76 lines
2.2 KiB
Text
76 lines
2.2 KiB
Text
.sp 1.5i
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.nr H1 7
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.NH
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Hints to change the standard
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.nh
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.sp
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.LP
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We encoutered some difficulties when the compiler was developed. In this
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chapter some hints are presented to change the standard, which would make
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the implementation of the compiler less difficult. The semantics of Pascal
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would not be altered by these adaptions.
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.sp 2
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.LP
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\- Some minor changes in the grammar of Pascal from the user's point of view,
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but which make the writing of an LL(1) parser considerably easier, could be:
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.in +3m
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.nf
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field-list : [ ( fixed-part [ variant-part ] | variant-part ) ] .
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fixed-part : record-section \fB;\fR { record-section \fB;\fR } .
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variant-part : \fBcase\fR variant-selector \fBof\fR variant \fB;\fR { variant \fB;\fR } .
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case-statement : \fBcase\fR case-index \fBof\fR case-list-element \fB;\fR { case-list-element \fB;\fR } \fBend\fR .
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.fi
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.in -3m
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.LP
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\- To ease the semantic checking on sets, the principle of qualified sets could
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be used, every set-constructor must be preceeded by its type-identifier:
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.nf
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.ti +3m
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set-constructor : type-identifier \fB[\fR [ member-designator { \fB,\fR member-designator } ] \fB]\fR .
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Example:
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t1 = set of 1..5;
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t2 = set of integer;
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The type of [3, 5] would be ambiguous, but the type of t1[3, 5] not.
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.fi
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.LP
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\- Another problem arises from the fact that a function name can appear in
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three distinct 'use' contexts: function call, assignment of function
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result and as function parameter.
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.br
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Example:
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.in +5m
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.nf
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\fBprogram\fR function_name;
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\fBfunction\fR p(x : integer; function y : integer) : integer;
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\fBbegin\fR .. \fBend\fR;
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\fBfunction\fR f : integer;
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\fBbegin\fR
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f := p(f, f); (*)
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\fBend\fR;
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\fBbegin\fR .. \fBend\fR.
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.fi
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.in -5m
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A possible solution in case of a call (also a procedure call) would be to
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make the (possibly empty) actual-parameter-list mandatory. The assignment
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of the function result could be changed in a \fIreturn\fR statement.
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Though this would change the semantics of the program slightly.
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.br
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The above statement (*) would look like this: return p(f(), f);
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.LP
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\- Another extension to the standard could be the implementation of an
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\fIotherwise\fR clause in a case-statement. This would behave exactly like
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the \fIdefault\fR clause in a switch-statement in C.
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.bp
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