Power Programmierung/Power-Programmierung (Tewi)(1994).iso/rexx/rextutor/rexx.txt

Text File | 1993-11-26 | 46KB | 1,151 lines

  The REXX Tutorial for Complete Beginners

This document is being ported from a UNIX version for IMC Unix.  It was
written by Ian Collier.  If you find any mistakes or unixisms in it, please
contact Paul Prescod: [e-mail].

Please also contact me if you have suggestions for improvement, or
text you would like to see added, or files you would like to see in
the zip file.  Also, if you could write a REXX script that turns
a readable text file into a .inf file, I would love to ship both text
and .inf versions.



1. Creating a Rexx program

Many programmers start by writing a program which displays the message
"Hello world!".  Here is how to do that in Rexx...

Write a file called "hello.cmd" containing the following text.  Use any
text editor, e, edit,  or epm will do.  To do this in the WorkPlace shell,
just go to the templates folder and drag a data file object out of it.
Bring up its settings and rename it to "hello.cmd."  The text, as with all
example text in this guide, starts at the first indented line and ends at
the last.  The four spaces at the start of each line of text should not be
entered (this is very important!):

      /* This program says "Hello world!" */
      say "Hello world!"

This program consists of a comment saying what the program does, and an
instruction which does it.  "say" is the Rexx instruction which displays
data on the terminal.

The method of executing a Rexx program varies greatly between
implementations.  On OS/2, you would just type "hello.cmd" in that
file's directory, or double click on its icon in the WorkPlace Shell.

When you execute your first Rexx program using the method detailed above,
you should see the message "Hello world!" appear on the screen.

  2. Doing arithmetic

Rexx has a wide range of arithmetic operators, which can deal with very
high precision numbers.  Here is an example of a program which does
arithmetic.  Make a file called "arith.cmd" containing the following:

      /* This program inputs a number and does some calculations on it */
      pull a
      b=a*a
      c=1/a
      d=3+a
      e=2**(a-1)
      say 'Results are:' a b c d e

in a positive integer.  Here is a sample run:

input:  5
output: Results are: 5 25 0.2 8 16

  The results you see are the original number (5), its square (25), its
  reciprocal (0.2), the number plus three (8) and two to the power of one
  less than the number (16).

  This example illustrates several things:

  * variables: in this example a, b, c, d and e are variables. You can
    assign a value to a variable by typing its name, "=", and a value, and
    you can use its value in an expression simply by typing its name.
  * input: by typing "pull a" you tell the interpreter to ask the user for
    input and to store it in the variable a.
  * arithmetic: the usual symbols (+ - * /) as well as ** (to-power) were
    used to perform calculations.  Parentheses (or "brackets") were used to
    group together an expression, as they are in mathematics.
  * string expressions: the last line of the program displays the results by
    saying the string expression

        'Results are:' a b c d e

    This has six components: the string constant 'Results are:' and the five
    variables. These components are attached together with spaces into one
    string expression, which the "say" command then displays on the
    terminal.  A string constant is any sequence of characters which starts
    and ends with a quotation mark - that is, either " or ' (it makes no
    difference which you use, as long as they are both the same).

If you supply the number 6 as input to the program, you should notice that
the number 1/6 is given to nine significant figures.  You can easily
change this.  Edit the program and insert before the second line:

      numeric digits 25

If you run this new program you should see that 25 significant figures are
produced.  In this way you can calculate numbers to whatever accuracy you
require, within the limits of the machine.

At this point it seems worth a mention that you can put more than one
instruction on a line in a Rexx program. You simply place a semicolon
between the instructions, like this:

      /* This program inputs a number and does some calculations on it */
      pull a; b=a*a; c=1/a; d=3+a; e=2**(a-1); say 'Results are:' a b c d e

Needless to say, that example went a bit over the top...

  3. Errors

Suppose you ignored the instructions of the previous section and typed a
non-integer such as 3.4 as input to the program.  Then you would get an
error, because the ** (to-power) operator is only designed to work when
the second parameter (that is, the power number) is an integer. You might
see this, for example:

      3.4
          6 +++ e=2**(a-1)
     Error 26 running arith.cmd, line 6: Invalid whole number

Or if you typed zero, you might see the following (because you cannot
divide by zero):

    rexx arith
    0
        4 +++ c=1/a
    Error 42 running arith.cmd, line 4: Arithmetic overflow or underflow

Perhaps most interestingly, if you type a sequence of characters which is
not a number, you might see this.  It does not complain about the
characters you entered, but at the fact that the program tries to use it
as a number.

      rexx arith
      hello
          3 +++ b=a*a
    Error 41 running arith.cmd, line 3: Bad arithmetic conversion

In each case, you have generated a "syntax error" (it is classified as a
syntax error, even though the problem was not directly related to the
program's syntax).  What you see is a "traceback" starting with the line
which caused the error (no other lines are listed in this traceback,
because we have not yet considered any Rexx control structures), and a
description of the error.  This information should help you to determine
where the error occurred and what caused it.  More difficult errors can be
traced with the "trace" instruction (see later).

  4. Untyped data

In the previous section, you found that you could input either a number or
a sequence of letters and store it in the variable a, although arithmetic
can only be performed on numbers.  That is because data in Rexx are
untyped.  In other words, the contents of a variable or the result of an
expression can be any sequence of characters.  What those characters are
used for matters only to you, the programmer.  However, if you try to
perform arithmetic on a random sequence of characters you will generate a
syntax error, as shown in the previous section.

You have seen that you can add strings together by placing a space in
between them.  There are two other ways: the abuttal and the concatenation
operator.  An abuttal is simply typing the two pieces of data next to each
other without a space.  This is not always appropriate: for example you
cannot type an "a" next to a "b", because that would give "ab", which is
the name of another unrelated variable.  Instead, it is safer to use the
concatenation operator, ||.  Both these operations concatenate the strings
without a space in between. For example:

      /* demonstrates concatenation and the use of untyped data */
      a='A string'
      b='123'
      c='456'
      d=a":" (b||c)*3
      say d

The above program says "A string: 370368".  This is because (b||c) is the
concatenation of strings b and c, which is "123456".  That sequence of
characters happens to represent a number, and so it can be multiplied by 3
to give 370368.  Finally, that is added with a space to the concatenation
of a with ":" and stored in d.

  5. More on variables

The previous examples only used single-letter variable names.  In fact it
is more useful to have whole words as variable names, and Rexx allows this
up to an implementation maximum (which should be suitably large, e.g. 250
characters).  Moreover, not only letters but numbers and the six characters
"@#$!?_" are allowed in variable names - or "symbols", as they are called
in the liturature.  These are valid symbols:

      fred
      Dan_Yr_0gof
      HI!

The case of letters is unimportant, so that for example "Hello", "HELLO"
and "hellO" all mean the same.

If you use a symbol in an expression when it has not been previously given
a value, that does not cause an error (unless "signal on novalue" is set -
see later).  Instead, it just results in its own name translated into
upper case.

      /* A demonstration of simple symbols */
      foo=3
      bar="Characters"
      say foo bar':' hi!

This program says "3 Characters: HI!".

As well as "simple symbols", which are variables like the above, there are
arrays.  Any ordinary variable name can also be used as the name of an
array:

      /* This program uses an array. */
      pull a
      array.1=a
      array.2=a*a
      array.3=a**3
      say array.1 array.2 array.3 array.1+array.2+array.3

An element of an array is accessed by typing the array name, a dot, and
the element number.  The array name and the dot are together known as the
"stem" of the array.  The name of the element is called a "compound
symbol".  Note that an array does not have to be declared before it is
used.

In fact not only numbers, but strings and variable names may be used as
element names.  Also, an element name can consist of two or more parts
separated by dots, so giving two or more dimensional arrays.

      /* This program uses an array with various elements */
      book.1.author="M. F. Cowlishaw"
      book.1.title="The REXX Language, a practical approach to programming"
      book.1.pub="Englewood Cliffs 1985"
      book.2.author="A. S. Rudd"
      book.2.title="Practical Usage of REXX"
      book.2.pub="Ellis Horwood 1990"
      /* insert lots more */
      say "Input a book number"
      pull i
      say "Author:   " book.i.author
      say "Title:    " book.i.title
      say "Publisher:" book.i.pub

In the above program, an array called "book" is created, containing a
number of records each with elements AUTHOR, TITLE and PUB.  Notice that
these three uppercase names are produced by symbols "author", "title" and
"pub", because those symbols have not been given values.  When a book
number i has been input, the elements of the ith record are printed out.
It is not an error to reference an undefined element of an array.  If you
type "3" into the above program, you will see this:

      Input a book number
      3
      Author:    BOOK.3.AUTHOR
      Title:     BOOK.3.TITLE
      Publisher: BOOK.3.PUB

As before, if a compound symbol has not been given a value, then its name
is used instead.

There is a way to initialise every element of an array: by assigning a
value to the stem itself.  Edit the above program and insert after the
comment line:

      book.="Undefined"

This gives every possible element of the array the value "Undefined", so
that if you again type "3" you will see the following:

      Input a book number
      3
      Author:    Undefined
      Title:     Undefined
      Publisher: Undefined

  6. Functions

The Online Rexx Summary contains a list of the functions which are available
in Rexx.  Each of these functions performs a specific operation upon the
parameters. For example,

      /* Invoke the date function with various parameters */
      say date("W")',' date()

  This might say, for example, "Friday, 22 May 1992".

  A function is called by typing its name immediately followed by "(". After
  that come the parameters, if any, and finally a closing ")".  In the above
  example, the "date" function is called twice.  The value of date("W") is
  the name of the weekday, and the value of date() is the date in "default"
  format.

  7. Conditionals

  It is time to use some Rexx control structures.  The first of these will
  be the conditional.  Here is an example:

      /* Use a conditional to tell whether a number is less than 50 */
      pull a
      if a<50 then say a "is less than 50"
      else say a "is not less than 50"

  The program is executed in the manner in which it reads - so, if a is less
  than 50 then the first instruction is executed, else the second
  instruction is executed.

  The "a<50" is a conditional expression.  It is like an ordinary
  expression (in fact conditional expressions and ordinary numeric
  expressions are interchangeable), but it contains a comparison operator.

  There are many comparison operators, as follows:

  = (equal to)  < (less than)  > (greater than)  <= (less or equal)
  >= (greater or equal)  <> (greater or less)  \= (not equal)
  \> (not greater)  \< (not less)

  All the above operators can compare numbers, deciding whether one is equal
  to, less than, or greater than the other.  They can also compare
  non-numeric strings, first stripping off leading and trailing blanks.

  There are analogous comparison operators to these for comparing strings
  strictly.  The main difference between them is that 0 is equal to 0.0,
  numerically speaking, but the two are different if compared as strings.
  The other difference is that the strict operators do not strip blanks
  before comparing.  The strict operators are

  == (equal to)  << (less than)  >> (greater than)  <<= (less or equal)
  >>= (greater or equal)  \== (not equal)  \>> (not greater)  \<< (not less)

  Conditional expressions may be combined with the boolean operators:
  & (and), | (or) and && (xor).  They may also be reversed with the \ (not)
  operator.

      /* Decide what range a number is in */
      pull a
      if a>0 & a<10 then say "1-9"
      if a\<10 & a<20 then say "10-19"
      if \ (a<20 | a>=30) then say "20-29"
      if a<=0 | a>=30 then say "Out of range"

  As well as demonstrating the boolean and comparison operators, this
  program shows that the "else" clause is not required to be present.

  The above program may also be written using Rexx's other conditional
  instruction, "select":

      /* Decide what range a number is in */
      pull a
      select
         when a>0 & a<10 then say "1-9"
     when a\<10 & a<20 then say "10-19"
     when \ (a<20 | a>=30) then say "20-29"
     otherwise say "Out of range"
      end

  The "select" instruction provides a means of selecting precisely one from
  a list of conditional instructions, with the option of providing a list of
  instructions to do when none of the above were true.  The difference is
  that if no part of a "select" instruction can be executed then a syntax
  error results, whereas it is OK to miss out the "else" part of an "if"
  instruction.

  Only one instruction may be placed after "then", "else" and "when", but
  Rexx provides a way of bracketing instructions together so that they can
  be treated as a single instruction.  To do this, place the instruction
  "do" before the list of instructions and "end" after it.

      /* execute some instructions conditionally */
      pull a
      if a=50 then
         do
            say "Congratulations!"
        say "You have typed the correct number."
     end
      else say "Wrong!"

  If you wish for one of the conditional instructions to do "nothing", then
  you must use the instruction "nop" (for "no operation").  Simply placing
  no instructions after the "then", "else" or "when" will not work.

  8. Loops

  Rexx has a comprehensive set of instructions for making loops, using the
  words "do" and "end" which you met briefly in the previous section.

    a. Counted loops

    The instructions within a counted loop are executed the specified number
    of times:

        /* Say "Hello" ten times */
    do 10
       say "Hello"
        end

    A variation of the counted loop is one which executes forever:

        /* This program goes on forever until the user halts it */
    do forever
       nop
    end

    b. Control loops

    Control loops are like counted loops, but they use a variable (called
    the control variable) as a counter.  The control variable may count
    simply in steps of 1:

        /* Count to 20 */
    do c=1 to 20
       say c
    end

    or in steps of some other value:

        /* Print all multiples of 2.3 not more than 20 */
    do m=0 to 20 by 2.3
       say m
    end

    It may take a specific number of steps:

        /* Print the first six multiples of 5.7 */
    do m=0 for 6 by 5.7
       say m
    end

    or it may go on forever:

        /* Print all the natural numbers */
    do n=0
       say n
    end n

    The "n" at the end of this last example is optional.  At the end of any
    controlled loop, the name of the control variable may be placed after
    the "end", where it will be checked to see if it matches the control
    variable.

    c. Conditional loops

    A set of instructions may be executed repeatedly until some condition is
    true. For example,

        /* I won't take no for an answer */
    do until answer \= "NO"
       pull answer
        end

    Alternatively, they may be executed as long as some condition is true:

        /* It's OK to repeat this as long as there is no error */
    do while error=0
       pull a
       if a="ERROR" then error=1
       else say a
    end

    Note that in this example, if there is already an error to start with
    then the set of instructions will not be executed at all.  However in
    the previous example the instructions will always be executed at least
    once.

    d. Controlled conditional loops

    It is possible to combine forms a or b with form c mentioned above, like
    this:

        /* I won't take no for an answer unless it is typed three times */
    do 3 until answer \= "NO"
       pull answer
    end

    or this:

        /* input ten answers, but stop when empty string is entered */
    do n=1 to 10 until ans==""
       pull ans
       a.n=ans
    end

  The "iterate" and "leave" instructions allow you to continue with, or to
  leave, a loop respectively.  For example:

      /* input ten answers, but stop when empty string is entered */
      do n=1 to 10
         pull a.n
     if a.n=="" then leave
      end

      /* print all integers from 1-10 except 3 */
      do n=1 to 10
         if n=3 then iterate
     say n
      end

  If a symbol is placed after the instructions "iterate" or "leave", then
  you can iterate or leave the loop whose control variable is that symbol.

      /* Print pairs (i,j) where 1 <= i,j <= 5, except (2,j) if j>=3 */
      do i=1 to 5
         do j=1 to 5
        if i=2 & j=3 then iterate i /* or "leave j" would work,
                                       or just "leave"           */
            say "("i","j")"
     end
      end

  9. Parsing

  The following program examines its arguments:

      /* Parse the arguments */
      parse arg a.1 a.2 a.3 a.4 .
      do i=1 to 4
         say "Argument" i "was:" a.i
      end

  Execute it as usual, except this time type "alpha beta gamma delta" after
  the program name on the command line, for example:

    arguments alpha beta gamma delta

  The program should print out:

      Argument 1 was: alpha
      Argument 2 was: beta
      Argument 3 was: gamma
      Argument 4 was: delta

  The argument "alpha beta gamma delta" has been parsed into four
  components.  The components were split up at the spaces in the input.  If
  you experiment with the program you should see that if you do not type
  four words as arguments then the last components printed out are empty, and
  that if you type more than four words then the last component contains all
  the extra data.  Also, even if multiple spaces appear between the words,
  only the last component contains spaces.  This is known as "tokenisation".

  It is not only possible to parse the arguments, but also the input.  In
  the above program, replace "arg" by "pull".  When you run this new program
  you will have to type in some input to be tokenised.

  Replace "parse" with "parse upper" in the program.  Now, when you supply
  input to be tokenised it will be uppercased.

  "arg" and "pull" are, respectively, abbreviations for the instructions
  "parse upper arg" and "parse upper pull".  That explains why the "pull"
  instruction appeared in previous examples, and why it was that input was
  always uppercased if you typed letters in response to it.

  Other pieces of data may be parsed as well.  "parse source" parses
  information about how the program was invoked, and what it is called.
  "parse version" parses information about the interpreter itself.  However,
  the two most useful uses of the parse instruction are
  "parse var [variable]" and "parse value [expression] with".  These allow
  you to parse arbitrary data supplied by the program.

  For example,

      /* Get information about the date and time */
      d=date()
      parse var d  day month year

      parse value time() with hour ':' min ':' sec

  The last line above illustrates a different way to parse data.  Instead of
  tokenising the result of evaluating time(), we split it up at the
  character ':'.  Thus, for example, "17:44:11" is split into 17, 44 and 11.

  Any search string may be specified in the "template" of a "parse"
  command.  The search string is simply placed in quotation marks, for
  example:

      parse arg first "beta" second "delta"

  This line assigns to variable first anything which appears before
  "beta", and to second anything which appears between "beta" and "delta".
  If "beta" does not appear in the argument string, then the entire string
  is assigned to first, and the empty string is assigned to "second".  If
  "beta" does appear, but "delta" does not, then everything after "beta"
  will be assigned to second.

  It is possible to tokenise the pieces of input appearing between search
  strings. For example,

      parse arg "alpha" first second "delta"

  This tokenises everything between "alpha" and "delta" and places the
  tokens in first and second.

  Placing a dot instead of a variable name during tokenising causes that
  token to be thrown away:

      parse pull a . c . e

  This keeps the first, third and last tokens, but throws away the second
  and fourth.  It is often a good idea to place a dot after the last
  variable name, thus:

      parse pull first second third .

  Not only does this throw away the unused tokens, but it also ensures that
  none of the tokens contain spaces (remember, only the last token may
  contain spaces; this is the one we are throwing away).

  Finally, it is possible to parse by numeric position instead of by
  searching for a string.  Numeric positions start at 1, for the first
  character, and range upwards for further characters.

      parse var a 6 piece1 +3 piece2 +5 piece3

  The value of piece1 will be the three characters of a starting at
  character 6; piece2 will be the next 5 characters, and piece3 will be the
  rest of a.


  10. Interpret

  Suppose you have a variable "inst" which contains the string "a=a+1".  You
  can execute that string as an instruction, by typing:

      interpret inst

  The interpret instruction may be used to accept Rexx instructions from the
  user, or to assign values to variables whose names are not known in
  advance.

      /* Input the name of a variable, and set that variable to 42 */
      parse pull var
      interpret var "=" 42

  11. The stack

  Rexx has a data stack, which is accessed via the "push", "queue" and
  "pull" instructions.  The "pull" instruction (or in full, "parse pull")
  inputs data from the user as we have seen before.  However, if there is
  some data on the stack then it will pull that instead.

      /* Access the Rexx stack */
      queue "Hello!"
      parse pull a      /* a contains "Hello!" */
      parse pull b      /* b is input from the user */
      push "67890"
      push "12345"
      parse pull c      /* c contains "12345" */
      /* there is one item left on the stack */

  The difference between "push" and "queue" is that when the items are
  pulled off the stack, the items which were queued appear in the same order
  that they were queued (FIFO, or first in, first out), and the items which
  were pushed appear in reverse order (LIFO, or last in, first out).

  12. Subroutines and functions

  The following program defines an internal function, and calls it with
  some data:

      /* Define a function */
      say "The results are:" square(3) square(5) square(9)
      exit

      square: /* function to square its argument */
      parse arg in
      return in*in

  The output from this program should be: "The results are: 9 25 81"

  When Rexx finds the function call "square(3)", it searches the program for
  a label called "square".  It finds the label on line 5 - the name followed
  by a colon.  The interpreter executes the code which starts at that line,
  until it finds the instruction "return".  While that code is being
  executed, the arguments to the function can be determined with "parse arg"
  in the same way as the arguments to a program.  When the "return"
  instruction is reached, the value specified is evaluated and used as the
  value of "square(3)".

  The "exit" instruction in this program causes it to finish executing
  instead of running into the function.

  A function which takes multiple arguments may be defined, simply by
  separating the arguments with a comma. That is, like this:

      /* Define a function with three arguments */
      say "The results are:" condition(5,"Yes","No") condition(10,"X","Y")
      exit

      condition: /* if the first argument is less than 10, return the second,
                    else return the third. */
      parse arg c,x,y
      if c<10 then return x
      else return y

  A subroutine is similar to a function, except that it need not give a
  value after the "return" instruction.  It is called with the "call"
  instruction.

      /* Define a subroutine to print a string in a box, then call it */
      call box "This is a sentence in a box"
      call box "Is this a question in a box?"
      exit

      box: /* Print the argument in a box */
      parse arg text
      say "+--------------------------------+"
      say "|"centre(text,32)"|"             /* centre the text in the box */
      say "+--------------------------------+"
      return

  It is possible to call a function, even a built-in function, as if it were
  a subroutine.  The result returned by the function is placed into the
  variable called "result".

      /* print the date, using the "call" instruction */
      call date "N"
      say result

  If a function or subroutine does not need to use the variables which the
  caller is using, or if it uses variables which the caller does not need,
  then you can start the function with the "procedure" instruction.  This
  clears all the existing variables away out of sight, and prepares for a
  new set of variables.  This new set will be destroyed when the function
  finishes executing.  The following program calculates the factorial of a
  number recursively:

      /* Calculate factorial x, that is, 1*2*3* ... *x  */
      parse pull x .
      say x"!="factorial(x)
      exit

      factorial: /* calculate the factorial of the argument */
      procedure
      parse arg p
      if p<3 then return p
      else return factorial(p-1) * p

  The variable p which holds the argument to funtion factorial is unaffected
  during the calculation of factorial(p-1), because it is hidden by the
  "procedure" instruction.

  If the subroutine or function needs access to just a few variables, then
  you can use "procedure expose" followed by the list of variable names to
  hide away all except those few variables.

  You can write functions and subroutines which are not contained in the
  same Rexx program.  In order to do this, write the function and save it
  into a file whose name will be recognised by the interpreter.  This type
  of function is called an "external" function, as opposed to an "internal"
  function which can be found inside the currently running program.

  If you want to call your function or subroutine using "call foobar" or
  "foobar()", then you should save it in a file named "foobar.cmd" which
  can be found in the current directory or in your path.

  The "procedure" instruction is automatically executed before running your
  external function, and so it should not be placed at the start of the
  function.  It is not possible for an external function to access any of
  its caller's variables, except by the passing of parameters.

  For returning from external functions, as well as the "return" instruction
  there is "exit".  The "exit" instruction may be used to return any data to
  the caller of the function in the same way as "return", but "exit" can be
  used return to the caller of the external function even when it is used
  inside an internal function (which is in turn in the external function).
  "exit" may be used to return from an ordinary Rexx program, as we have
  seen.  In this case, a number may be supplied after "exit", which will be
  used as the exit code of the interpreter.

  13. Executing commands

  Rexx can be used as a control language for a variety of command-based
  systems.  The way that Rexx executes commands in these systems is as
  follows.  When Rexx encounters a program line which is nether an
  instruction nor an assignment, it treats that line as a string expression
  which is to be evaluated and passed to the environment.  For example:

      /* Execute a given command upon a set of files */
      parse arg command
      command "file1"
      command "file2"
      command "file3"
      exit

  Each of the three similar lines in this program is a string expression
  which adds the name of a file (contained in the string constants) to the
  name of a command (given as a parameter).  The resulting string is passed
  to the environment to be executed as a command.  When the command has
  finished, the variable "rc" is set to the exit code of the command.

  The environment to which a command is given varies widely between systems,
  but in most systems you can select from a set of possible environments by
  using the "address" instruction.

  14. Signal

  Where other programming languages have the "goto" command, Rexx has
  "signal".  The instruction "signal label" makes the program jump to the
  specified label.  However, once this has been done, it is not possible to
  resume any "select" or "do" control structures that have recently been
  entered.  Thus the main use of "signal" is to jump to an error handling
  routine when something goes wrong, so that the program can clean up and
  exit.

  There is a much more useful way of using "signal", however.  That is to
  trap certain kinds of error condition.  The conditions which may be
  trapped include: "syntax" (that is, any syntax error), "error" (any
  environment command that results in a non-zero exit code), "halt" (when
  the user interrrupts execution) and "novalue" (which is when a symbol is
  used without having been given a value).

  Error trapping for one of these conditions is turned on by

      signal on <condition>

  and is turned off by

      signal off <condition>

  When one of these conditions occurs, the program immediately signals to a
  label whose name is the same as that of the condition.  Trapping is turned
  off, so another "signal on" will be required if you want to continue to
  trap that condition.

  Whenever a "signal" occurs, the variable "sigl" is set to the line number
  of the instruction which caused the jump.  If the signal was due to an
  error trap, then the variable "rc" will be set to an error code.

      /* This program goes on forever until someone stops it. */
      say "Press Control-C to halt"
      signal on halt
      do i=1
         say i
     do 10000
     end
      end

      halt:
      say "Ouch!"
      say "Died at line" sigl


  15. Tracing

  Full details of how to use Rexx's tracing facility are contained in
  the online rexx reference.

  If a program goes wrong and you need more information in order to work out
  why, then Rexx provides you with the ability to trace all or part of your
  program to see what is happening.

  The most common form of tracing is turned on by

      trace r

  This causes each instruction to be listed before it is executed. Also, it
  displays the results of each calculation after it has been found.

  Another useful trace command is:

      trace ?a

  This makes the interpreter list each instruction and stop before it is
  executed.  You can execute the instruction by pressing return, or you can
  type in some Rexx to be interpreted, after which the interpreter will
  pause again.  You can use this to examine the program's variables and so
  forth.

  If you want the interpreter to stop only when passing a label, use:

     trace ?l

  This is useful for setting breakpoints in the program, or for making the
  program stop at a function call.




The REXX Summary

Summary of the summary

 * Section 1: Removed
 * Section 2: Summary of expression syntax
 * Section 3: Summary of instructions
 * Section 4: Summary of builtin functions

[1] The Rexx Language, a practical approach to programming
    M. F. Cowlishaw
    Englewood Cliffs 1985

2 Summary of expression syntax

Strings:          "String constants" 'enclosed in quotes'
Hex strings:      '616263 64'x
Numbers:          123.4e-56
Constant symbols: .abc 1d4 7!
Simple symbols:   foo bar xyz3
Stems:            array.
Compound symbols: c.3 element.bar.45
Operators:        arithmetic: + - * / ** (to-power) % (div) // (mod)
                  string:     [abuttal] [space] || (concatenation)
                  logical:    \ (not) & (and) | (or) && (xor)
                  comparison: = > < <> <= >= \> \< \=
                              == >> << <<= >>= \>> \<< \== (strong comparison)
Function calls:   fn(1,2,3)    bar(,5)  'DATE'()

3 Summary of instructions

/* this is a comment */

expression                     - execute a host command

symbol=value                   - assignment

ADDRESS [VALUE] [environment]  - change the current environment
ADDRESS environment command    - execute a command in an environment

CALL name [arg][,[arg],...]]   - call a function or subroutine
CALL ON condition [NAME symbol]- turn on condition handling
CALL OFF condition             - turn off condition handling

    condition = ERROR | FAILURE | NOTREADY | HALT

DO [ symbol=start   [TO  finish]  ]     [WHILE expression_w]
   [                [BY  step  ]  ]
   [                [FOR count ]  ]     [UNTIL expression_u]

   [ expression_c                 ]     [FOREVER           ]

                               - block or repetitive block instruction

DROP symbol [symbol...]        - de-assignment

END [symbol]                   - end block or repetitive block instruction

EXIT [expression]              - exit program or external function

IF expression [;] THEN [;] statement [ ; ELSE [;] statement]
                               - conditional instruction

INTERPRET expression           - dynamically execute a string

ITERATE [symbol]               - continue repetitive block

LEAVE [symbol]                 - leave repetitive block

NOP                            - do nothing

NUMERIC DIGITS n               - set parameters for numeric formatting
NUMERIC FUZZ n
NUMERIC FORM   SCIENTIFIC
             | ENGINEERING
             | "string constant"
             | [VALUE] expression

OPTIONS expression             - Control system-dependent things

[PARSE [UPPER]] ARG template
[PARSE [UPPER]] PULL [template]
PARSE [UPPER] LINEIN [template]
PARSE [UPPER] SOURCE template
PARSE [UPPER] VERSION template
PARSE [UPPER] NUMERIC template
PARSE [UPPER] VAR symbol template
PARSE [UPPER] VALUE expression WITH template

    template        -> [firstPosition] assignments [assignments]
    assignments     -> [nextPosition] varlist [stopPosition]
    varlist         -> varname ' ' [varlist]
    varname         ->   "non-constant symbol"
                       | '.'
    firstPosition   -> position
    nextPosition    -> position [nextPosition]
    stopPosition    -> position
    position        ->   searchPosition
                       | absPosition
                       | relPosition
                       | '(' "expression" ')'
    searchPosition  -> "string constant"
    absPosition     ->   "integer"
                       | '=' numexpr
    relPosition     ->   '+' numexpr
                       | '-' numexpr
    numexpr         ->   "integer"
                       | '(' "integer expression" ')'

PROCEDURE                      - hide the caller's symbols
PROCEDURE EXPOSE var1 [var2...]
PROCEDURE HIDE var1 [var2...]

PUSH  expression               - stack a value in LIFO order
QUEUE expression               - stack a value in FIFO order

RETURN [value]                 - return from a function or subroutine

SAY [expression]               - echo data
SAYN expression                - echo data without newline

SELECT [expression]
   WHEN expression THEN statements
   WHEN expression THEN statements
   ...
   [OTHERWISE statements]
END [SELECT]                   - switch on a list of conditions

SIGNAL [VALUE] name            - jump to a label
SIGNAL ON condition [NAME symbol] - turn on condition handling
SIGNAL OFF condition              - turn off condition handling

    condition = ERROR | FAILURE | NOTREADY | HALT | SYNTAX | NOVALUE

TRACE [symbol]                 - control program tracing. Values are:
TRACE "string"                   A (all clauses) C (commands) E (error)
TRACE VALUE expression           F (failure) I (intermediate values)
                                 L (labels) N (normal, =F) O (off)
                                 R (results)


4 Summary of builtin functions

Standard functions

ABBREV(information,info[,length])  - check for valid abbreviations
ABS(number)                        - return the absolute value
ADDRESS()                          - return the current environment
ARG([n][,opt])                     - return or test an argument
BITAND(string1,string2[,pad])
BITOR (string1,string2[,pad])      - combine two strings with bit operations
BITXOR(string1,string2[,pad])
B2D(binary)
B2X(binary)
D2B(decimal)
C2X(string)
C2D(string[,n])
D2C(decimal[,n])                   - convert between data formats
D2X(decimal[,n])
X2B(hex)
X2C(hex)
X2D(hex)
CENTER(s,n[,pad])                  - centre a string in a field
CENTRE(s,n[,pad])
COMPARE(s1,s2[,pad])               - compare two strings
CONDITION([option])                - return information about trapped condition
                                     Option can be (default I):
                                   C (condition name) D (description)
                                   I (instruction)    S (status)

COPIES(s,n)                        - replicate a string
DATATYPE(string[,type])            - test datatype. Type can be:
                                   A (alphanumeric) B (bits) L (lowercase)
                                   M (mixed case) N (number) S (symbol chars)
                                   U (upper case) W (whole number) X (hex)

DATE([format])                     - get date. Format can be:
                                   B (base date - days since 1/1/1 AD)
                                   C (days in century) D (days in year)
                                   E (European) J (Julian) M (month name)
                                   N (normal: dd Mon yyyy) O (ordered)
                                   S (sorted) U (USA) W (day of week)

DELSTR(string,n[,length])          - delete substring
DELWORD(string,n[,length])         - delete words
DIGITS()                           - NUMERIC DIGITS setting
ERRORTEXT(i)                       - Rexx error message
FORM()                             - NUMERIC FORM setting

FORMAT(number [,[before] [,[after] [,[expp] [,expt]]]] )
                                   - format a number as specified
FUZZ()                             - NUMERIC FUZZ setting
INSERT(new,target[,[n][,[length][,pad]]])  - insert new string into target
JUSTIFY(s,n[,pad])                 - justify text to given width
LASTPOS(needle,haystack[,start])   - find last occurrence of a string
LEFT(string,num[,pad])             - return an initial substring
LENGTH(string)                     - find the length of a string
LINESIZE()                         - find the terminal width
MAX(number[,number...])            - find the maximum of a set
MIN(number[,number...])            - find the minimum of a set
OVERLAY(new,target[,[n][,[length][,pad]]])  - overlay new string on to target
POS(needle,haystack[,start])       - find the first occurance of a string
QUEUED()                           - return the number of items on the stack
RANDOM([min][,[max][,seed]])       - return a random number
REVERSE(string)                    - find the reverse of a string
RIGHT(string,num[,pad])            - return a final substring
SOURCELINE([i])                    - return a line of the source program
SPACE(s[,[n][,pad]])               - evenly space words in a sentence
STRIP(string[,[opt][,char]])       - remove leading/trailing spaces
SUBSTR(string,n[,length[,pad]])    - return a substring
SUBWORD(string,n[,k])              - return a substring of words
SYMBOL(name)                       - test to see if a symbol is defined
TIME([format])                     - get the time. Format can be:
                                   C (civil time) N (normal) L (long)
                                   H (hours since midnight)
                                   M (minutes since midnight)
                                   S (seconds since midnight) E (elapsed time)
                                   R (elapsed time then reset)

TRACE([setting])                   - get and/or set trace mode (see trace
                                     instruction)

TRANSLATE(string[,[tableo][,[tablei][,pad]]])
                                   - translate characters using given tables
TRUNC(number[,n])                  - truncate floating point number
VALUE(s[,[newvalue][,selector]])   - get or set value of a symbol

VERIFY(string,reference[,[option][,start]])
                                   - verify string for valid characters
WORD(string,n)                     - return a word from a string
WORDINDEX(string,n)                - return the position of a word in a string
WORDLENGTH(string,n)               - return the length of a word in a string
WORDPOS(phrase,string[,start])     - find a phrase in a string
WORDS(string)                      - return the number of words in a string
XRANGE([a[,b]])                    - return a range of characters

I/O functions (some of which are UNIX-specific)

CHARIN([stream] [,[position] [,count]])  - read characters
CHAROUT([stream] [,[string] [,position] ]- write characters
CHARS([stream])                          - number of characters available
CLOSE(stream)                            - close a stream
FDOPEN(fd [,[mode] [,stream]])           - open an fd number
FILENO(stream)                           - find an fd number
FTELL(stream)                            - return the current file pointer
LINEIN([stream] [,[line] [,count]])      - read a line
LINEOUT([stream] [,[string] [,line]])    - write a line
LINES([stream])                          - determine whether lines may be read
OPEN(file [,[mode] [,stream]])           - open a file
PCLOSE(stream)                           - close a pipe
POPEN(command [,[mode] [,stream]])       - open a pipe to a shell command
STREAM(stream [,[option] [,command]])    - miscellaneous stream operations

UNIX-specific functions

CHDIR(directory)                   - change to new directory
GETCWD()                           - return current working directory
GETENV(name)                       - get an environment variable
PUTENV(string)                     - set an environment variable
SYSTEM(s)                          - return the output of a shell command
USERID()                           - return the process owner's login name

Mathematical functions (implemented as separate package)

ACOS(x)      the arc-cosine of x in radians (0<=acs(x)<=pi)
ASIN(x)      the arc-sine of x in radians (-pi/2<=asn(x)<=pi/2)
ATAN(x)      the arc-tangent of x in radians (-pi/2<=atn(x)<=pi/2)
COS(x)       the cosine of x radians
EXP(x)       the exponential of x (2.718281... to the power x)
LN(x)        the natural logarithm of x (x>0)
SIN(x)       the sine of x radians
SQRT(x)      the square root of x (x>=0) [arbitrary precision possible]
TAN(x)       the tangent of x radians (x <> pi/2)
TOPOWER(x,y) x to the power y