52 lines
2.4 KiB
Plaintext
52 lines
2.4 KiB
Plaintext
BEGIN
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# Algol 68 doesn't have classes and inheritence as such, however structures #
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# can contain procedures and different instances of a structure can have #
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# different versons of the procedure #
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# this allows us to simulate inheritence by creating structure instances #
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# with different procedures #
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# the following declares a type (MODE) HORSE with two constructors, one for #
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# a standard horse and one for a zebra (the "derived class"). #
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# The standard horse has its print procedure set to thw print horse #
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# procedure (the "base class" method). zebras get the print zebra procedure #
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# (the "derived class" method). A convenience operator (PRINT) is defined #
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# to simplify calling the horse/zebra print method of its horse parameter #
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# = this PRINT operator is independent of which actual print method the #
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# horse has - it just saved typeing "( print OF h )( h )" everywhere we #
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# need to call the print method (euivalent to h.print(h) in e.g. C, Java, #
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# etc.) #
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# "class" #
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MODE HORSE = STRUCT( STRING name, PROC(HORSE)VOID print );
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# constructors #
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PROC new horse = ( STRING name )HORSE: ( name, print horse );
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PROC new zebra = ( STRING name )HORSE: ( name, print zebra );
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# print methods: one for a standard horse and one for a zebra #
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PROC print horse = ( HORSE h )VOID: print( ( "horse: ", name OF h ) );
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PROC print zebra = ( HORSE h )VOID: print( ( "zebra: ", name OF h ) );
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# print operator #
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OP PRINT = ( HORSE h )VOID: ( print OF h )( h );
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# declare and construct some horses and zebras #
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HORSE h1 := new horse( "silver blaze" );
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HORSE z1 := new zebra( "stripy" );
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HORSE z2 := new zebra( "second zebra" );
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# show their values #
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PRINT h1; print( ( newline ) );
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PRINT z1; print( ( newline ) );
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PRINT z2; print( ( newline ) );
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print( ( "----", newline ) );
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# change the second zebra to be a copy of the first zebra #
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z2 := z1;
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PRINT z1; print( ( newline ) );
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PRINT z2; print( ( newline ) );
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print( ( "----", newline ) );
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# change the name of the first zebra leaving z2 unchanged #
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name OF z1 := "ed";
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PRINT z1; print( ( newline ) );
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PRINT z2; print( ( newline ) )
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END
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