GAP (changes) - Chapter 2: Changes between GAP 4.7 and GAP 4.8

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2 Changes between GAP 4.7 and GAP 4.8
2.1 GAP 4.8.2 (February 2016)
2.1-1 Changes in the core GAP system introduced in GAP 4.8
2.1-2 New and updated packages since GAP 4.7.8
2.2 GAP 4.8.3 (March 2016)
2.2-1 Changes in the core GAP system introduced in GAP 4.8.3

### 2 Changes between GAP 4.7 and GAP 4.8

This chapter contains an overview of the most important changes introduced in GAP 4.8.2 release (the 1st public release of GAP 4.8). Later it will also contain information about subsequent update releases for GAP 4.8. First of all, the GAP development repository is now hosted on GitHub at https://github.com/gap-system/gap, and GAP 4.8 is the first major GAP release made from this repository. The public issue tracker for the core GAP system is located at https://github.com/gap-system/gap/issues, and you may use appropriate milestones from https://github.com/gap-system/gap/milestones to see all changes that were introduced in corresponding GAP releases. An overview of the most significant ones is provided below.

#### 2.1 GAP 4.8.2 (February 2016)

##### 2.1-1 Changes in the core GAP system introduced in GAP 4.8

New features:

- Added support for profiling which tracks how much time in spent on each line of GAP code. This can be used to show where code is spending a long time and also check which lines of code are even executed. See the documentation for `ProfileLineByLine` (Reference: ProfileLineByLine) and `CoverageLineByLine` (Reference: CoverageLineByLine) for details on generating profiles, and the Profiling package for transforming these profiles into a human-readable form.

- Added ability to install (in the library or packages) methods for accessing lists using multiple indices and indexing into lists using indices other than positive small integers. Such methods could allow, for example, to support expressions like

      m[1,2];
      m[1,2,3] := x;
      IsBound(m["a","b",Z(7)]);
      Unbind(m[1][2,3])

- Added support for partially variadic functions to allow function expressions like

      function( a, b, c, x... ) ... end;

  which would require at least three arguments and assign the first three to a, b and c and then a list containing any remaining ones to x.

  The former special meaning of the argument arg is still supported and is now equivalent to `function( arg... )`, so no changes in the existing code are required.

- Introduced `CallWithTimeout` (Reference: CallWithTimeout) and `CallWithTimeoutList` (Reference: CallWithTimeoutList) to call a function with a limit on the CPU time it can consume. This functionality may not be available on all systems and you should check `GAPInfo.TimeoutsSupported` before using this functionality.

- GAP now displays the filename and line numbers of statements in backtraces when entering the break loop.

- Introduced `TestDirectory` (Reference: TestDirectory) function to find (recursively) all `.tst` files from a given directory or a list of directories and run them using `Test` (Reference: Test).

Improved and extended functionality:

- Method tracing shows the filename and line of function during tracing.

- `TraceAllMethods` (Reference: TraceAllMethods) and `UntraceAllMethods` (Reference: UntraceAllMethods) to turn on and off tracing all methods in GAP. Also, for the uniform approach `UntraceImmediateMethods` (Reference: UntraceImmediateMethods) has been added as an equivalent of `TraceImmediateMethods(false)`.

- The most common cases of `AddDictionary` (Reference: AddDictionary) on three arguments now bypass method selection, avoiding the cost of determining homogeneity for plain lists of mutable objects.

- Improved methods for symmetric and alternating groups in the "natural" representations and removed some duplicated code.

- Package authors may optionally specify the source code repository, issue tracker and support email address for their package using new components in the `PackageInfo.g` file, which will be used to create hyperlinks from the package overview page (see `PackageInfo.g` from the Example package which you may use as a template).

Changed functionality:

- As a preparation for the future developments to support multithreading, some language extensions from the HPC-GAP project were backported to the GAP library to help to unify the codebase of both GAP 4 and HPC-GAP. The only change which is not backwards compatible is that `atomic`, `readonly` and `readwrite` are now keywords, and thus are no longer valid identifiers. So if you have any variables or functions using that name, you will have to change it in GAP 4.8.

- There was inconsistent use of the following properties of semigroups: `IsGroupAsSemigroup`, `IsMonoidAsSemigroup`, and `IsSemilatticeAsSemigroup`. `IsGroupAsSemigroup` was true for semigroups that mathematically defined a group, and for semigroups in the category `IsGroup` (Reference: IsGroup); `IsMonoidAsSemigroup` was only true for semigroups that mathematically defined monoids, but did not belong to the category `IsMonoid` (Reference: IsMonoid); and `IsSemilatticeAsSemigroup` was simply a property of semigroups, as there is no category `IsSemilattice`.

  From version 4.8 onwards, `IsSemilatticeAsSemigroup` is renamed to `IsSemilattice`, and `IsMonoidAsSemigroup` returns true for semigroups in the category `IsMonoid` (Reference: IsMonoid).

  This way all of the properties of the type `IsXAsSemigroup` are consistent. It should be noted that the only methods installed for `IsMonoidAsSemigroup` belong to the Semigroups and Smallsemi packages.

- `ReadTest` became obsolete and for backwards compatibility is replaced by `Test` (Reference: Test) with the option to compare the output up to whitespaces.

- The function `ErrorMayQuit`, which differs from `Error` (Reference: Error) by not allowing execution to continue, has been renamed to `ErrorNoReturn` (Reference: ErrorNoReturn).

Fixed bugs:

- A combination of two bugs could lead to a segfault. First off, `NullMat` (Reference: NullMat) (and various other GAP functions), when asked to produce matrix over a small field, called `ConvertToMatrixRep` (Reference: ConvertToMatrixRep (for a list (and a field))). After this, if the user tried to change one of the entries to a value from a larger extension field, this resulted in an error. (This is now fixed).

  Unfortunately, the C code catching this error had a bug and allowed users to type "return" to continue while ignoring the conversion error. This was a bad idea, as the C code would be in an inconsistent state at this point, subsequently leading to a crash.

  This, too, has been fixed, by not allowing the user to ignore the error by entering "return".

- The Fitting-free code and code inheriting PCGS is now using `IndicesEANormalSteps` (Reference: IndicesEANormalSteps) instead of `IndicesNormalSteps` (Reference: IndicesNormalSteps), as these indices are neither guaranteed, nor required to be maximally refined when restricting to subgroups.

- A bug that caused a break loop in the computation of the Hall subgroup for groups having a trivial Fitting subgroup.

- Including a `break` or `continue` statement in a function body but not in a loop now gives a syntax error instead of failing at run time.

- `GroupGeneralMappingByImages` (Reference: GroupGeneralMappingByImages) now verifies that that image of a mapping is contained in its range.

- Fixed a bug in caching the degree of transformation that could lead to a non-identity transformation accidentally changing its value to the identity transformation.

- Fixed the problem with using Windows default browser as a help viewer using `SetHelpViewer("browser");`.

##### 2.1-2 New and updated packages since GAP 4.7.8

At the time of the release of GAP 4.7.8 there were 119 packages redistributed with GAP. New packages that have been added to the redistribution since the release of GAP 4.7.8 are:

- CAP (Categories, Algorithms, Programming) package by Sebastian Gutsche, Sebastian Posur and Øystein Skartsæterhagen, together with three associated packages GeneralizedMorphismsForCAP, LinearAlgebraForCAP and ModulePresentationsForCAP (all three - by Sebastian Gutsche and Sebastian Posur).

- Digraphs package by Jan De Beule, Julius Jonušas, James Mitchell, Michael Torpey and Wilf Wilson, which provides functionality to work with graphs, digraphs, and multidigraphs.

- FinInG package by John Bamberg, Anton Betten, Philippe Cara, Jan De Beule, Michel Lavrauw and Max Neunhöffer for computation in Finite Incidence Geometry.

- HeLP package by Andreas Bächle and Leo Margolis, which computes constraints on partial augmentations of torsion units in integral group rings using a method developed by Luthar, Passi and Hertweck. The package can be employed to verify the Zassenhaus Conjecture and the Prime Graph Question for finite groups, once their characters are known. It uses an interface to the software package 4ti2 to solve integral linear inequalities.

- matgrp package by Alexander Hulpke, which provides an interface to the solvable radical functionality for matrix groups, building on constructive recognition.

- NormalizInterface package by Sebastian Gutsche, Max Horn and Christof Söger, which provides a GAP interface to Normaliz, enabling direct access to the complete functionality of Normaliz, such as computations in affine monoids, vector configurations, lattice polytopes, and rational cones.

- profiling package by Christopher Jefferson for transforming profiles produced by `ProfileLineByLine` (Reference: ProfileLineByLine) and `CoverageLineByLine` (Reference: CoverageLineByLine) into a human-readable form.

- Utils package by Sebastian Gutsche, Stefan Kohl and Christopher Wensley, which provides a collection of utility functions gleaned from many packages.

- XModAlg package by Zekeriya Arvasi and Alper Odabas, which provides a collection of functions for computing with crossed modules and Cat1-algebras and morphisms of these structures.

#### 2.2 GAP 4.8.3 (March 2016)

##### 2.2-1 Changes in the core GAP system introduced in GAP 4.8.3

New features:

- New function `TestPackage` (Reference: TestPackage) to run standard tests (if available) for a single package in the current GAP session (also callable via `make testpackage PKGNAME=pkgname` to run package tests in the same settings that are used for testing GAP releases).

Improved and extended functionality:

- `TestDirectory` (Reference: TestDirectory) now prints a special status message to indicate the outcome of the test (this is convenient for automated testing). If necessary, this message may be suppressed by using the option `suppressStatusMessage`

- Improved output of tracing methods (which may be invoked, for example, with `TraceAllMethods` (Reference: TraceAllMethods)) by displaying filename and line number in some more cases.

Changed functionality:

- Fixed some inconsistencies in the usage of `IsGeneratorsOfSemigroup` (Reference: IsGeneratorsOfSemigroup).

Fixed bugs that could lead to incorrect results:

- Fallback methods for conjugacy classes, that were never intended for infinite groups, now use `IsFinite` (Reference: IsFinite) filter to prevent them being called for infinite groups. [Reported by Gabor Horvath]

Fixed bugs that could lead to break loops:

- Calculating stabiliser for the alternating group caused a break loop in the case when it defers to the corresponding symmetric group.

- It was not possible to use `DotFileLatticeSubgroups` (Reference: DotFileLatticeSubgroups) for a trivial group. [Reported by Sergio Siccha]

- A break loop while computing `AutomorphismGroup` (Reference: AutomorphismGroup) for `TransitiveGroup(12,269)`. [Reported by Ignat Soroko]

- A break loop while computing conjugacy classes of `PSL(6,4)`. [Reported by Martin Macaj]

Other fixed bugs:

- Fix for using Firefox as a default help viewer with `SetHelpViewer` (Reference: SetHelpViewer). [Reported by Tom McDonough]

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