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PL/0

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Template:Short description Template:About PL/0 is a programming language, intended as an educational programming language, that is similar to but much simpler than Pascal, a general-purpose programming language. It serves as an example of how to construct a compiler. It was originally introduced in the book, Algorithms + Data Structures = Programs, by Niklaus Wirth in 1976. It features quite limited language constructs: there are no real numbers, very few basic arithmetic operations and no control-flow constructs other than "if" and "while" blocks. While these limitations make writing real applications in this language impractical, it helps the compiler remain compact and simple.

FeaturesEdit

All constants and variables used must be declared explicitly.

The only data types are integers. The only operators are arithmetic and comparison operators. There is an odd function that tests whether the argument is odd.

In the original implementation presented by Wirth, there are no input and output routines. The compiler prints the value as a given variable changes. So the program: <syntaxhighlight lang="pascal"> var i, s; begin 

 i := 0; s := 0;
 while i < 5 do
 begin
   i := i + 1;
   s := s + i * i
 end

end. </syntaxhighlight> gives the output: 

          0
          0
          1
          1
          2
          5
          3
         14
          4
         30
          5
         55

However, most implementations have single input and single output routines.

Flow control structures are if-then and while-do constructs and user-defined procedures. Procedures cannot accept parameters.

GrammarEdit

The following is the syntax rules of the model language defined in EBNF:

<syntaxhighlight lang="ebnf"> program = block "." ;

block = [ "const" ident "=" number {"," ident "=" number} ";"] 

       [ "var" ident {"," ident} ";"]
       { "procedure" ident ";" block ";" } statement ;

statement = [ ident ":=" expression | "call" ident 

             | "?" ident | "!" expression 
             | "begin" statement {";" statement } "end" 
             | "if" condition "then" statement 
             | "while" condition "do" statement ];

condition = "odd" expression | 

           expression ("="|"#"|"<"|"<="|">"|">=") expression ;

expression = [ "+"|"-"] term { ("+"|"-") term};

term = factor {("*"|"/") factor};

factor = ident | number | "(" expression ")"; </syntaxhighlight>

It is rather easy for students to write a recursive descent parser for such a simple syntax. Therefore, the PL/0 compiler is still widely used in courses on compiler construction throughout the world. Due to the lack of features in the original specification, students usually spend most of their time with extending the language and their compiler. They usually start with introducing REPEAT .. UNTIL and continue with more advanced features like parameter passing to procedures or data structures like arrays, strings or floating point numbers.

Use in educationEdit

The main article on compilers honours PL/0Template:Citation needed for introducing several influential concepts (stepwise refinement, recursive descent parsing, EBNF, P-code, T-diagrams) to the field by educating students to use these concepts. Over the last 3 decades, most university courses on compiler construction that used PL/0 have followed Wirth strictly in employing these techniques (see references below). Some years ago university courses deviated from the course set by Wirth with the replacement of the classical recursive descent parsing technique by a (nonetheless classical) Unix-like approach of employing lex and yacc. Only recently an implementation (PL/0 Language Tools) along this way has also combined modern concepts like object-orientation and design patterns with a modern scripting language (Python), allowing students to consume the source text of the implementation in a contemporary programming style.

Compiler constructionEdit

In December 1976, Wirth wrote a small booklet about compiler construction, containing the full source code of the PL/0 compiler. The syntax rules above were taken from this first edition of Wirth's book Compilerbau.<ref name="Wirth, 1986">Wirth, 1986</ref> In later editions of this book (under the influence of his ongoing research) Wirth changed the syntax of PL/0. He changed the spelling of keywords like const and procedure to uppercase. This change made PL/0 resemble Modula-2 more closely. At the same time, Wirth's friend and collaborator C. A. R. Hoare was working on his influential communicating sequential processes concept, which used the exclamation mark ! and the question mark ? to denote communication primitives. Wirth added both symbols to the PL/0 language, but he did not mention their semantics in the book.

ExamplesEdit

This program<ref>PL/0 Template:Webarchive</ref> outputs the squares of numbers from 1 to 10. Most courses in compiler construction today have replaced the exclamation mark with the WriteLn procedure. <syntaxhighlight lang="pascal"> VAR x, squ;

PROCEDURE square; BEGIN 

  squ:= x * x

END;

BEGIN 

  x := 1;
  WHILE x <= 10 DO
  BEGIN
     CALL square;
     ! squ;
     x := x + 1
  END

END. </syntaxhighlight>

This following program prints the prime numbers from 1 to 100. The write statement corresponds to '!' statement in the EBNF syntax above.

<syntaxhighlight lang="pascal"> const max = 100; var arg, ret;

procedure isprime; var i; begin ret := 1; i := 2; while i < arg do begin if arg / i * i = arg then begin ret := 0; i := arg end; i := i + 1 end end;

procedure primes; begin arg := 2; while arg < max do begin call isprime; if ret = 1 then write arg; arg := arg + 1 end end;

call primes . </syntaxhighlight>

The following example was taken from the second edition of Wirth's book Compilerbau,<ref name="Wirth, 1986"/> which appeared in 1986 in Germany.

<syntaxhighlight lang="pascal"> VAR x, y, z, q, r, n, f;

PROCEDURE multiply; VAR a, b; BEGIN 

 a := x;
 b := y;
 z := 0;
 WHILE b > 0 DO
 BEGIN
   IF ODD b THEN z := z + a;
   a := 2 * a;
   b := b / 2
 END

END;

PROCEDURE divide; VAR w; BEGIN 

 r := x;
 q := 0;
 w := y;
 WHILE w <= r DO w := 2 * w;
 WHILE w > y DO
 BEGIN
   q := 2 * q;
   w := w / 2;
   IF w <= r THEN
   BEGIN
     r := r - w;
     q := q + 1
   END
 END

END;

PROCEDURE gcd; VAR f, g; BEGIN 

 f := x;
 g := y;
 WHILE f # g DO
 BEGIN
   IF f < g THEN g := g - f;
   IF g < f THEN f := f - g
 END;
 z := f

END;

PROCEDURE fact; BEGIN 

 IF n > 1 THEN
 BEGIN
   f := n * f;
   n := n - 1;
   CALL fact
 END

END;

BEGIN 

 ?x; ?y; CALL multiply; !z;
 ?x; ?y; CALL divide; !q; !r;
 ?x; ?y; CALL gcd; !z;
 ?n; f := 1; CALL fact; !f

END. </syntaxhighlight>

Oberon-0Edit

In the third and last edition of his book on compiler construction, Wirth replaced PL/0 with Oberon-0. The language Oberon-0 is much more complex than PL/0. For example, Oberon-0 offers arrays, records, type declarations and procedure parameters. The publisher of Wirth's books (Addison-Wesley) has decided to phase out all his books, but Wirth has published revised editions of his book beginning in 2004.<ref>The revised third edition Template:Webarchive (2005) of Compiler Construction, Niklaus Wirth, 1996, Template:ISBN has never seen the printing press, but it is available online.</ref> Template:As of, the most recent revision available is from May 2017.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

See alsoEdit

NotesEdit

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ReferencesEdit

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External linksEdit

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