Category:PL/0: Difference between revisions

Lang tag(s):	pl0
See Also:	PL/0 on the HOPL; ;

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Revision as of 00:36, 18 February 2023

PL/0 is an educational programming language. It was originally introduced in the book, Algorithms + Data Structures = Programs, by Niklaus Wirth in 1976. Wirth uses PL/0 as an example of how to construct a compiler. This language has little constructs. Writing real applications in PL/0 is not practical, but the compiler can remain compact and simple.

Features

One must explicitly declare all used constants and variables.

The only data type are integer numbers. The only operators are arithmetical and comparison ones. There is a function odd which examines if the integer argument is odd.

In the original implementation presented by Wirth, there are no input and output routines. The interpreter prints the new value of each variable when it becomes changed. So, the program:

var a, b;
begin
  a := 0; b := 10;
  while a < 5 do 
  begin
    a := a + 1;
    b := b - 1;
  end
end.

gives the output:

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

The flow control structures are if-then and while-do constructs, and procedures defined by the user. Procedures cannot accept any parameters.

Syntax

The syntax rules of PL/0 can be specified in EBNF as follows:

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 ")";

Wirth in his book presents the implementation without ? ("receive an integer value and assign it to the variable") and ! ("display a value of the expression") routines. Some implementations use other routines for receiving and displaying data, usually read and write.

Due to typograhic conventions, Wirth uses non-ASCII symbols ≠, ≤, and ≥. Some implementations use # for "not equal", [ for "less or equal", and ] for "greater or equal".

Some implementations accept only upper-case letters or only lower-case letters in keywords and identifiers.

External links

Wikipedia has an article about PL/0.
PL/0 - Pascal for small machines

Pages in category "PL/0"

The following 25 pages are in this category, out of 25 total.

@@ Line 8: / Line 8: @@
 The only data type are integer numbers. The only operators are arithmetical and comparison ones. There is a function <code>odd</code> which examines if the integer argument is odd.
-In the original implementation presented by Wirth, there are no input and output routines. The compiler prints the new value of each variable when it becomes changed. So, the program:
+In the original implementation presented by Wirth, there are no input and output routines. The interpreter prints the new value of each variable when it becomes changed. So, the program:
 <syntaxhighlight lang="pascal">
 var a, b;
@@ Line 35: / Line 35: @@
 </pre>
-Most implementations have single input and single output routines.
+However, most implementations have single input and single output routines.
 The flow control structures are ''if-then'' and ''while-do'' constructs, and procedures defined by the user. Procedures cannot accept any parameters.