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# 6 Function-Operation-Attribute Triples

### Sections

1. Key Dependent Operations
2. In Parent Attributes
3. Operation Functions

GAP is eager to maintain information that it has gathered about an object, possibly by lengthy calculations. The most important mechanism for information maintenance is the automatic storage and look-up that takes place for attributes; and this was already mentioned in section Attributes in the tutorial. In this chapter we will describe further mechanisms that allow storage of results that are not values of attributes.

The idea which is common to all sections is that certain operations, which are not themselves attributes, have an attribute associated with them. To automatically delegate tasks to the attribute, GAP knows, in addition to the operation and the attributes also a function, which is ``wrapped around'' the other two. This ``wrapper function'' is called by the user and decides whether to call the operation or the attribute or possibly both. The whole function-operation-attribute triple (or FOA triple) is set up by a single GAP command which writes the wrapper function and already installs some methods, e.g., for the attribute to fall back on the operation. The idea is then that subsequent methods, which perform the actual computation, are installed only for the operation, whereas the wrapper function remains unaltered, and in general no additional methods for the attribute are required either.

## 6.1 Key Dependent Operations

There are several functions that require as first argument a domain, e.g., a group, and as second argument something much simpler, e.g., a prime. `SylowSubgroup` is an example. Since its value depends on two arguments, it cannot be an attribute, yet one would like to store Sylow subgroups once they have been computed.

The idea is to provide an attribute of the group, called `ComputedSylowSubgroups`, and to store the groups in this list. The name implies that the value of this attribute may change in the course of a GAP session, whenever a newly-computed Sylow subgroup is put into the list. Therefore, this is a mutable attribute (see Creating Attributes and Properties in ``Programming in GAP''). The list contains primes in each bound odd position and a corresponding Sylow subgroup in the following even position. More precisely, if ``p`= ComputedSylowSubgroups(` G`)[` even`- 1 ]` then `ComputedSylowSubgroups(` G`)[` even`]` holds the value of `SylowSubgroup(` G`,` p`)`. The pairs are sorted in increasing order of p, in particular at most one Sylow p subgroup of G is stored for each prime p. This attribute value is maintained by the operation `SylowSubgroup`, which calls the operation `SylowSubgroupOp(` G`,` p`)` to do the real work, if the prime p cannot be found in the list. So methods that do the real work should be installed for `SylowSubgroupOp` and not for `SylowSubgroup`.

The same mechanism works for other functions as well, e.g., for `PCore`, but also for `HallSubgroup`, where the second argument is not a prime but a set of primes.

- `KeyDependentOperation(` name`,` dom-req`,` key-req`,` key-test`)`

  declares at the same time all three: two operations with names name and ``name `Op`, respectively, and an attribute with name and the attribute as described above, with names name, ``name `Op`, and `Computed` name `s`. dom-req and key-req specify the required filters for the first and second argument of the operation ``name `Op`, which are needed to create this operation with `NewOperation` (see NewOperation). dom-req is also the required filter for the corresponding attribute `Computed` name `s`. The fourth argument key-test is in general a function to which the second argument info of ``name`(` D`,` info`)` will be passed. This function can perform tests on info, and raise an error if appropriate.

  For example, to set up the three objects `SylowSubgroup`, `SylowSubgroupOp`, and `ComputedSylowSubgroups` together, the declaration file ``lib/grp.gd'' contains the following line of code.

      KeyDependentOperation( "SylowSubgroup", IsGroup, IsPosInt, "prime" );

  In this example, key-test has the value `"prime"`, which is silently replaced by a function that tests whether its argument is a prime.

      gap> s4 := Group((1,2,3,4),(1,2));;
      gap> SylowSubgroup( s4, 5 );;  ComputedSylowSubgroups( s4 );
      [ 5, Group(()) ]
      gap> SylowSubgroup( s4, 2 );;  ComputedSylowSubgroups( s4 );
      [ 2, Group([(3,4),(1,4)(2,3),(1,3)(2,4)]), 5, Group(()) ]

      gap> SylowSubgroup( s4, 6 );
      Error, SylowSubgroup: <p> must be a prime called from
      <compiled or corrupted call value>  called from
      <function>( <arguments> ) called from read-eval-loop
      Entering break read-eval-print loop ...
      you can 'quit;' to quit to outer loop, or
      you can 'return;' to continue
      brk> quit;

  Thus the prime test need not be repeated in the methods for the operation `SylowSubgroupOp` (which are installed to do the real work). Note that no methods need be installed for `SylowSubgroup` and `ComputedSylowSubgroups`. If a method is installed with `InstallMethod` for a wrapper operation such as `SylowSubgroup` then a warning is signalled provided the `InfoWarning` level is at least 1. (Use `InstallOtherMethod` in order to suppress the warning.)

  ## 6.2 In Parent Attributes

  This section describes how you can add new ``in parent attributes'' (see Constructing Subdomains and Parents in the Reference Manual). As an example, we describe how `Index` and its related functions are implemented.

  There are two operations `Index` and `IndexOp`, and an attribute `IndexInParent`. They are created together as shown below, and after they have been created, methods need be installed only for `IndexOp`. In the creation process, `IndexInParent` already gets one default method installed (in addition to the usual system getter of each attribute, see Attributes in the Reference Manual), namely `D -> IndexOp( Parent( D ), D )`.

  The operation `Index` proceeds as follows.

  - If it is called with the two arguments super and sub, and if `HasParent(` sub`)` and `IsIdenticalObj(` super`, Parent(` sub`) )` are `true`, `IndexInParent` is called with argument sub, and the result is returned.
  - Otherwise, `IndexOp` is called with the same arguments that `Index` was called with, and the result is returned.
  (Note that it is in principle possible to install even `Index` and `IndexOp` methods for a number of arguments different from two, with `InstallOtherMethod`, see Creating Attributes and Properties in ``Programming in GAP'').

- `InParentFOA(` name`,` super-req`,` sub-req`, DeclareAttribute )`
- `InParentFOA(` name`,` super-req`,` sub-req`, DeclareProperty )`

  declares the operations and the attribute as described above, with names name, ``name `Op`, and ``name `InParent`. super-req and sub-req specify the required filters for the first and second argument of the operation ``name `Op`, which are needed to create this operation with `NewOperation` (see NewOperation). sub-req is also the required filter for the corresponding attribute ``name `InParent`; note that `HasParent` is not required for the argument U of ``name `InParent`, because even without a parent stored, `Parent(` U`)` is legal, meaning U itself (see Parents in the Reference Manual). The fourth argument is `DeclareProperty` if ``name `InParent` takes only boolean values (for example in the case `IsNormalInParent`), and `DeclareAttribute` otherwise.

  For example, to set up the three objects `Index`, `IndexOp`, and `IndexInParent` together, the declaration file ``lib/domain.gd'' contains the following line of code.

      InParentFOA( "Index", IsGroup, IsGroup, DeclareAttribute );

  Note that no methods need be installed for `Index` and `IndexInParent`.

  ## 6.3 Operation Functions

  Chapter Group Actions of the Reference Manual and, in particular, the Section About Group Actions explain that certain operations such as `Orbits` (see Orbits), besides their usual usage with arguments G, D, and opr, can also be applied to an external set (G-set), in which case they can be interpreted as attributes. Moreover, they can also be interpreted as attributes for permutation group, meaning the natural action on the set of its moved points.

  The definition of `Orbits` says that a method should be a function with arguments G, D, gens, oprs, and opr, as in the case of the operation `ExternalSet` when specified via gens and oprs (see External Sets in the Reference Manual). All other syntax variants allowed for `Orbits` (e.g., leaving out gens and oprs) are handled by default methods.

  The default methods for `Orbits` support the following behaviour.

  1. If the only argument is an external set xset and the attribute tester `HasOrbits(` xset`)` returns `true`, the stored value of that attribute is returned.
  2. If the only argument is an external set xset and the attribute value is not known, the default arguments are obtained from the data of xset.
  3. If gens and oprs are not specified, gens is set to `Pcgs(` G`)` if `CanEasilyComputePcgs(` G`)` is `true`, and to `GeneratorsOfGroup(` G`)` otherwise; oprs is set to gens.
  4. The default value of opr is `OnPoints`.
  5. In the case of an operation of a permutation group G on `MovedPoints(` G`)` via `OnPoints`, if the attribute tester `HasOrbits(` G`)` returns `true`, the stored attribute value is returned.
  6. The operation is called as ``result`:= Orbits(` G`,` D`,` gens`,` oprs`,` opr`)`.
  7. In the case of an external set xset or a permutation group G in its natural action, the attribute setter is called to store result.
  8. result is returned.

  The declaration of operations that match the above pattern is done as follows.

- `OrbitsishOperation(` name`,` reqs`,` usetype`, NewAttribute ) F`
- `OrbitsishOperation(` name`,` reqs`,` usetype`, NewProperty ) F`

  declares an attribute op as described above, with name name. The second argument reqs specifies the list of required filters for the usual (five-argument) methods that do the real work.

  If the third argument usetype is `true`, the function call ``op`(` xset`)` will --- if the value of op for xset is not yet known --- delegate to the five-argument call of op with second argument xset rather than with D (cf. step 6 above). This allows certain methods for op to make use of the type of xset, in which the types of the external subsets of xset and of the external orbits in xset are stored. (This is used to avoid repeated calls of `NewType` in functions like `ExternalOrbits(` xset`)`, which call `ExternalOrbit(` xset`,` pnt`)` for several values of pnt.)

  For property testing functions such as `IsTransitive`, the fourth argument is `NewProperty`, otherwise it is `NewAttribute`.

  For example, to set up the operation `Orbits`, the declaration file ``lib/oprt.gd'' contains the following line of code:

      OrbitsishOperation( "Orbits", OrbitsishReq, false, NewAttribute );

  The variable `OrbitsishReq` contains the standard requirements

      OrbitsishReq := [ IsGroup, IsList,
      		  IsList,
      		  IsList,
      		  IsFunction ];

  which are usually entered in calls to `OrbitsishOperation`.

  A similar mechanism is provided for operations such as `Orbit` that do not have an associated attribute but still need a wrapper function to standardize the arguments for the associated operation.

- `OrbitishFO(` name`,` reqs`,` famrel`,` usetype`) F`

  declares a wrapper function and its operation, with names name and ``name `Op`. The second argument reqs specifies the list of required filters for the operation ``name `Op`.

  The third argument famrel is used to test the family relation between the second and third argument of ``name`(` G`,` D`,` elm`)`. For example, in the call `Orbit(` G`,` D`,` pnt`)`, pnt must be an element of D, so ``famrel`= IsCollsElms` is appropriate, and in the call `Blocks(` G`,` D`,` seed`)`, seed must be a subset of D, and the family relation is `IsIdenticalObj`. The fourth argument usetype serves the same purpose as in the case of `OrbitsishOperation`.

  For example, to setup the function `Orbit` and its operation `OrbitOp`, the declaration file ``lib/oprt.gd'' contains the following line of code:

      OrbitishFO( "Orbit", OrbitishReq, IsCollsElms, false );

  The variable `OrbitishReq` contains the standard requirements

      OrbitishReq  := [ IsGroup, IsList, IsObject,
      		  IsList,
      		  IsList,
      		  IsFunction ];

  which are usually entered in calls to `OrbitishFO`.

  The relation test via famrel is used to provide a uniform construction of the wrapper functions created by `OrbitishFO`, in spite of the different syntax of the specific functions. For example, `Orbit` admits the calls `Orbit(` G`,` D`,` pnt`,` opr`)` and `Orbit(` G`,` pnt`,` opr`)`, i.e., the second argument D may be omitted; `Blocks` admits the calls `Blocks(` G`,` D`,` seed`,` opr`)` and `Blocks(` G`,` D`,` opr`)`, i.e., the third argument may be omitted. The translation to the appropriate call of `OrbitOp` or `BlocksOp`, for either operation with five or six arguments, is handled via famrel.

  As a consequence, there must not only be methods for `OrbitOp` with the six arguments corresponding to `OrbitishReq`, but also methods for only five arguments (i.e., without D). Plenty of examples are contained in the implementation file ``lib/oprt.gi''.

  In order to handle a few special cases (currently `Blocks` and `MaximalBlocks`), also the following form is supported.

  `OrbitishFO(` name`,` reqs`,` famrel`,` attr`) F`

  The functions in question depend upon an argument seed, so they cannot be regarded as attributes. However, they are most often called without giving seed, meaning ``choose any minimal resp. maximal block system''. In this case, the result can be stored as the value of the attribute attr that was entered as fourth argument of `OrbitishFO`. This attribute is considered by a call `Blocks(` G`,` D`,` opr`)` (i.e., without seed) in the same way as `Orbits` considers `OrbitsAttr`.

  To set this up, the declaration file ``lib/oprt.gd'' contains the following lines:

      DeclareAttribute( "BlocksAttr", IsExternalSet );
      OrbitishFO( "Blocks",
          [ IsGroup, IsList, IsList,
            IsList,
            IsList,
            IsFunction ], IsIdenticalObj, BlocksAttr );

  And this extraordinary FOA triple works as follows:

      gap> s4 := Group((1,2,3,4),(1,2));; Blocks( s4, MovedPoints(s4), [1,2] );
      [ [ 1, 2, 3, 4 ] ]
      gap> Tester( BlocksAttr )( s4 );
      false

      gap> Blocks( s4, MovedPoints(s4) );
      [ [ 1, 2, 3, 4 ] ]
      gap> Tester( BlocksAttr )( s4 );  BlocksAttr( s4 );
      true
      [ [ 1, 2, 3, 4 ] ]

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  GAP 4 manual
  May 2002