Editing PL/0

{{Short description|Programming language}}
{{about|the programming language introduced by Niklaus Wirth|the subset of a programming language from IBM|PL/I}}
'''PL/0''' is a [[programming language]], intended as an [[educational programming language]], that is similar to but much simpler than [[Pascal (programming language)|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.

== Features ==
All [[Constant (computer programming)|constants]] and [[Variable (computer science)|variables]] used must be declared explicitly.

The only [[data type]]s are [[Integer (computer science)|integers]]. The only operators are arithmetic and comparison operators. There is an <code>odd</code> function that tests whether the argument is odd.

In the original implementation presented by Wirth, there are no [[Input/output|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:
<pre>
          0
          0
          1
          1
          2
          5
          3
         14
          4
         30
          5
         55
</pre>
However, most implementations have single input and single output routines.

[[Control flow|Flow control structures]] are <code>[[Conditional (computer programming)|if-then]]</code> and <code>[[While loop|while-do]]</code> constructs and user-defined [[Function (computer programming)|procedures]]. Procedures cannot accept parameters.

==Grammar==
The following is the syntax rules of the model language defined in [[extended Backus–Naur form|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 <code>REPEAT .. UNTIL</code> and continue with more advanced features like parameter passing to procedures or data structures like arrays, strings or floating point numbers.

==Use in education==
The main article on [[compiler]]s honours PL/0{{Citation needed|reason=No mention of PL/0 in that article|date=November 2022}} 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 (software)|lex]] and [[yacc]]. Only recently an implementation ([https://programming.dojo.net.nz/study/pl0-language-tools/index PL/0 Language Tools]) along this way has also combined modern concepts like object-orientation and design patterns with a modern scripting language ([[Python (programming language)|Python]]), allowing students to consume the source text of the implementation in a contemporary programming style.

==Compiler construction==
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 <code>const</code> and <code>procedure</code> 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.

==Examples==

This program<ref>[http://www.ntecs.de/old-hp/uu9r/lang/html/pl0.en.html PL/0] {{webarchive|url=https://web.archive.org/web/20120221073635/http://www.ntecs.de/old-hp/uu9r/lang/html/pl0.en.html|date=2012-02-21}}</ref> outputs the squares of numbers from 1 to 10. Most courses in compiler construction today have replaced the exclamation mark with the <code>WriteLn</code> 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-0==
In the third and last edition of his book on compiler construction, Wirth replaced PL/0 with [[Oberon (programming language)|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 [http://www.ethoberon.ethz.ch/WirthPubl/CBEAll.pdf revised third edition] {{Webarchive|url=https://web.archive.org/web/20170217071020/http://www.ethoberon.ethz.ch/WirthPubl/CBEAll.pdf |date=2017-02-17 }} (2005) of ''Compiler Construction'', Niklaus Wirth, 1996, {{ISBN|0-201-40353-6}} has never seen the printing press, but it is available online.</ref>  {{As of|2017|August}}, the most recent revision available is from May 2017.<ref>{{cite web|title=Compiler Construction|first=Niklaus|last=Wirth|author-link=Niklaus Wirth|date=2017-05-28|access-date=2017-08-25|url=https://www.inf.ethz.ch/personal/wirth/CompilerConstruction/}}</ref><ref>{{cite web|title=news.txt|first=Niklaus|last=Wirth|author-link=Niklaus Wirth|date=2017-08-18|access-date=2017-08-25|archive-date=2017-08-25|archive-url=https://web.archive.org/web/20170825151832/https://www.inf.ethz.ch/personal/wirth/news.txt|url=https://www.inf.ethz.ch/personal/wirth/news.txt}}</ref>

== See also ==
*[[P-code machine]]
*[[City & Guilds Mnemonic Code]]

==Notes==
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{{reflist}}

==References==
{{refbegin}}
* Liffick, Blaise W., Ed (1979), ''The Byte Book of Pascal'', {{ISBN|0-07-037823-1}}
* Wirth, Niklaus (1975), [https://web.archive.org/web/20130207170133/http://www.inf.ethz.ch/personal/wirth/books/AlgorithmE0/ ''Algorithms + Data Structures = Programs''], {{ISBN|0-13-022418-9}}
* Wirth, Niklaus (1986), [https://web.archive.org/web/20120603011556/http://www.inf.ethz.ch/personal/wirth/books/Compilerbau0/ ''Compilerbau''], B.G. Teubner, Stuttgart {{ISBN|3-519-32338-9}}
{{refend}}

==External links==
*The [http://www.standardpascaline.org/plzero.pas compiler (.pas file)] from the first edition of the ''Compilerbau'' book, written in [[Pascal (programming language)|Pascal]]
* [http://pascal.hansotten.com/niklaus-wirth/pl0/ Another copy of the compiler] at Pascal for small machines site
*The [[P-code machine#Example machine|interpreter]] from "Algorithms + Data Structures = Programs" book, written in [[Pascal (programming language)|Pascal]]
*Development of a [http://fruttenboel.verhoeven272.nl/m4m/index.html PL/0 style compiler] based on 'Compiler construction' written in Mocka (Modula-2 for Linux)
*A paper explaining the use of [https://web.archive.org/web/20120717010607/http://www.cs.rochester.edu/courses/254/PLzero/guide.pdf PL/0 at the University of Rochester] 
*The homepage of the PL/0 reference book, "Algorithms + Data Structures = Programs" [https://web.archive.org/web/20130207170133/http://www.inf.ethz.ch/personal/wirth/books/AlgorithmE0/]
*http://sourceforge.net/projects/pl0-compiler (written in C/C++, uses QT framework)
*https://modernc.org/pl0 (written in Go, runs in terminal, cross platform)
*https://github.com/dodobyte/plzero (a very small compiler produces windows executable)
*https://github.com/MarcRochkind/pl0compiler (compiler for IBM 701 written in C; generates 701 assembler)
* [http://rosettacode.org/wiki/Category:PL/0 Category:PL/0] Tasks implemented in PL/0 on [http://rosettacode.org rosettacode.org]

{{DEFAULTSORT:Pl 0}}
[[Category:Pascal programming language family]]
[[Category:Procedural programming languages]]
[[Category:Structured programming languages]]
[[Category:Educational programming languages]]
[[Category:Compilers]]
[[Category:Programming languages created in 1975]]