Skip to content

Latest commit

 

History

History
139 lines (128 loc) · 5.98 KB

rec.md

File metadata and controls

139 lines (128 loc) · 5.98 KB

REC

REC and CONVERT are symbol manipulation languages implemented on a variety of microprocessors, including REC86, the version of REC for MS-DOS on 8086 1.

REC86 CLI usage 1.

RXPCOM compiler

REC has compilers for converting regular expressions, push-down automata, Markov algorithms, Turing machines, Post production systems, and Lisp to REC programs.

AUTOMATA executes the RXPCOM compiler to transform a regular expression contained in a file whose extension is .RXP into a REC program which accepts strings read from the keyboard if they belong to the language denoted by the regular expression or rejects them otherwise.

Regular expressions accepted by this compiler may include any printing ASCII characters [between the exclamation point (!) and the tilde (~)], all of them taken as alphabet letters except for the following characters, which have the indicated meaning:

  • # the empty set
  • $ the null string
  • () grouping symbols
  • | alternation operator
  • * closure operator

Spaces and tabs may be used freely, and bracket-enclosed comments may appear at the beginning of the file. The first non-blank, non-left-bracket character will mark the beginning of the regular expression for which a parser is to be generated; the expression is taken to include all non-blank characters up to the end of the file. For example, the following lines

[ZAT3RD.RXP] [[Binary strings w/3rd-from-last digit = 0]]

(0 | 1)* 0 (0 | 1) (0 | 1)

may be placed in a file ZAT3RD.RXP for which RXPCOM will generate a file ZAT3RD.REC, which when executed will read strings and determine whether or not they are binary numbers whose third-from-last digit is a 0. 1

REC language

REC Syntax and Semantics.

REC is a compact language with a simple control structure based on four control symbols [5]:

  • (EXP) left and right parenthesis are used to specify expressions to be evaluated. If the evaluation of a expression is True, then a function associated with that expression is executed (the association of functions to execute to expressions to evaluate is described shortly). If the expression is False, no funtion is executed and the REC program control continues after the right parentheses.
  • EXP; semicolon is used to specify the evaluation of an expression (and the execution of its associated function) and continue.
  • EXP: colon is used to specify the repeated evaluation of an expression (and the execution of its associated function), while the evaluation is True.

Some examples will clarify the use of REC. In these, P is a predicate (condition) to be evaluated, and W, W1 and W2 are operators, which always are evaluated as true:

REC Expression Control Sentence
(P1; P2; ... Pn; ) P1 or P2 or ... Pn
(P1 P2 ... Pn; ) P1 and P2 and ... Pn
(P W1 ; W2;) if P then W1 else W2
(P W : ;) while P do W
(W P : ;) repeat W until P

When the compiler finds a predicate (P) false, the compiler jumps to the right and continues with to evaluate the expression in the next pair of left-right parentheses. Note that a pair of left-right parentheses can contain a compound expression using the control symbols colon and semicolon.

In the first example, if P1 is false, P2 is evaluated; if P2 is false, P3 is evaluated, and so on. But if any of Pi (1<= i <= n) is true, only that Pi is executed (i.e. its associated function), and control leaves this pair of left-right parentheses.

In the second example, the and operator, all the predicates are evaluated, since the semicolon is at the end of the expression. Obviously, only the functions associated to the predicates that evaluate True will be executed.

The third example is the codification of a typical if-then-else sentence. If P is true, then W1 is executed; if P is false then the compiler executes W2.

For the while sentence, fourth example , P is first evaluated; if it is true, W is executed, but colon control causes execution to jump to the beginning of the expression and start again to evaluate P and, if is True, to execute W, and so on. In the case of the repeat operator, fifth example, the only thing that changes is the order: W will be repeatedly executed until P becomes false. 2

({("field""table""conditión"&;)}{("field""table"&;)} {("*""tabla"&;)})

({("coleccion""ACRONIMOCAT"&;)}{("*""AISLA"&;)}{ ("numero""ACRONI" "numero < 629"&;)})

Versions

A CONVERT compiler of REC for PDP-8, Harold V. McIntosh, 1968 A FORTRAN coded Regular Expression Compiler for the IBM 1130 Computing System, Gerardo Cisneros, 1970

Footnotes

  1. REC and Convert as aids in teaching Automata Theory, Gerardo Cisneros and Harold V. McIntosh, 1989 The programming languages REC and convert, Harold V. McIntosh and Gerardo Cisneros, 1990 ↩ ↩2 ↩3

  2. LIDA/REC Visual Language for Databases interface PostgreSQL, A. Hernández-Montoya and S. V. Chapa-Vergara, 2005 REC language is a live on IBM1130 simulator, Ignacio Vega-Paez, Jose Angel Ortega, and Georgina G. Pulido, 2009 ↩