as `copiable' or `duplicable'. So far there is no definite answer to allow this. What is being proposed is either one of the following:
- Define twin, copy, ... in ANY like today, but exported to NONE.
- Remove twin, copy, ... from ANY and add them to a new abstract class COPIABLE
The second solution is simple as at runtime we simply have to check for conformance to COPIABLE.
Rationale
If you duplicate an object graph that has some external memory reference, causing a duplication of the object holding that reference could cause a double-free of that external memory reference when Eiffel objects are disposed by the GC.
In addition, some objects are complicated to duplicate such as: Files, Windows, Widgets, databases connections,...
Issues
For client code, it should still compile as before. For user defined code that is known to allow copy/duplication, the code will have to be rewritten to allow copy/duplication.
Change the type describing the dynamic type of objects
Before
In non-void-safe mode and in void-safe mode, the dynamic type of an object was detachable X.
After
The dynamic type of an object would always be attached X.
Rationale
It is common sense that an object presence indicates implicitly that it is attached and it will simplify code that test for the type of an object. Currently one had to write:
if attached {TYPE [detachable X]} obj.generating_type then
when really it makes more sense to write:
if attached {TYPE [X]} obj.generating_type then
Issues
It was initially detachable X for reasons that are too complex and vague as of this writing (mostly related to backward compatibility, storable, reflection). Changing the type to attached has some consequences:
- IDs won't be contiguous as they used to be
- Storables won't be retrievable by old systems
I/O and external operations
Before
Any I/O operations could raise an exception upon failure.
After
No I/O operations will raise an exception upon failure.
Rationale
Writing exception free code for manipulating external resources is difficult at the moment, forcing you to use rescue clauses too many times in code that may have a very complex flow. Performing a check after the operation to see if it was successful is exception free but allows for a better and more accurate error reporting.
Issues
Changing the code would break too much code. The idea is to keep the existing as is, but add a new set of classes from a new I/O library outside of EiffelBase which will be exception free. It has mostly an impact on IO_MEDIUM descendants.
Remove explicit creation procedure from TUPLE
Scope: void-safety
Before
After
Rationale
Explicit creation of a TUPLE with attached reference fields introduces objects that break void-safety rules and make a system void-unsafe.
Issues
Some code needs to be redesigned to remove explicit TUPLE creation.
Impact
Low: explicit tuple creation can be flagged as obsolete by using regular Eiffel mechanism. After 1-2 releases the feature can be removed.
Make TUPLE argument of features {ROUTINE}.call and {FUNCTION}.item attached
Scope: void-safety
Before
{ROUTINE}.call and {FUNCTION}.item take argument of a type detachable TUPLE.
After
{ROUTINE}.call and {FUNCTION}.item take argument of a type TUPLE.
Rationale
It is possible that an agent object expects some arguments, but if it is passed Void, the compiler does not warn user about the issue.
Issues
All calls .call (Void) and .item (Void) need to be updated.
Impact
Very low: code can be updated by replacing .call (Void) with .call.
