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6 <TITLE>Prefixing</TITLE>
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15 <U><I>Loglan 82, A micro-manual of the programming language -
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16 Basic constructs and facilities</I></U>
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17 <H1><IMG ALIGN=MIDDLE SRC="gifs/logo2.gif"> 8) Prefixing</H1>
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21 Classes and prefixing are ingenius inventions of Simula-67(cf
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22 [1]). Unfortunately they were hardly ever known and, perhaps,
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23 by this have not been introduced into many programming language
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24 that gained certain popularity. Moreover, implementation constraints
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25 of Simula-67 bind prefixing and classes workableness to such a
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26 degree that both facilities cannot be used in all respects. We
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27 hope that LOGLAN-82, adopting merits and rooting up deficiencies
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28 of these constructs, will smooth their variations and vivify theirs
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31 What is prefixing ? First of all it is a method for unit extending.
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32 Consider the simplest example:
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34 <IMG SRC="gifs/pr01.gif">
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36 Assume the user desires to extend this class with new attributes.
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37 Instead of writing a completely new class, he may enlarge the
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40 <IMG SRC="gifs/pr02.gif">
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42 Class gas_bill is prefixed by class bill. This new declaration
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43 may appear anywhere within the scope of declaration of class bill.
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44 (In Simula-67 such a prefixing is forbidden in nested units.)
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45 Class gas_bill has all the attributes of class bill and additionally
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46 its own attributes (in this case the only one: cube_meters). The
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47 generation statement of this class has the form:
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49 <IMG SRC="gifs/pr03.gif">
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51 where z is a reference variable of type gas_bill. Remote access
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52 to the attributes of prefixed class is standard:
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54 <IMG SRC="gifs/pr04.gif">
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56 Consider now the example of a class with parameters.
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58 Assume that in a program a class:
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60 <IMG SRC="gifs/pr05.gif">
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64 <IMG SRC="gifs/pr06.gif">
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68 Then for variable z of type id_card and variable t of type idf_card
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69 the corresponding generation statement may be the following:
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71 <IMG SRC="gifs/pr07.gif">
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73 Thus the formal parameters of a class are concatenated with the
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74 formal parameters of its prefix.
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76 One can still extend class idf_card. For instance:
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78 <IMG SRC="gifs/pr08.gif">
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80 Prefixing allows to build up hierarchies of classes. Each one
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81 hierarchy has a tree structure. A root of such a tree is a class
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82 without prefix. One class is a successor of another class iff
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83 the first is prefixed by the latter one.
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85 Consider the prefix structure:
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87 <IMG SRC="gifs/Fig0701.gif">
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89 Class H has a prefix sequence A, B, E, F, H. Let a, b, ... , h
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90 denote the corresponding unique attributes of classes A, B, ...
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91 , H, respectively. The objects of these classes have the following
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94 <IMG SRC="gifs/Fig0702.gif">
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96 Let Ra, Rb,..., Rh denote reference variables of types A, B,...,
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97 H, respectively. Then the following expressions are correct:
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99 <IMG SRC="gifs/pr09.gif">
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101 Variable Ra may designate the object of class B (or C,..., H),
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102 i.e. the statement:
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104 <IMG SRC="gifs/pr10.gif">
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106 is legal. But then attribute b is not accessible through dot via
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107 Ra, i.e. Ra.b is incorrect. This follows from insecurity of such
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108 a remote access. In fact, variable Ra may point any object of
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109 a class prefixed by A, in particular, Ra may point the object
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110 of A itself, which has no attribute b. If Ra.b had been correct,
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111 a compiler should have distiguish the cases Ra points to the object
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112 of A or not. But this, of course, is undistinguishable at compilation
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115 To allow, however, the user's access to attribute b (after instruction
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116 Ra:=<B>new</B> B), the instantaneous type modification is provided
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117 within the language:
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119 <IMG SRC="gifs/pr11.gif">
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121 The correctness of this expression is checked at run time. If
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122 Ra designates an object of B or prefixed ba B, the type of the
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123 expression is B. Otherwise the expression is erroneous. Thus,
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124 for instance, the expressions:
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126 <IMG SRC="gifs/pr12.gif">
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128 enable remote access to the attributes b, c, ... via Ra.
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130 So far the question of attribute concatenation was merely discussed.
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131 However the sequences of statements can be also concatenated.
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133 Consider class B prefixed with class A. In the sequence of statements
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134 of class A the keyword inner may occur anywhere, but only once.
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135 The sequence of statements of class B consists of the sequence
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136 of statements of class A with inner replaced by the sequence of
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137 statements of class B.
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139 <IMG SRC="gifs/Fig0703.gif">
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141 In this case inner in class B is equivalent to the empty statement.
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142 If class B prefixes another class, say C, then inner in B is replaced
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143 by the sequence of statements of class C, and so on. If inner
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144 does not occur explicitly, an implicit occurrence of inner before
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145 the final end of a class is assumed.
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149 Let class complex be declared as usual:
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151 <IMG SRC="gifs/pr13.gif">
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153 and assume one desires to declare a class mcomplex with the additional
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154 attribute module. In order the generation of class mcomplex define
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155 the value of attribute module, one can declare a class:
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157 <IMG SRC="gifs/pr14.gif">
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159 Class mcomplex may be still extended:
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161 <IMG SRC="gifs/pr15.gif">
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163 For these declarations each generation of class mcomplex defines
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164 the value of attribute module, each generation of class pcomplex
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165 defines the values of attributes module and alfa.
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167 For reference variables z1, z2 z3 of type complex, the following
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168 sequence of statements illustrates the presented constructs:
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170 <IMG SRC="gifs/pr16.gif">
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174 Binary search tree (Bst) is a binary tree where for each node
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175 x the nodes in the left subtree are less than x, the nodes in
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176 the right subtree are greater than x. It is the well-known exercise
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177 to program the algorithms for the following operations on Bst:
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179 member(x) = true iff x belongs to Bst
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181 insert(x), enlarge Bst with x, if x does not yet belong to Bst
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183 We define both these operations in a class:
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185 <IMG SRC="gifs/pr17.gif">
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187 In the example the common actions of member and insert are programmed
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188 in class help. Then it suffices to use class help as a prefix
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189 of function member and procedure insert, instead of redundant
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190 occurrences of the corresponding sequence of statements in both
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193 Class Bst may be applied as follows:
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195 <IMG SRC="gifs/pr18.gif">
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197 As shown in the declaration of Bst, class may prefix not only
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198 other classes but also procedures and functions. Class may prefix
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203 Let class push_down (p. 5) prefix a block:
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205 <IMG SRC="gifs/pr19.gif">
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207 In the above block prefixed with class push_down one can use pop
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208 and push as local attributes. (They are local since the block
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209 is embedded in the prefix push down.)
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213 <IMG SRC="gifs/pr20.gif">
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215 In place where classes push_down and Bst are visible together
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216 a block prefixed with Bst may be nested in a block prefixed with
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217 push_down (or vice versa). In the inner block both data structures
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218 are directly accessible. Note that this construct is illegal in
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227 Last update 02/07/95
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231 Comments, suggestions and critiques are welcome to : <A HREF="mailto:linfo062@crisv2.univ-pau.fr">linfo062@crisv2.univ-pau.fr</A>
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