151 lines
5.1 KiB
Text
151 lines
5.1 KiB
Text
.NH
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The Compiler
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.PP
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The compiler is written in \fBC\fP using LLgen and Lex and compiles
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Occam programs to EM code, using the procedural interface as defined for EM.
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In the following sub-sections we describe the LLgen parser generator and
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the aspect of indentation.
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.NH 2
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The LLgen Parser Generator
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.PP
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LLgen accepts a Context Free syntax extended with the operators `\f5*\fP', `\f5?\fP' and `\f5+\fP'
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that have effects similar to those in regular expressions.
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The `\f5*\fP' is the closure set operator without an upperbound; `\f5+\fP' is the positive
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closure operator without an upperbound; `\f5?\fP' is the optional operator;
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`\f5[\fP' and `\f5]\fP' can be used for grouping.
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For example, a comma-separated list of expressions can be described as:
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.DS
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.ft 5
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expression_list:
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expression [ ',' expression ]*
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;
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.ft
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.DE
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.LP
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Alternatives must be separated by `\f5|\fP'.
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C code (``actions'') can be inserted at all points between the colon and the
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semicolon.
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Variables global to the complete rule can be declared just in front of the
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colon enclosed in the brackets `\f5{\fP' and `\f5}\fP'. All other declarations are local to
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their actions.
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Nonterminals can have parameters to pass information.
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A more mature version of the above example would be:
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.DS
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.ft 5
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expression_list(expr *e;) { expr e1, e2; } :
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expression(&e1)
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[ ',' expression(&e2)
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{ e1=append(e1, e2); }
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]*
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{ *e=e1; }
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;
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.ft
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.DE
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As LLgen generates a recursive-descent parser with no backtrack, it must at all
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times be able to determine what to do, based on the current input symbol.
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Unfortunately, this cannot be done for all grammars. Two kinds of conflicts
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are possible, viz. the \fBalternation\fP and \fBrepetition\fP conflict.
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An alternation confict arises if two sides of an alternation can start with the
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same symbol. E.g.
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.DS
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.ft 5
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plus: '+' | '+' ;
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.ft
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.DE
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The parser doesn't know which `\f5+\fP' to choose (neither do we).
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Such a conflict can be resolved by putting an \fBif-condition\fP in front of
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the first conflicting production. It consists of a \fB``%if''\fP followed by a
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C-expression between parentheses.
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If a conflict occurs (and only if it does) the C-expression is evaluated and
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parsing continues along this path if non-zero. Example:
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.DS
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.ft 5
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plus:
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%if (some_plusses_are_more_equal_than_others())
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'+'
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'+'
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;
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.ft
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.DE
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A repetition conflict arises when the parser cannot decide whether
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``\f5productionrule\fP'' in e.g. ``\f5[ productionrule ]*\fP'' must be chosen
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once more, or that it should continue.
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This kind of conflicts can be resolved by putting a \fBwhile-condition\fP right
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after the opening parentheses. It consists of a \fB``%while''\fP
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followed by a C-expression between parentheses. As an example, we can look at
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the \fBcomma-expression\fP in C. The comma may only be used for the
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comma-expression if the total expression is not part of another comma-separated
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list:
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.DS
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.nf
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.ft 5
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comma_expression:
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sub_expression
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[ %while (not_part_of_comma_separated_list())
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',' sub_expression
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]*
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;
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.ft
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.fi
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.DE
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Again, the \fB``%while''\fP is only used in case of a conflict.
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.LP
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Error recovery is done almost completely automatically. All you have to do
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is to write a routine called \fILLmessage\fP to give the necessary error
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messages and supply information about terminals found missing.
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.NH 2
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Indentation
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.PP
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The way conflicts can be resolved are of great use to Occam. The use of
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indentation, to group statements, leads to many conflicts because the spaces
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used for indentation are just token separators to the lexical analyzer, i.e.
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``white space''. The lexical analyzer can be instructed to generate `BEGIN' and
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`END' tokens at each indentation change, but that leads to great difficulties
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as expressions may occupy several lines, thus leading to indentation changes
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at the strangest moments. So we decided to resolve the conflicts by looking
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at the indentation ourselves. The lexical analyzer puts the current indentation
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level in the global variable \fIind\fP for use by the parser. The best example
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is the \fBSEQ\fP construct, which exists in two flavors, one with a replicator
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and one process:
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.DS
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.nf
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.ft 5
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seq i = [ 1 for str[byte 0] ]
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out ! str[byte i]
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.ft
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.fi
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.DE
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and one without a replicator and several processes:
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.DS
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.nf
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.ft 5
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seq
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in ? c
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out ! c
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.ft
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.fi
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.DE
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The LLgen skeleton grammar to handle these two is:
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.DS
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.nf
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.ft 5
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SEQ { line=yylineno; oind=ind; }
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[ %if (line==yylineno)
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replicator
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process
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[ %while (ind>oind) process ]*
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]
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.ft
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.fi
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.DE
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This shows clearly that, a replicator must be on the same line as the \fBSEQ\fP,
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and new processes are collected as long as the indentation level of each process
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is greater than the indentation level of \fBSEQ\fP (with appropriate checks on this
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identation).
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.PP
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Different indentation styles are accepted, as long as the same amount of spaces
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is used for each indentation shift. The ascii tab character sets the indentation
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level to an eight space boundary. The first indentation level found in a file
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is used to compare all other indentation levels to.
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