GAP (ref) - Chapter 78: Method Selection

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78 Method Selection
78.1 Operations and Methods
78.2 Method Installation
78.2-1 InstallMethod
78.2-2 InstallOtherMethod
78.3 Applicable Methods and Method Selection
78.4 Partial Methods
78.4-1 TryNextMethod
78.5 Redispatching
78.5-1 RedispatchOnCondition
78.6 Immediate Methods
78.6-1 InstallImmediateMethod
78.7 Logical Implications
78.7-1 InstallTrueMethod
78.8 Operations and Mathematical Terms

### 78 Method Selection

This chapter explains how GAP decides which function to call for which types of objects. It assumes that you have read the chapters about objects (Chapter 12) and types (Chapter 13).

An operation is a special GAP function that bundles a set of functions, its methods.

All methods of an operation compute the same result. But each method is installed for specific types of arguments.

If an operation is called with a tuple of arguments, one of the applicable methods is selected and called.

Special cases of methods are partial methods, immediate methods, and logical implications.

#### 78.1 Operations and Methods

Operations are functions in the category `IsOperation` (5.4-2).

So on the one hand, operations are GAP functions, that is, they can be applied to arguments and return a result or cause a side-effect.

On the other hand, operations are more. Namely, an operation corresponds to a set of GAP functions, called the methods of the operation.

Each call of an operation causes a suitable method to be selected and then called. The choice of which method to select is made according to the types of the arguments, the underlying mechanism is described in the following sections.

Examples of operations are the binary infix operators `=`, `+` etc., and `PrintObj` (6.3-5) is the operation that is called for each argument of `Print` (6.3-4).

Also all attributes and properties are operations. Each attribute has a special method which is called if the attribute value is already stored; this method of course simply returns this value.

The setter of an attribute is called automatically if an attribute value has been computed. Attribute setters are operations, too. They have a default method that ignores the request to store the value. Depending on the type of the object, there may be another method to store the value in a suitable way, and then set the attribute tester for the object to `true`.

#### 78.2 Method Installation

In order to describe what it means to select a method of an operation, we must describe how the methods are connected to their operations.

For attributes and properties there is `InstallImmediateMethod` (78.6-1).

For declaring that a filter is implied by other filters there is `InstallTrueMethod` (78.7-1).

##### 78.2-1 InstallMethod

`‣ InstallMethod`( opr[, info][, famp], args-filts[, val], method ) | ( function )

installs a function method method for the operation opr; args-filts should be a list of requirements for the arguments, each entry being a filter; if supplied info should be a short but informative string that describes for what situation the method is installed, famp should be a function to be applied to the families of the arguments, and val should be an integer that measures the priority of the method.

The default values for info, famp, and val are the empty string, the function `ReturnTrue` (5.3-1), and the integer zero, respectively.

The exact meaning of the arguments famp, args-filts, and val is explained in Section 78.3.

opr expects its methods to require certain filters for their arguments. For example, the argument of a method for the operation `Zero` (31.10-3) must be in the category `IsAdditiveElementWithZero` (31.14-5). It is not possible to use `InstallMethod` to install a method for which the entries of args-filts do not imply the respective requirements of the operation opr. If one wants to override this restriction, one has to use `InstallOtherMethod` (78.2-2) instead.

##### 78.2-2 InstallOtherMethod

`‣ InstallOtherMethod`( opr[, info][, famp], args-filts[, val], method ) | ( function )

installs a function method method for the operation opr, in the same way as for `InstallMethod` (78.2-1), but without the restriction that the number of arguments must match a declaration of opr and without the restriction that args-filts imply the respective requirements of the operation opr.

#### 78.3 Applicable Methods and Method Selection

A method installed as above is applicable for an arguments tuple if the following conditions are satisfied.

The number of arguments equals the length of the list args-filts, the i-th argument lies in the filter args-filts[i], and famp returns `true` when applied to the families of the arguments. The maximal number of arguments supported for methods is six, one gets an error message if one tries to install a method with at least seven arguments.

So args-filt describes conditions for each argument, and famp describes a relation between the arguments.

For unary operations such as attributes and properties, there is no such relation to postulate, famp is `ReturnTrue` (5.3-1) for these operations, a function that always returns `true`. For binary operations, the usual value of famp is `IsIdenticalObj` (12.5-1), which means that both arguments must lie in the same family.

Note that any properties which occur among the filters in the filter list will not be tested by the method selection if they are not yet known. (More exact: if prop is a property then the filter implicitly uses not prop but `Hasprop and prop`.) If this is desired you must explicitly enforce a test (see section 78.5) below.

If no method is applicable, the error message "no method found" is signaled.

Otherwise, the applicable method with highest rank is selected and then called. This rank is given by the sum of the ranks of the filters in the list args-filt, including involved filters, plus the number val used in the call of `InstallMethod` (78.2-1). So the argument val can be used to raise the priority of a method relative to other methods for opr.

Note that from the applicable methods, an efficient one shall be selected. This is a method that needs only little time and storage for the computations.

It seems to be impossible for GAP to select an optimal method in all cases. The present ranking of methods is based on the assumption that a method installed for a special situation shall be preferred to a method installed for a more general situation.

For example, a method for computing a Sylow subgroup of a nilpotent group is expected to be more efficient than a method for arbitrary groups. So the more specific method will be selected if GAP knows that the group given as argument is nilpotent.

Of course there is no obvious way to decide between the efficiency of incommensurable methods. For example, take an operation with one method for permutation groups, another method for nilpotent groups, but no method for nilpotent permutation groups, and call this operation with a permutation group known to be nilpotent.

#### 78.4 Partial Methods

##### 78.4-1 TryNextMethod

`‣ TryNextMethod`( ) | ( function )

After a method has been selected and called, the method may recognize that it cannot compute the desired result, and give up by calling `TryNextMethod()`.

In effect, the execution of the method is terminated, and the method selection calls the next method that is applicable w.r.t. the original arguments. In other words, the applicable method is called that is subsequent to the one that called `TryNextMethod`, according to decreasing rank of the methods.

For example, since every finite group of odd order is solvable, one may install a method for the property `IsSolvableGroup` (39.15-6) that checks whether the size of the argument is an odd integer, returns `true` if so, and gives up otherwise.

Care is needed if a partial method might modify the type of one of its arguments, for example by computing an attribute or property. If this happens, and the type has really changed, then the method should not exit using `TryNextMethod()` but should call the operation again, as the new information in the type may cause some methods previously judged inapplicable to be applicable. For example, if the above method for `IsSolvableGroup` (39.15-6) actually computes the size, (rather than just examining a stored size), then it must take care to check whether the type of the group has changed.

#### 78.5 Redispatching

As mentioned above the method selection will not test unknown properties. In situations, in which algorithms are only known (or implemented) under certain conditions, however such a test might be actually desired.

One way to achieve this would be to install the method under weaker conditions and explicitly test the properties first, exiting via `TryNextMethod` (78.4-1) if some of them are not fulfilled. A problem of this approach however is that such methods then automatically are ranked lower and that the code does not look nice.

A much better way is to use redispatching: Before deciding that no method has been found one tests these properties and if they turn out to be true the method selection is started anew (and will then find a method).

This can be achieved via the following function:

##### 78.5-1 RedispatchOnCondition

`‣ RedispatchOnCondition`( oper, fampred, reqs, cond, val ) | ( function )

This function installs a method for the operation oper under the conditions fampred and reqs which has absolute value val; that is, the value of the filters reqs is disregarded. cond is a list of filters. If not all the values of properties involved in these filters are already known for actual arguments of the method, they are explicitly tested and if they are fulfilled and stored after this test, the operation is dispatched again. Otherwise the method exits with `TryNextMethod` (78.4-1). This can be used to enforce tests like `IsFinite` (30.4-2) in situations when all existing methods require this property. The list cond may have unbound entries in which case the corresponding argument is ignored for further tests.

#### 78.6 Immediate Methods

Usually a method is called only if its operation has been called and if this method has been selected, see `InstallMethod` (78.2-1).

For attributes and properties, one can install also immediate methods.

##### 78.6-1 InstallImmediateMethod

`‣ InstallImmediateMethod`( opr[, info], filter, rank, method ) | ( function )

`InstallImmediateMethod` installs method as an immediate method for opr, which must be an attribute or a property, with requirement filter and rank rank. The rank must be an integer value that measures the priority of method among the immediate methods for opr. If supplied, info should be a short but informative string that describes the situation in which the method is called.

An immediate method is called automatically as soon as the object lies in filter, provided that the value is not yet known. Afterwards the attribute setter is called in order to store the value, unless the method exits via `TryNextMethod` (78.4-1).

Note the difference to `InstallMethod` (78.2-1) that no family predicate occurs because opr expects only one argument, and that filter is not a list of requirements but the argument requirement itself.

Immediate methods are thought of as a possibility for objects to gain useful knowledge. They must not be used to force the storing of "defining information" in an object. In other words, GAP should work even if all immediate methods are completely disabled. Therefore, the call to `InstallImmediateMethod` installs method also as an ordinary method for opr with requirement filter.

Note that in such a case GAP executes a computation for which it was not explicitly asked by the user. So one should install only those methods as immediate methods that are extremely cheap. To emphasize this, immediate methods are also called zero cost methods. The time for their execution should really be approximately zero.

For example, the size of a permutation group can be computed very cheaply if a stabilizer chain of the group is known. So it is reasonable to install an immediate method for `Size` (30.4-6) with requirement `IsGroup and Tester( stab )`, where stab is the attribute corresponding to the stabilizer chain.

Another example would be the implementation of the conclusion that every finite group of prime power order is nilpotent. This could be done by installing an immediate method for the attribute `IsNilpotentGroup` (39.15-3) with requirement `IsGroup and Tester( Size )`. This method would then check whether the size is a finite prime power, return `true` in this case and otherwise call `TryNextMethod` (78.4-1). But this requires factoring of an integer, which cannot be guaranteed to be very cheap, so one should not install this method as an immediate method.

#### 78.7 Logical Implications

##### 78.7-1 InstallTrueMethod

`‣ InstallTrueMethod`( newfil, filt ) | ( function )

It may happen that a filter newfil shall be implied by another filter filt, which is usually a meet of other properties, or the meet of some properties and some categories. Such a logical implication can be installed as an "immediate method" for newfil that requires filt and that always returns `true`. (This should not be mixed up with the methods installed via `InstallImmediateMethod` (78.6-1), which have to be called at runtime for the actual objects.)

`InstallTrueMethod` has the effect that newfil becomes an implied filter of filt, see 13.2.

For example, each cyclic group is abelian, each finite vector space is finite dimensional, and each division ring is integral. The first of these implications is installed as follows.

    InstallTrueMethod( IsCommutative, IsGroup and IsCyclic );

Contrary to the immediate methods installed with `InstallImmediateMethod` (78.6-1), logical implications cannot be switched off. This means that after the above implication has been installed, one can rely on the fact that every object in the filter `IsGroup and IsCyclic` will also be in the filter `IsCommutative` (35.4-9).

#### 78.8 Operations and Mathematical Terms

Usually an operation stands for a mathematical concept, and the name of the operation describes this uniquely. Examples are the property `IsFinite` (30.4-2) and the attribute `Size` (30.4-6). But there are cases where the same mathematical term is used to denote different concepts, for example `Degree` is defined for polynomials, group characters, and permutation actions, and `Rank` is defined for matrices, free modules, p-groups, and transitive permutation actions.

It is in principle possible to install methods for the operation `Rank` that are applicable to the different types of arguments, corresponding to the different contexts. But this is not the approach taken in the GAP library. Instead there are operations such as `RankMat` (24.7-1) for matrices and `DegreeOfCharacter` (72.8-4) (in fact these are attributes) which are installed as methods of the "ambiguous" operations `Rank` and `Degree`.

The idea is to distinguish between on the one hand different ways to compute the same thing (e.g. different methods for `\=` (31.11-1), `Size` (30.4-6), etc.), and on the other hand genuinely different things (such as the degree of a polynomial and a permutation action).

The former is the basic purpose of operations and attributes. The latter is provided as a user convenience where mathematical usage forces it on us and where no conflicts arise. In programming the library, we use the underlying mathematically precise operations or attributes, such as `RankMat` (24.7-1) and `RankOperation`. These should be attributes if appropriate, and the only role of the operation `Rank` is to decide which attribute the user meant. That way, stored information is stored with "full mathematical precision" and is less likely to be retrieved for a wrong purpose later.

One word about possible conflicts. A typical example is the mathematical term "centre", which is defined as { x ∈ M | a * x = x * a ∀ a ∈ M } for a magma M, and as { x ∈ L | l * x = 0 ∀ l ∈ L } for a Lie algebra L. Here it is not possible to introduce an operation `Centre` (35.4-5) that delegates to attributes `CentreOfMagma` and `CentreOfLieAlgebra`, depending on the type of the argument. This is because any Lie algebra in GAP is also a magma, so both `CentreOfMagma` and `CentreOfLieAlgebra` would be defined for a Lie algebra, with different meaning if the characteristic is two. So we cannot achieve that one operation in GAP corresponds to the mathematical term "centre".

"Ambiguous" operations such as `Rank` are declared in the library file `lib/overload.g`.

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