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<p><a id="X7E8202627B421DB1" name="X7E8202627B421DB1"></a></p>
<div class="ChapSects"><a href="chap13.html#X7E8202627B421DB1">13 <span class="Heading">Types of Objects</span></a>
<div class="ContSect"><span class="tocline"><span class="nocss"> </span><a href="chap13.html#X846063757EC05986">13.1 <span class="Heading">Families</span></a>
</span>
<div class="ContSSBlock">
<span class="ContSS"><br /><span class="nocss"> </span><a href="chap13.html#X7CF70EAC84284919">13.1-1 FamilyObj</a></span>
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<div class="ContSect"><span class="tocline"><span class="nocss"> </span><a href="chap13.html#X84EFA4C07D4277BB">13.2 <span class="Heading">Filters</span></a>
</span>
<div class="ContSSBlock">
<span class="ContSS"><br /><span class="nocss"> </span><a href="chap13.html#X82E62B997C05E05E">13.2-1 RankFilter</a></span>
<span class="ContSS"><br /><span class="nocss"> </span><a href="chap13.html#X7A78ECC67E2C9D78">13.2-2 NamesFilter</a></span>
<span class="ContSS"><br /><span class="nocss"> </span><a href="chap13.html#X7F9568A67F3840DE">13.2-3 ShowImpliedFilters</a></span>
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<div class="ContSect"><span class="tocline"><span class="nocss"> </span><a href="chap13.html#X7CC6903E78F24167">13.3 <span class="Heading">Categories</span></a>
</span>
<div class="ContSSBlock">
<span class="ContSS"><br /><span class="nocss"> </span><a href="chap13.html#X85C6EB707A406A5A">13.3-1 CategoriesOfObject</a></span>
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<div class="ContSect"><span class="tocline"><span class="nocss"> </span><a href="chap13.html#X8698205F8648EB33">13.4 <span class="Heading">Representation</span></a>
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<div class="ContSSBlock">
<span class="ContSS"><br /><span class="nocss"> </span><a href="chap13.html#X7BBE93BE7977750F">13.4-1 RepresentationsOfObject</a></span>
</div></div>
<div class="ContSect"><span class="tocline"><span class="nocss"> </span><a href="chap13.html#X7C701DBF7BAE649A">13.5 <span class="Heading">Attributes</span></a>
</span>
<div class="ContSSBlock">
<span class="ContSS"><br /><span class="nocss"> </span><a href="chap13.html#X7F7960338163AA88">13.5-1 KnownAttributesOfObject</a></span>
</div></div>
<div class="ContSect"><span class="tocline"><span class="nocss"> </span><a href="chap13.html#X79DE5208877AE42A">13.6 <span class="Heading">Setter and Tester for Attributes</span></a>
</span>
<div class="ContSSBlock">
<span class="ContSS"><br /><span class="nocss"> </span><a href="chap13.html#X87D5B5AC7DAF932D">13.6-1 Tester</a></span>
<span class="ContSS"><br /><span class="nocss"> </span><a href="chap13.html#X7FD8952C841D2B1F">13.6-2 Setter</a></span>
<span class="ContSS"><br /><span class="nocss"> </span><a href="chap13.html#X8529F8A17884A32C">13.6-3 AttributeValueNotSet</a></span>
<span class="ContSS"><br /><span class="nocss"> </span><a href="chap13.html#X79120CE37BB69D11">13.6-4 InfoAttributes</a></span>
<span class="ContSS"><br /><span class="nocss"> </span><a href="chap13.html#X7851E2DB79656DB0">13.6-5 DisableAttributeValueStoring</a></span>
<span class="ContSS"><br /><span class="nocss"> </span><a href="chap13.html#X7E5DACBE7A9A9AD1">13.6-6 EnableAttributeValueStoring</a></span>
</div></div>
<div class="ContSect"><span class="tocline"><span class="nocss"> </span><a href="chap13.html#X871597447BB998A1">13.7 <span class="Heading">Properties</span></a>
</span>
<div class="ContSSBlock">
<span class="ContSS"><br /><span class="nocss"> </span><a href="chap13.html#X7E51C08286E03E7F">13.7-1 KnownPropertiesOfObject</a></span>
<span class="ContSS"><br /><span class="nocss"> </span><a href="chap13.html#X86711BC77B62EB02">13.7-2 KnownTruePropertiesOfObject</a></span>
</div></div>
<div class="ContSect"><span class="tocline"><span class="nocss"> </span><a href="chap13.html#X7997705185C7E720">13.8 <span class="Heading">Other Filters</span></a>
</span>
</div>
<div class="ContSect"><span class="tocline"><span class="nocss"> </span><a href="chap13.html#X7E340B8C833BC440">13.9 <span class="Heading">Types</span></a>
</span>
<div class="ContSSBlock">
<span class="ContSS"><br /><span class="nocss"> </span><a href="chap13.html#X7D3E6B6482BE5B16">13.9-1 TypeObj</a></span>
<span class="ContSS"><br /><span class="nocss"> </span><a href="chap13.html#X85A60A7F8083C1C4">13.9-2 DataType</a></span>
</div></div>
</div>
<h3>13 <span class="Heading">Types of Objects</span></h3>
<p>Every <strong class="pkg">GAP</strong> object has a <em>type</em>. The type of an object is the information which is used to decide whether an operation is admissible or possible with that object as an argument, and if so, how it is to be performed (see Chapter <a href="chap78.html#X8058CC8187162644"><span class="RefLink">78</span></a>).</p>
<p>For example, the types determine whether two objects can be multiplied and what function is called to compute the product. Analogously, the type of an object determines whether and how the size of the object can be computed. It is sometimes useful in discussing the type system, to identify types with the set of objects that have this type. Partial types can then also be regarded as sets, such that any type is the intersection of its parts.</p>
<p>The type of an object consists of two main parts, which describe different aspects of the object.</p>
<p>The <em>family</em> determines the relation of the object to other objects. For example, all permutations form a family. Another family consists of all collections of permutations, this family contains the set of permutation groups as a subset. A third family consists of all rational functions with coefficients in a certain family.</p>
<p>The other part of a type is a collection of <em>filters</em> (actually stored as a bit-list indicating, from the complete set of possible filters, which are included in this particular type). These filters are all treated equally by the method selection, but, from the viewpoint of their creation and use, they can be divided (with a small number of unimportant exceptions) into categories, representations, attribute testers and properties. Each of these is described in more detail below.</p>
<p>This chapter does not describe how types and their constituent parts can be created. Information about this topic can be found in Chapter <a href="chap79.html#X83548994805AD1C9"><span class="RefLink">79</span></a>.</p>
<p><em>Note:</em> Detailed understanding of the type system is not required to use <strong class="pkg">GAP</strong>. It can be helpful, however, to understand how things work and why <strong class="pkg">GAP</strong> behaves the way it does.</p>
<p>A discussion of the type system can be found in <a href="chapBib.html#biBBreuerLinton98">[BL98]</a>.</p>
<p><a id="X846063757EC05986" name="X846063757EC05986"></a></p>
<h4>13.1 <span class="Heading">Families</span></h4>
<p>The family of an object determines its relationship to other objects.</p>
<p>More precisely, the families form a partition of all <strong class="pkg">GAP</strong> objects such that the following two conditions hold: objects that are equal w.r.t. <code class="code">=</code> lie in the same family; and the family of the result of an operation depends only on the families of its operands.</p>
<p>The first condition means that a family can be regarded as a set of elements instead of a set of objects. Note that this does not hold for categories and representations (see below), two objects that are equal w.r.t. <code class="code">=</code> need not lie in the same categories and representations. For example, a sparsely represented matrix can be equal to a densely represented matrix. Similarly, each domain is equal w.r.t. <code class="code">=</code> to the sorted list of its elements, but a domain is not a list, and a list is not a domain.</p>
<p><a id="X7CF70EAC84284919" name="X7CF70EAC84284919"></a></p>
<h5>13.1-1 FamilyObj</h5>
<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ FamilyObj</code>( <var class="Arg">obj</var> )</td><td class="tdright">( function )</td></tr></table></div>
<p>returns the family of the object <var class="Arg">obj</var>.</p>
<p>The family of the object <var class="Arg">obj</var> is itself an object, its family is <code class="code">FamilyOfFamilies</code>.</p>
<p>It should be emphasized that families may be created when they are needed. For example, the family of elements of a finitely presented group is created only after the presentation has been constructed. Thus families are the dynamic part of the type system, that is, the part that is not fixed after the initialisation of <strong class="pkg">GAP</strong>.</p>
<p>Families can be parametrized. For example, the elements of each finitely presented group form a family of their own. Here the family of elements and the finitely presented group coincide when viewed as sets. Note that elements in different finitely presented groups lie in different families. This distinction allows <strong class="pkg">GAP</strong> to forbid multiplications of elements in different finitely presented groups.</p>
<p>As a special case, families can be parametrized by other families. An important example is the family of <em>collections</em> that can be formed for each family. A collection consists of objects that lie in the same family, it is either a nonempty dense list of objects from the same family or a domain.</p>
<p>Note that every domain is a collection, that is, it is not possible to construct domains whose elements lie in different families. For example, a polynomial ring over the rationals cannot contain the integer <code class="code">0</code> because the family that contains the integers does not contain polynomials. So one has to distinguish the integer zero from each zero polynomial.</p>
<p>Let us look at this example from a different viewpoint. A polynomial ring and its coefficients ring lie in different families, hence the coefficients ring cannot be embedded "naturally" into the polynomial ring in the sense that it is a subset. But it is possible to allow, e.g., the multiplication of an integer and a polynomial over the integers. The relation between the arguments, namely that one is a coefficient and the other a polynomial, can be detected from the relation of their families. Moreover, this analysis is easier than in a situation where the rationals would lie in one family together with all polynomials over the rationals, because then the relation of families would not distinguish the multiplication of two polynomials, the multiplication of two coefficients, and the multiplication of a coefficient with a polynomial. So the wish to describe relations between elements can be taken as a motivation for the introduction of families.</p>
<p><a id="X84EFA4C07D4277BB" name="X84EFA4C07D4277BB"></a></p>
<h4>13.2 <span class="Heading">Filters</span></h4>
<p>A <em>filter</em> is a special unary <strong class="pkg">GAP</strong> function that returns either <code class="keyw">true</code> or <code class="keyw">false</code>, depending on whether or not the argument lies in the set defined by the filter. Filters are used to express different aspects of information about a <strong class="pkg">GAP</strong> object, which are described below (see <a href="chap13.html#X7CC6903E78F24167"><span class="RefLink">13.3</span></a>, <a href="chap13.html#X8698205F8648EB33"><span class="RefLink">13.4</span></a>, <a href="chap13.html#X7C701DBF7BAE649A"><span class="RefLink">13.5</span></a>, <a href="chap13.html#X79DE5208877AE42A"><span class="RefLink">13.6</span></a>, <a href="chap13.html#X871597447BB998A1"><span class="RefLink">13.7</span></a>, <a href="chap13.html#X7997705185C7E720"><span class="RefLink">13.8</span></a>).</p>
<p>Presently any filter in <strong class="pkg">GAP</strong> is implemented as a function which corresponds to a set of positions in the bitlist which forms part of the type of each <strong class="pkg">GAP</strong> object, and returns <code class="keyw">true</code> if and only if the bitlist of the type of the argument has the value <code class="keyw">true</code> at all of these positions.</p>
<p>The intersection (or meet) of two filters <var class="Arg">filt1</var>, <var class="Arg">filt2</var> is again a filter, it can be formed as</p>
<p><var class="Arg">filt1</var> <code class="keyw">and</code> <var class="Arg">filt2</var></p>
<p>See <a href="chap20.html#X79AD41A185FD7213"><span class="RefLink">20.4</span></a> for more details.</p>
<p>For example, <code class="code">IsList and IsEmpty</code> is a filter that returns <code class="keyw">true</code> if its argument is an empty list, and <code class="keyw">false</code> otherwise. The filter <code class="func">IsGroup</code> (<a href="chap39.html#X7939B3177BBD61E4"><span class="RefLink">39.2-7</span></a>) is defined as the intersection of the category <code class="func">IsMagmaWithInverses</code> (<a href="chap35.html#X82CBFF648574B830"><span class="RefLink">35.1-4</span></a>) and the property <code class="func">IsAssociative</code> (<a href="chap35.html#X7C83B5A47FD18FB7"><span class="RefLink">35.4-7</span></a>).</p>
<p>A filter that is not the meet of other filters is called a <em>simple filter</em>. For example, each attribute tester (see <a href="chap13.html#X79DE5208877AE42A"><span class="RefLink">13.6</span></a>) is a simple filter. Each simple filter corresponds to a position in the bitlist currently used as part of the data structure representing a type.</p>
<p>Every filter has a <em>rank</em>, which is used to define a ranking of the methods installed for an operation, see Section <a href="chap78.html#X795EE8257848B438"><span class="RefLink">78.2</span></a>. The rank of a filter can be accessed with <code class="func">RankFilter</code> (<a href="chap13.html#X82E62B997C05E05E"><span class="RefLink">13.2-1</span></a>).</p>
<p><a id="X82E62B997C05E05E" name="X82E62B997C05E05E"></a></p>
<h5>13.2-1 RankFilter</h5>
<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ RankFilter</code>( <var class="Arg">filt</var> )</td><td class="tdright">( function )</td></tr></table></div>
<p>For simple filters, an <em>incremental rank</em> is defined when the filter is created, see the sections about the creation of filters: <a href="chap79.html#X78DD5C237960B40B"><span class="RefLink">79.1</span></a>, <a href="chap79.html#X7858E2848048F99D"><span class="RefLink">79.2</span></a>, <a href="chap79.html#X7A38E7E87CCCEDD1"><span class="RefLink">79.3</span></a>, <a href="chap79.html#X80B191247B4287FC"><span class="RefLink">79.4</span></a>. For an arbitrary filter, its rank is given by the sum of the incremental ranks of the <em>involved</em> simple filters; in addition to the implied filters, these are also the required filters of attributes (again see the sections about the creation of filters). In other words, for the purpose of computing the rank and <em>only</em> for this purpose, attribute testers are treated as if they would imply the requirements of their attributes.</p>
<p><a id="X7A78ECC67E2C9D78" name="X7A78ECC67E2C9D78"></a></p>
<h5>13.2-2 NamesFilter</h5>
<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ NamesFilter</code>( <var class="Arg">filt</var> )</td><td class="tdright">( function )</td></tr></table></div>
<p><code class="func">NamesFilter</code> returns a list of names of the <em>implied</em> simple filters of the filter <var class="Arg">filt</var>, these are all those simple filters <code class="code">imp</code> such that every object in <var class="Arg">filt</var> also lies in <code class="code">imp</code>. For implications between filters, see <code class="func">ShowImpliedFilters</code> (<a href="chap13.html#X7F9568A67F3840DE"><span class="RefLink">13.2-3</span></a>) as well as sections <a href="chap78.html#X7FB5016E83DB4349"><span class="RefLink">78.7</span></a>, <a href="chap79.html#X78DD5C237960B40B"><span class="RefLink">79.1</span></a>, <a href="chap79.html#X7858E2848048F99D"><span class="RefLink">79.2</span></a>, <a href="chap79.html#X7A38E7E87CCCEDD1"><span class="RefLink">79.3</span></a>.</p>
<p><a id="X7F9568A67F3840DE" name="X7F9568A67F3840DE"></a></p>
<h5>13.2-3 ShowImpliedFilters</h5>
<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ ShowImpliedFilters</code>( <var class="Arg">filter</var> )</td><td class="tdright">( function )</td></tr></table></div>
<p>Displays information about the filters that may be implied by <var class="Arg">filter</var>. They are given by their names. <code class="code">ShowImpliedFilters</code> first displays the names of all filters that are unconditionally implied by <var class="Arg">filter</var>. It then displays implications that require further filters to be present (indicating by <code class="code">+</code> the required further filters). The function displays only first-level implications, implications that follow in turn are not displayed (though <strong class="pkg">GAP</strong> will do these).</p>
<div class="example"><pre>
<span class="GAPprompt">gap></span> <span class="GAPinput">ShowImpliedFilters(IsMatrix);</span>
Implies:
IsGeneralizedRowVector
IsNearAdditiveElementWithInverse
IsAdditiveElement
IsMultiplicativeElement
May imply with:
+IsGF2MatrixRep
IsOrdinaryMatrix
+CategoryCollections(CategoryCollections(IsAdditivelyCommutativeElement))
IsAdditivelyCommutativeElement
+IsInternalRep
IsOrdinaryMatrix
</pre></div>
<p><a id="X7CC6903E78F24167" name="X7CC6903E78F24167"></a></p>
<h4>13.3 <span class="Heading">Categories</span></h4>
<p>The <em>categories</em> of an object are filters (see <a href="chap13.html#X84EFA4C07D4277BB"><span class="RefLink">13.2</span></a>) that determine what operations an object admits. For example, all integers form a category, all rationals form a category, and all rational functions form a category. An object which claims to lie in a certain category is accepting the requirement that it should have methods for certain operations (and perhaps that their behaviour should satisfy certain axioms). For example, an object lying in the category <code class="func">IsList</code> (<a href="chap21.html#X7C4CC4EA8299701E"><span class="RefLink">21.1-1</span></a>) must have methods for <code class="func">Length</code> (<a href="chap21.html#X780769238600AFD1"><span class="RefLink">21.17-5</span></a>), <code class="func">IsBound\[\]</code> (<a href="chap21.html#X8297BBCD79642BE6"><span class="RefLink">21.2-1</span></a>) and the list element access operation <code class="func">\[\]</code> (<a href="chap21.html#X8297BBCD79642BE6"><span class="RefLink">21.2-1</span></a>).</p>
<p>An object can lie in several categories. For example, a row vector lies in the categories <code class="func">IsList</code> (<a href="chap21.html#X7C4CC4EA8299701E"><span class="RefLink">21.1-1</span></a>) and <code class="func">IsVector</code> (<a href="chap31.html#X802F34F280B29DF4"><span class="RefLink">31.14-14</span></a>); each list lies in the category <code class="func">IsCopyable</code> (<a href="chap12.html#X811EFD727EBD1ADC"><span class="RefLink">12.6-1</span></a>), and depending on whether or not it is mutable, it may lie in the category <code class="func">IsMutable</code> (<a href="chap12.html#X7999AD1D7A4F1F46"><span class="RefLink">12.6-2</span></a>). Every domain lies in the category <code class="func">IsDomain</code> (<a href="chap31.html#X86B4AC017FAF4D12"><span class="RefLink">31.9-1</span></a>).</p>
<p>Of course some categories of a mutable object may change when the object is changed. For example, after assigning values to positions of a mutable non-dense list, this list may become part of the category <code class="func">IsDenseList</code> (<a href="chap21.html#X870AA9D8798C93DD"><span class="RefLink">21.1-2</span></a>).</p>
<p>However, if an object is immutable then the set of categories it lies in is fixed.</p>
<p>All categories in the library are created during initialization, in particular they are not created dynamically at runtime.</p>
<p>The following list gives an overview of important categories of arithmetic objects. Indented categories are to be understood as subcategories of the non indented category listed above it.</p>
<div class="example"><pre>
IsObject
IsExtLElement
IsExtRElement
IsMultiplicativeElement
IsMultiplicativeElementWithOne
IsMultiplicativeElementWithInverse
IsExtAElement
IsAdditiveElement
IsAdditiveElementWithZero
IsAdditiveElementWithInverse
</pre></div>
<p>Every object lies in the category <code class="func">IsObject</code> (<a href="chap12.html#X7B130AC98415CAFB"><span class="RefLink">12.1-1</span></a>).</p>
<p>The categories <code class="func">IsExtLElement</code> (<a href="chap31.html#X860D1E387DD5CCCF"><span class="RefLink">31.14-8</span></a>) and <code class="func">IsExtRElement</code> (<a href="chap31.html#X809E0C097E480AF1"><span class="RefLink">31.14-9</span></a>) contain objects that can be multiplied with other objects via <code class="code">*</code> from the left and from the right, respectively. These categories are required for the operands of the operation <code class="code">*</code>.</p>
<p>The category <code class="func">IsMultiplicativeElement</code> (<a href="chap31.html#X797D3B2A7A2B2F53"><span class="RefLink">31.14-10</span></a>) contains objects that can be multiplied from the left and from the right with objects from the same family. <code class="func">IsMultiplicativeElementWithOne</code> (<a href="chap31.html#X82BC294F7D388AE8"><span class="RefLink">31.14-11</span></a>) contains objects <code class="code">obj</code> for which a multiplicatively neutral element can be obtained by taking the <span class="SimpleMath">0</span>-th power <code class="code">obj^0</code>. <code class="func">IsMultiplicativeElementWithInverse</code> (<a href="chap31.html#X7FDB14E57814FA3B"><span class="RefLink">31.14-13</span></a>) contains objects <code class="code">obj</code> for which a multiplicative inverse can be obtained by forming <code class="code">obj^-1</code>.</p>
<p>Likewise, the categories <code class="func">IsExtAElement</code> (<a href="chap31.html#X7FBD4F65861C2DF2"><span class="RefLink">31.14-1</span></a>), <code class="func">IsAdditiveElement</code> (<a href="chap31.html#X78D042B486E1D7F7"><span class="RefLink">31.14-3</span></a>), <code class="func">IsAdditiveElementWithZero</code> (<a href="chap31.html#X87F3552A789C572D"><span class="RefLink">31.14-5</span></a>) and <code class="func">IsAdditiveElementWithInverse</code> (<a href="chap31.html#X7C0E4AE883947778"><span class="RefLink">31.14-7</span></a>) contain objects that can be added via <code class="code">+</code> to other objects, objects that can be added to objects of the same family, objects for which an additively neutral element can be obtained by multiplication with zero, and objects for which an additive inverse can be obtained by multiplication with <code class="code">-1</code>.</p>
<p>So a vector lies in <code class="func">IsExtLElement</code> (<a href="chap31.html#X860D1E387DD5CCCF"><span class="RefLink">31.14-8</span></a>), <code class="func">IsExtRElement</code> (<a href="chap31.html#X809E0C097E480AF1"><span class="RefLink">31.14-9</span></a>) and <code class="func">IsAdditiveElementWithInverse</code> (<a href="chap31.html#X7C0E4AE883947778"><span class="RefLink">31.14-7</span></a>). A ring element must additionally lie in <code class="func">IsMultiplicativeElement</code> (<a href="chap31.html#X797D3B2A7A2B2F53"><span class="RefLink">31.14-10</span></a>).</p>
<p>As stated above it is not guaranteed by the categories of objects whether the result of an operation with these objects as arguments is defined. For example, the category <code class="func">IsMatrix</code> (<a href="chap24.html#X7E1AE46B862B185F"><span class="RefLink">24.2-1</span></a>) is a subcategory of <code class="func">IsMultiplicativeElementWithInverse</code> (<a href="chap31.html#X7FDB14E57814FA3B"><span class="RefLink">31.14-13</span></a>). Clearly not every matrix has a multiplicative inverse. But the category <code class="func">IsMatrix</code> (<a href="chap24.html#X7E1AE46B862B185F"><span class="RefLink">24.2-1</span></a>) makes each matrix an admissible argument of the operation <code class="func">Inverse</code> (<a href="chap31.html#X78EE524E83624057"><span class="RefLink">31.10-8</span></a>), which may sometimes return <code class="keyw">fail</code>. Likewise, two matrices can be multiplied only if they are of appropriate shapes.</p>
<p>Analogous to the categories of arithmetic elements, there are categories of domains of these elements.</p>
<div class="example"><pre>
IsObject
IsDomain
IsMagma
IsMagmaWithOne
IsMagmaWithInversesIfNonzero
IsMagmaWithInverses
IsAdditiveMagma
IsAdditiveMagmaWithZero
IsAdditiveMagmaWithInverses
IsExtLSet
IsExtRSet
</pre></div>
<p>Of course <code class="func">IsDomain</code> (<a href="chap31.html#X86B4AC017FAF4D12"><span class="RefLink">31.9-1</span></a>) is a subcategory of <code class="func">IsObject</code> (<a href="chap12.html#X7B130AC98415CAFB"><span class="RefLink">12.1-1</span></a>). A domain that is closed under multiplication <code class="code">*</code> is called a magma and it lies in the category <code class="func">IsMagma</code> (<a href="chap35.html#X87D3F38B7EAB13FA"><span class="RefLink">35.1-1</span></a>). If a magma is closed under taking the identity, it lies in <code class="func">IsMagmaWithOne</code> (<a href="chap35.html#X86071DE7835F1C7C"><span class="RefLink">35.1-2</span></a>), and if it is also closed under taking inverses, it lies in <code class="func">IsMagmaWithInverses</code> (<a href="chap35.html#X82CBFF648574B830"><span class="RefLink">35.1-4</span></a>). The category <code class="func">IsMagmaWithInversesIfNonzero</code> (<a href="chap35.html#X83E4903D7FBB2E24"><span class="RefLink">35.1-3</span></a>) denotes closure under taking inverses only for nonzero elements, every division ring lies in this category.</p>
<p>Note that every set of categories constitutes its own notion of generation, for example a group may be generated as a magma with inverses by some elements, but to generate it as a magma with one it may be necessary to take the union of these generators and their inverses.</p>
<p><a id="X85C6EB707A406A5A" name="X85C6EB707A406A5A"></a></p>
<h5>13.3-1 CategoriesOfObject</h5>
<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ CategoriesOfObject</code>( <var class="Arg">object</var> )</td><td class="tdright">( operation )</td></tr></table></div>
<p>returns a list of the names of the categories in which <var class="Arg">object</var> lies.</p>
<div class="example"><pre>
<span class="GAPprompt">gap></span> <span class="GAPinput">g:=Group((1,2),(1,2,3));;</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">CategoriesOfObject(g);</span>
[ "IsListOrCollection", "IsCollection", "IsExtLElement",
"CategoryCollections(IsExtLElement)", "IsExtRElement",
"CategoryCollections(IsExtRElement)",
"CategoryCollections(IsMultiplicativeElement)",
"CategoryCollections(IsMultiplicativeElementWithOne)",
"CategoryCollections(IsMultiplicativeElementWithInverse)",
"CategoryCollections(IsAssociativeElement)",
"CategoryCollections(IsFiniteOrderElement)", "IsGeneralizedDomain",
"CategoryCollections(IsPerm)", "IsMagma", "IsMagmaWithOne",
"IsMagmaWithInversesIfNonzero", "IsMagmaWithInverses" ]
</pre></div>
<p><a id="X8698205F8648EB33" name="X8698205F8648EB33"></a></p>
<h4>13.4 <span class="Heading">Representation</span></h4>
<p>The <em>representation</em> of an object is a set of filters (see <a href="chap13.html#X84EFA4C07D4277BB"><span class="RefLink">13.2</span></a>) that determines how an object is actually represented. For example, a matrix or a polynomial can be stored sparsely or densely; all dense polynomials form a representation. An object which claims to lie in a certain representation is accepting the requirement that certain fields in the data structure be present and have specified meanings.</p>
<p><strong class="pkg">GAP</strong> distinguishes four essentially different ways to represent objects. First there are the representations <code class="code">IsInternalRep</code> for internal objects such as integers and permutations, and <code class="code">IsDataObjectRep</code> for other objects that are created and whose data are accessible only by kernel functions. The data structures underlying such objects cannot be manipulated at the <strong class="pkg">GAP</strong> level.</p>
<p>All other objects are either in the representation <code class="code">IsComponentObjectRep</code> or in the representation <code class="code">IsPositionalObjectRep</code>, see <a href="chap79.html#X866E223484649E5A"><span class="RefLink">79.10</span></a> and <a href="chap79.html#X834893D07FAA6FD2"><span class="RefLink">79.11</span></a>.</p>
<p>An object can belong to several representations in the sense that it lies in several subrepresentations of <code class="code">IsComponentObjectRep</code> or of <code class="code">IsPositionalObjectRep</code>. The representations to which an object belongs should form a chain and either two representations are disjoint or one is contained in the other. So the subrepresentations of <code class="code">IsComponentObjectRep</code> and <code class="code">IsPositionalObjectRep</code> each form trees. In the language of Object Oriented Programming, we support only single inheritance for representations.</p>
<p>These trees are typically rather shallow, since for one representation to be contained in another implies that all the components of the data structure implied by the containing representation, are present in, and have the same meaning in, the smaller representation (whose data structure presumably contains some additional components).</p>
<p>Objects may change their representation, for example a mutable list of characters can be converted into a string.</p>
<p>All representations in the library are created during initialization, in particular they are not created dynamically at runtime.</p>
<p>Examples of subrepresentations of <code class="code">IsPositionalObjectRep</code> are <code class="code">IsModulusRep</code>, which is used for residue classes in the ring of integers, and <code class="code">IsDenseCoeffVectorRep</code>, which is used for elements of algebras that are defined by structure constants.</p>
<p>An important subrepresentation of <code class="code">IsComponentObjectRep</code> is <code class="code">IsAttributeStoringRep</code>, which is used for many domains and some other objects. It provides automatic storing of all attribute values (see below).</p>
<p><a id="X7BBE93BE7977750F" name="X7BBE93BE7977750F"></a></p>
<h5>13.4-1 RepresentationsOfObject</h5>
<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ RepresentationsOfObject</code>( <var class="Arg">object</var> )</td><td class="tdright">( operation )</td></tr></table></div>
<p>returns a list of the names of the representations <var class="Arg">object</var> has.</p>
<div class="example"><pre>
<span class="GAPprompt">gap></span> <span class="GAPinput">g:=Group((1,2),(1,2,3));;</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">RepresentationsOfObject(g);</span>
[ "IsComponentObjectRep", "IsAttributeStoringRep" ]
</pre></div>
<p><a id="X7C701DBF7BAE649A" name="X7C701DBF7BAE649A"></a></p>
<h4>13.5 <span class="Heading">Attributes</span></h4>
<p>The attributes of an object describe knowledge about it.</p>
<p>An attribute is a unary operation without side-effects.</p>
<p>An object may store values of its attributes once they have been computed, and claim that it knows these values. Note that "store" and "know" have to be understood in the sense that it is very cheap to get such a value when the attribute is called again.</p>
<p>The stored value of an attribute is in general immutable (see <a href="chap12.html#X7F0C119682196D65"><span class="RefLink">12.6</span></a>), except if the attribute had been specially constructed as "mutable attribute".</p>
<p>It depends on the representation of an object (see <a href="chap13.html#X8698205F8648EB33"><span class="RefLink">13.4</span></a>) which attribute values it stores. An object in the representation <code class="code">IsAttributeStoringRep</code> stores <em>all</em> attribute values once they are computed. Moreover, for an object in this representation, subsequent calls to an attribute will return the <em>same</em> object; this is achieved via a special method for each attribute setter that stores the attribute value in an object in <code class="code">IsAttributeStoringRep</code>, and a special method for the attribute itself that fetches the stored attribute value. (These methods are called the "system setter" and the "system getter" of the attribute, respectively.)</p>
<p>Note also that it is impossible to get rid of a stored attribute value because the system may have drawn conclusions from the old attribute value, and just removing the value might leave the data structures in an inconsistent state. If necessary, a new object can be constructed.</p>
<p>Several attributes have methods for more than one argument. For example <code class="func">IsTransitive</code> (<a href="chap41.html#X79B15750851828CB"><span class="RefLink">41.10-1</span></a>) is an attribute for a <span class="SimpleMath">G</span>-set that can also be called for the two arguments, being a group <span class="SimpleMath">G</span> and its action domain. If attributes are called with more than one argument then the return value is not stored in any of the arguments.</p>
<p>Properties are a special form of attributes that have the value <code class="keyw">true</code> or <code class="keyw">false</code>, see section <a href="chap13.html#X871597447BB998A1"><span class="RefLink">13.7</span></a>.</p>
<p>Examples of attributes for multiplicative elements are <code class="func">Inverse</code> (<a href="chap31.html#X78EE524E83624057"><span class="RefLink">31.10-8</span></a>), <code class="func">One</code> (<a href="chap31.html#X8046262384895B2A"><span class="RefLink">31.10-2</span></a>), and <code class="func">Order</code> (<a href="chap31.html#X84F59A2687C62763"><span class="RefLink">31.10-10</span></a>). <code class="func">Size</code> (<a href="chap30.html#X858ADA3B7A684421"><span class="RefLink">30.4-6</span></a>) is an attribute for domains, <code class="func">Centre</code> (<a href="chap35.html#X847ABE6F781C7FE8"><span class="RefLink">35.4-5</span></a>) is an attribute for magmas, and <code class="func">DerivedSubgroup</code> (<a href="chap39.html#X7CC17CF179ED7EF2"><span class="RefLink">39.12-3</span></a>) is an attribute for groups.</p>
<p><a id="X7F7960338163AA88" name="X7F7960338163AA88"></a></p>
<h5>13.5-1 KnownAttributesOfObject</h5>
<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ KnownAttributesOfObject</code>( <var class="Arg">object</var> )</td><td class="tdright">( operation )</td></tr></table></div>
<p>returns a list of the names of the attributes whose values are known for <var class="Arg">object</var>.</p>
<div class="example"><pre>
<span class="GAPprompt">gap></span> <span class="GAPinput">g:=Group((1,2),(1,2,3));;Size(g);;</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">KnownAttributesOfObject(g);</span>
[ "Size", "OneImmutable", "NrMovedPoints", "MovedPoints",
"GeneratorsOfMagmaWithInverses", "MultiplicativeNeutralElement",
"HomePcgs", "Pcgs", "GeneralizedPcgs", "StabChainMutable",
"StabChainOptions" ]
</pre></div>
<p><a id="X79DE5208877AE42A" name="X79DE5208877AE42A"></a></p>
<h4>13.6 <span class="Heading">Setter and Tester for Attributes</span></h4>
<p>For every attribute, the <em>attribute setter</em> and the <em>attribute tester</em> are defined.</p>
<p>To check whether an object belongs to an attribute <var class="Arg">attr</var>, the tester of the attribute is used, see <code class="func">Tester</code> (<a href="chap13.html#X87D5B5AC7DAF932D"><span class="RefLink">13.6-1</span></a>). To store a value for the attribute <var class="Arg">attr</var> in an object, the setter of the attribute is used, see <code class="func">Setter</code> (<a href="chap13.html#X7FD8952C841D2B1F"><span class="RefLink">13.6-2</span></a>).</p>
<p><a id="X87D5B5AC7DAF932D" name="X87D5B5AC7DAF932D"></a></p>
<h5>13.6-1 Tester</h5>
<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ Tester</code>( <var class="Arg">attr</var> )</td><td class="tdright">( function )</td></tr></table></div>
<p>For an attribute <var class="Arg">attr</var>, <code class="code">Tester(<var class="Arg">attr</var>)</code> is a filter (see <a href="chap13.html#X84EFA4C07D4277BB"><span class="RefLink">13.2</span></a>) that returns <code class="keyw">true</code> or <code class="keyw">false</code>, depending on whether or not the value of <var class="Arg">attr</var> for the object is known. For example, <code class="code">Tester( Size )( <var class="Arg">obj</var> )</code> is <code class="keyw">true</code> if the size of the object <var class="Arg">obj</var> is known.</p>
<p><a id="X7FD8952C841D2B1F" name="X7FD8952C841D2B1F"></a></p>
<h5>13.6-2 Setter</h5>
<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ Setter</code>( <var class="Arg">attr</var> )</td><td class="tdright">( function )</td></tr></table></div>
<p>For an attribute <var class="Arg">attr</var>, <code class="code">Setter(<var class="Arg">attr</var>)</code> is called automatically when the attribute value has been computed for the first time. One can also call the setter explicitly, for example, <code class="code">Setter( Size )( <var class="Arg">obj</var>, <var class="Arg">val</var> )</code> sets <var class="Arg">val</var> as size of the object <var class="Arg">obj</var> if the size was not yet known.</p>
<p>For each attribute <var class="Arg">attr</var> that is declared with <code class="func">DeclareAttribute</code> (<a href="chap79.html#X7A00FC8A7A677A56"><span class="RefLink">79.18-3</span></a>) resp. <code class="func">DeclareProperty</code> (<a href="chap79.html#X7F4602F082682A04"><span class="RefLink">79.18-4</span></a>) (see <a href="chap79.html#X87E29BA57C8208A4"><span class="RefLink">79.18</span></a>), tester and setter are automatically made accessible by the names <code class="code">Has<var class="Arg">attr</var></code> and <code class="code">Set<var class="Arg">attr</var></code>, respectively. For example, the tester for <code class="func">Size</code> (<a href="chap30.html#X858ADA3B7A684421"><span class="RefLink">30.4-6</span></a>) is called <code class="code">HasSize</code>, and the setter is called <code class="code">SetSize</code>.</p>
<div class="example"><pre>
<span class="GAPprompt">gap></span> <span class="GAPinput">g:=Group((1,2,3,4),(1,2));;Size(g);</span>
24
<span class="GAPprompt">gap></span> <span class="GAPinput">HasSize(g);</span>
true
<span class="GAPprompt">gap></span> <span class="GAPinput">SetSize(g,99);</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">Size(g);</span>
24
</pre></div>
<p>For two properties <var class="Arg">prop1</var> and <var class="Arg">prop2</var>, the intersection <code class="code"><var class="Arg">prop1</var> and <var class="Arg">prop2</var></code> (see <a href="chap13.html#X84EFA4C07D4277BB"><span class="RefLink">13.2</span></a>) is again a property for which a setter and a tester exist. Setting the value of this intersection to <code class="keyw">true</code> for a <strong class="pkg">GAP</strong> object means to set the values of <var class="Arg">prop1</var> and <var class="Arg">prop2</var> to <code class="keyw">true</code> for this object.</p>
<div class="example"><pre>
<span class="GAPprompt">gap></span> <span class="GAPinput">prop:= IsFinite and IsCommutative;</span>
<Property "<<and-filter>>">
<span class="GAPprompt">gap></span> <span class="GAPinput">g:= Group( (1,2,3), (4,5) );;</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">Tester( prop )( g );</span>
false
<span class="GAPprompt">gap></span> <span class="GAPinput">Setter( prop )( g, true );</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">Tester( prop )( g ); prop( g );</span>
true
true
</pre></div>
<p>It is <em>not allowed</em> to set the value of such an intersection to <code class="keyw">false</code> for an object.</p>
<div class="example"><pre>
<span class="GAPprompt">gap></span> <span class="GAPinput">Setter( prop )( Rationals, false );</span>
You cannot set an "and-filter" except to true
not in any function
Entering break read-eval-print loop ...
you can 'quit;' to quit to outer loop, or
you can type 'return true;' to set all components true
(but you might really want to reset just one component) to continue
<span class="GAPbrkprompt">brk></span> <span class="GAPinput"></span>
</pre></div>
<p><a id="X8529F8A17884A32C" name="X8529F8A17884A32C"></a></p>
<h5>13.6-3 AttributeValueNotSet</h5>
<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ AttributeValueNotSet</code>( <var class="Arg">attr</var>, <var class="Arg">obj</var> )</td><td class="tdright">( function )</td></tr></table></div>
<p>If the value of the attribute <var class="Arg">attr</var> is already stored for <var class="Arg">obj</var>, <code class="code">AttributeValueNotSet</code> simply returns this value. Otherwise the value of <code class="code"><var class="Arg">attr</var>( <var class="Arg">obj</var> )</code> is computed and returned <em>without storing it</em> in <var class="Arg">obj</var>. This can be useful when "large" attribute values (such as element lists) are needed only once and shall not be stored in the object.</p>
<div class="example"><pre>
<span class="GAPprompt">gap></span> <span class="GAPinput">HasAsSSortedList(g);</span>
false
<span class="GAPprompt">gap></span> <span class="GAPinput">AttributeValueNotSet(AsSSortedList,g);</span>
[ (), (4,5), (1,2,3), (1,2,3)(4,5), (1,3,2), (1,3,2)(4,5) ]
<span class="GAPprompt">gap></span> <span class="GAPinput">HasAsSSortedList(g);</span>
false
</pre></div>
<p>The normal behaviour of attributes (when called with just one argument) is that once a method has been selected and executed, and has returned a value the setter of the attribute is called, to (possibly) store the computed value. In special circumstances, this behaviour can be altered dynamically on an attribute-by-attribute basis, using the functions <code class="code">DisableAttributeValueStoring</code> and <code class="code">EnableAttributeValueStoring</code>.</p>
<p>In general, the code in the library assumes, for efficiency, but not for correctness, that attribute values <em>will</em> be stored (in suitable objects), so disabling storing may cause substantial computations to be repeated.</p>
<p><a id="X79120CE37BB69D11" name="X79120CE37BB69D11"></a></p>
<h5>13.6-4 InfoAttributes</h5>
<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ InfoAttributes</code></td><td class="tdright">( info class )</td></tr></table></div>
<p>This info class (together with <code class="func">InfoWarning</code> (<a href="chap7.html#X7A28F77C82D6A3E0"><span class="RefLink">7.4-7</span></a>) is used for messages about attribute storing being disabled (at level 2) or enabled (level 3). It may be used in the future for other messages concerning changes to attribute behaviour.</p>
<p><a id="X7851E2DB79656DB0" name="X7851E2DB79656DB0"></a></p>
<h5>13.6-5 DisableAttributeValueStoring</h5>
<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ DisableAttributeValueStoring</code>( <var class="Arg">attr</var> )</td><td class="tdright">( function )</td></tr></table></div>
<p>disables the usual call of <code class="code">Setter( <var class="Arg">attr</var> )</code> when a method for <var class="Arg">attr</var> returns a value. In consequence the values will never be stored. Note that <var class="Arg">attr</var> must be an attribute and <em>not</em> a property.</p>
<p><a id="X7E5DACBE7A9A9AD1" name="X7E5DACBE7A9A9AD1"></a></p>
<h5>13.6-6 EnableAttributeValueStoring</h5>
<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ EnableAttributeValueStoring</code>( <var class="Arg">attr</var> )</td><td class="tdright">( function )</td></tr></table></div>
<p>enables the usual call of <code class="code">Setter( <var class="Arg">attr</var> )</code> when a method for <var class="Arg">attr</var> returns a value. In consequence the values may be stored. This will usually have no effect unless <code class="code">DisableAttributeValueStoring</code> has previously been used for <var class="Arg">attr</var>. Note that <var class="Arg">attr</var> must be an attribute and <em>not</em> a property.</p>
<div class="example"><pre>
<span class="GAPprompt">gap></span> <span class="GAPinput">g := Group((1,2,3,4,5),(1,2,3));</span>
Group([ (1,2,3,4,5), (1,2,3) ])
<span class="GAPprompt">gap></span> <span class="GAPinput">KnownAttributesOfObject(g);</span>
[ "LargestMovedPoint", "GeneratorsOfMagmaWithInverses",
"MultiplicativeNeutralElement" ]
<span class="GAPprompt">gap></span> <span class="GAPinput">SetInfoLevel(InfoAttributes,3);</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">DisableAttributeValueStoring(Size);</span>
#I Disabling value storing for Size
<span class="GAPprompt">gap></span> <span class="GAPinput">Size(g);</span>
60
<span class="GAPprompt">gap></span> <span class="GAPinput">KnownAttributesOfObject(g);</span>
[ "OneImmutable", "LargestMovedPoint", "NrMovedPoints",
"MovedPoints", "GeneratorsOfMagmaWithInverses",
"MultiplicativeNeutralElement", "StabChainMutable",
"StabChainOptions" ]
<span class="GAPprompt">gap></span> <span class="GAPinput">Size(g);</span>
60
<span class="GAPprompt">gap></span> <span class="GAPinput">EnableAttributeValueStoring(Size);</span>
#I Enabling value storing for Size
<span class="GAPprompt">gap></span> <span class="GAPinput">Size(g);</span>
60
<span class="GAPprompt">gap></span> <span class="GAPinput">KnownAttributesOfObject(g);</span>
[ "Size", "OneImmutable", "LargestMovedPoint", "NrMovedPoints",
"MovedPoints", "GeneratorsOfMagmaWithInverses",
"MultiplicativeNeutralElement", "StabChainMutable",
"StabChainOptions" ]
</pre></div>
<p><a id="X871597447BB998A1" name="X871597447BB998A1"></a></p>
<h4>13.7 <span class="Heading">Properties</span></h4>
<p>The properties of an object are those of its attributes (see <a href="chap13.html#X7C701DBF7BAE649A"><span class="RefLink">13.5</span></a>) whose values can only be <code class="keyw">true</code> or <code class="keyw">false</code>.</p>
<p>The main difference between attributes and properties is that a property defines two sets of objects, namely the usual set of all objects for which the value is known, and the set of all objects for which the value is known to be <code class="keyw">true</code>.</p>
<p>(Note that it makes no sense to consider a third set, namely the set of objects for which the value of a property is <code class="keyw">true</code> whether or not it is known, since there may be objects for which the containment in this set cannot be decided.)</p>
<p>For a property <var class="Arg">prop</var>, the containment of an object <var class="Arg">obj</var> in the first set is checked again by applying <code class="code">Tester( <var class="Arg">prop</var> )</code> to <var class="Arg">obj</var>, and <var class="Arg">obj</var> lies in the second set if and only if <code class="code">Tester( <var class="Arg">prop</var> )( <var class="Arg">obj</var> ) and <var class="Arg">prop</var>( <var class="Arg">obj</var> )</code> is <code class="keyw">true</code>.</p>
<p>If a property value is known for an immutable object then this value is also stored, as part of the type of the object. To some extent, property values of mutable objects also can be stored, for example a mutable list all of whose entries are immutable can store whether it is strictly sorted. When the object is mutated (for example by list assignment) the type may need to be adjusted.</p>
<p>Important properties for domains are <code class="func">IsAssociative</code> (<a href="chap35.html#X7C83B5A47FD18FB7"><span class="RefLink">35.4-7</span></a>), <code class="func">IsCommutative</code> (<a href="chap35.html#X830A4A4C795FBC2D"><span class="RefLink">35.4-9</span></a>), <code class="func">IsAnticommutative</code> (<a href="chap56.html#X82DECD237D49D937"><span class="RefLink">56.4-6</span></a>), <code class="func">IsLDistributive</code> (<a href="chap56.html#X7D4BB44187C55BF2"><span class="RefLink">56.4-3</span></a>) and <code class="func">IsRDistributive</code> (<a href="chap56.html#X79A5AEE786AED315"><span class="RefLink">56.4-4</span></a>), which mean that the multiplication of elements in the domain satisfies <span class="SimpleMath">( a * b ) * c = a * ( b * c )</span>, <span class="SimpleMath">a * b = b * a</span>, <span class="SimpleMath">a * b = - ( b * a )</span>, <span class="SimpleMath">a * ( b + c ) = a * b + a * c</span>, and <span class="SimpleMath">( a + b ) * c = a * c + b * c</span>, respectively, for all <span class="SimpleMath">a</span>, <span class="SimpleMath">b</span>, <span class="SimpleMath">c</span> in the domain.</p>
<p><a id="X7E51C08286E03E7F" name="X7E51C08286E03E7F"></a></p>
<h5>13.7-1 KnownPropertiesOfObject</h5>
<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ KnownPropertiesOfObject</code>( <var class="Arg">object</var> )</td><td class="tdright">( operation )</td></tr></table></div>
<p>returns a list of the names of the properties whose values are known for <var class="Arg">object</var>.</p>
<p><a id="X86711BC77B62EB02" name="X86711BC77B62EB02"></a></p>
<h5>13.7-2 KnownTruePropertiesOfObject</h5>
<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ KnownTruePropertiesOfObject</code>( <var class="Arg">object</var> )</td><td class="tdright">( operation )</td></tr></table></div>
<p>returns a list of the names of the properties known to be <code class="keyw">true</code> for <var class="Arg">object</var>.</p>
<div class="example"><pre>
<span class="GAPprompt">gap></span> <span class="GAPinput">g:=Group((1,2),(1,2,3));;</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">KnownPropertiesOfObject(g);</span>
[ "IsFinite", "CanEasilyCompareElements", "CanEasilySortElements",
"IsDuplicateFree", "IsGeneratorsOfMagmaWithInverses",
"IsAssociative", "IsGeneratorsOfSemigroup", "IsSimpleSemigroup",
"IsRegularSemigroup", "IsCompletelyRegularSemigroup",
"IsCompletelySimpleSemigroup", "IsFinitelyGeneratedGroup",
"IsSubsetLocallyFiniteGroup", "KnowsHowToDecompose",
"IsNilpotentByFinite" ]
<span class="GAPprompt">gap></span> <span class="GAPinput">Size(g);</span>
6
<span class="GAPprompt">gap></span> <span class="GAPinput">KnownPropertiesOfObject(g);</span>
[ "IsEmpty", "IsTrivial", "IsNonTrivial", "IsFinite",
"CanEasilyCompareElements", "CanEasilySortElements",
"IsDuplicateFree", "IsGeneratorsOfMagmaWithInverses",
"IsAssociative", "IsGeneratorsOfSemigroup", "IsSimpleSemigroup",
"IsRegularSemigroup", "IsCompletelyRegularSemigroup",
"IsCompletelySimpleSemigroup", "IsFinitelyGeneratedGroup",
"IsSubsetLocallyFiniteGroup", "KnowsHowToDecompose",
"IsPerfectGroup", "IsSolvableGroup", "IsPolycyclicGroup",
"IsNilpotentByFinite", "IsTorsionFree", "IsFreeAbelian" ]
<span class="GAPprompt">gap></span> <span class="GAPinput">KnownTruePropertiesOfObject(g);</span>
[ "IsNonTrivial", "IsFinite", "CanEasilyCompareElements",
"CanEasilySortElements", "IsDuplicateFree",
"IsGeneratorsOfMagmaWithInverses", "IsAssociative",
"IsGeneratorsOfSemigroup", "IsSimpleSemigroup",
"IsRegularSemigroup", "IsCompletelyRegularSemigroup",
"IsCompletelySimpleSemigroup", "IsFinitelyGeneratedGroup",
"IsSubsetLocallyFiniteGroup", "KnowsHowToDecompose",
"IsSolvableGroup", "IsPolycyclicGroup", "IsNilpotentByFinite" ]
</pre></div>
<p><a id="X7997705185C7E720" name="X7997705185C7E720"></a></p>
<h4>13.8 <span class="Heading">Other Filters</span></h4>
<p>There are situations where one wants to express a kind of knowledge that is based on some heuristic.</p>
<p>For example, the filters (see <a href="chap13.html#X84EFA4C07D4277BB"><span class="RefLink">13.2</span></a>) <code class="func">CanEasilyTestMembership</code> (<a href="chap39.html#X798F13EA810FB215"><span class="RefLink">39.25-1</span></a>) and <code class="func">CanEasilyComputePcgs</code> (<a href="chap45.html#X7B561B1685CEC2AB"><span class="RefLink">45.2-3</span></a>) are defined in the <strong class="pkg">GAP</strong> library. Note that such filters do not correspond to a mathematical concept, contrary to properties (see <a href="chap13.html#X871597447BB998A1"><span class="RefLink">13.7</span></a>). Also it need not be defined what "easily" means for an arbitrary <strong class="pkg">GAP</strong> object, and in this case one cannot compute the value for an arbitrary <strong class="pkg">GAP</strong> object. In order to access this kind of knowledge as a part of the type of an object, <strong class="pkg">GAP</strong> provides filters for which the value is <code class="keyw">false</code> by default, and it is changed to <code class="keyw">true</code> in certain situations, either explicitly (for the given object) or via a logical implication (see <a href="chap78.html#X7FB5016E83DB4349"><span class="RefLink">78.7</span></a>) from other filters.</p>
<p>For example, a <code class="keyw">true</code> value of <code class="func">CanEasilyComputePcgs</code> (<a href="chap45.html#X7B561B1685CEC2AB"><span class="RefLink">45.2-3</span></a>) for a group means that certain methods are applicable that use a pcgs (see <a href="chap45.html#X7F18A01785DBAC4E"><span class="RefLink">45.1</span></a>) for the group. There are logical implications to set the filter value to <code class="keyw">true</code> for permutation groups that are known to be solvable, and for groups that have already a (sufficiently nice) pcgs stored. In the case one has a solvable matrix group and wants to enable methods that use a pcgs, one can set the <code class="func">CanEasilyComputePcgs</code> (<a href="chap45.html#X7B561B1685CEC2AB"><span class="RefLink">45.2-3</span></a>) value to <code class="keyw">true</code> for this particular group.</p>
<p>A filter <var class="Arg">filt</var> of the kind described here is different from the other filters introduced in the previous sections. In particular, <var class="Arg">filt</var> is not a category (see <a href="chap13.html#X7CC6903E78F24167"><span class="RefLink">13.3</span></a>) or a property (see <a href="chap13.html#X871597447BB998A1"><span class="RefLink">13.7</span></a>) because its value may change for a given object, and <var class="Arg">filt</var> is not a representation (see <a href="chap13.html#X8698205F8648EB33"><span class="RefLink">13.4</span></a>) because it has nothing to do with the way an object is made up from some data. <var class="Arg">filt</var> is similar to an attribute tester (see <a href="chap13.html#X79DE5208877AE42A"><span class="RefLink">13.6</span></a>), the only difference is that <var class="Arg">filt</var> does not refer to an attribute value; note that <var class="Arg">filt</var> is also used in the same way as an attribute tester; namely, the <code class="keyw">true</code> value may be required for certain methods to be applicable.</p>
<p><a id="X7E340B8C833BC440" name="X7E340B8C833BC440"></a></p>
<h4>13.9 <span class="Heading">Types</span></h4>
<p>We stated above (see <a href="chap13.html#X7E8202627B421DB1"><span class="RefLink">13</span></a>) that, for an object <var class="Arg">obj</var>, its <em>type</em> is formed from its family and its filters. There is a also a third component, used in a few situations, namely defining data of the type.</p>
<p><a id="X7D3E6B6482BE5B16" name="X7D3E6B6482BE5B16"></a></p>
<h5>13.9-1 TypeObj</h5>
<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ TypeObj</code>( <var class="Arg">obj</var> )</td><td class="tdright">( function )</td></tr></table></div>
<p>returns the type of the object <var class="Arg">obj</var>.</p>
<p>The type of an object is itself an object.</p>
<p>Two types are equal if and only if the two families are identical, the filters are equal, and, if present, also the defining data of the types are equal.</p>
<p><a id="X85A60A7F8083C1C4" name="X85A60A7F8083C1C4"></a></p>
<h5>13.9-2 DataType</h5>
<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ DataType</code>( <var class="Arg">type</var> )</td><td class="tdright">( function )</td></tr></table></div>
<p>The last part of the type, defining data, has not been mentioned before and seems to be of minor importance. It can be used, e.g., for cosets <span class="SimpleMath">U g</span> of a group <span class="SimpleMath">U</span>, where the type of each coset may contain the group <span class="SimpleMath">U</span> as defining data. As a consequence, two such cosets mod <span class="SimpleMath">U</span> and <span class="SimpleMath">V</span> can have the same type only if <span class="SimpleMath">U = V</span>. The defining data of the type <var class="Arg">type</var> can be accessed via <code class="func">DataType</code>.</p>
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