PIL - Language features

In the last post we saw some of the history of PIL and how to run it on MTS. We'll now take a closer look at the features of PIL. Examples shown can be entered directly into PIL after starting it with $run *pil.

Direct mode

PIL starts up in direct mode, where statements entered are immediately executed when you press RETURN. * is used by PIL as the prompt to enter input. You can use the TYPE statement and simple arithmetic expressions to make PIL act as a calculator:

* type 22 * 2
  22 * 2 =  44.0
* type 2 ** 16
  2 ** 16 =  65536.0

On MTS, PIL is case-insensitive for keywords and lines can optionally end with a period. Errors are immediately reported, usually starting with Eh?.

* TYPE 2+2.
  2+2 =  4.0
* TYPE
  Eh? IMPROPERLY FORMED STATEMENT

If the last character entered on the line is - then input will continue on the next line before it is executed. The prompt will change to & to show this continuation. * at the start of line can be used to make comments.

 * type 1 + 2 + 3 +-
 & 5 + 6
   1 + 2 + 3 +5 + 6 =  17.0
 * * comment here
 *

Variables and types

Variables can be introduced by the optional keyword SET followed by a variable name and an assignment.

* set a = 2
* b = 3
* type a, b, a+b
  a =  2.0
  b =  3.0
  a+b =  5.0

PIL understands three types: numerical, which are stored as floating point values, character strings and Boolean values.

* a = 1 / 3
* b = "PIL"
* c = The True
* type a, b, c
  a =  0.3333333
  b = "PIL"
  c = The True

Booleans constants are 'The True' or 'The False' - something I've not seen in any other language. Strings can be up to 255 characters long and can be entered with single or double quotes as delimiters. Floats have 7 digits of precision and can be entered with exponential notation. Volume 12 mentions type 9.999999e64 as the maxomum value but on the version I'm running it seems 62 is the maximum exponent.

* type 9.999999e62
  9.999999e62 =  9.999999E+62
* type 9.999999e63
  Eh? EXPONENT OUT OF RANGE

Variable names are up to 8 characters long, are case sensitive and are distinguished from keywords, as this silly example shows.

* set set = 1
* set SET = 2
* type set, SET
  set =  1.0
  SET =  2.0

Arrays are allowed with any number of dimensions, though once set the number of dimensions cannot be changed.

* x(1, -2) = 3
* x(3, 44.0) = 4
* type x
  x(1,-2) =  3.0
  x(3,44) =  4.0
* x(3,4,5) = 42
  Eh? ??-

Expressions

Arithmetic expressions work mostly as expected. The absolute value can be taken by surrounding an expression with |; exponentiation is done with **. Functions for arithmetic operations such as square root, cosine, log etc generally have a short and long form and do not need parentheses unless needed to resolve ambiguities.

* type 1 + |-41|
  1 + |-41| =  42.0
* type the square root of 9
  the square root of 9 =  3.0
* type sqrt of 16
  sqrt of 16 =  4.0
* type sqrt of 25+1
  sqrt of 25+1 =  6.0
* type sqrt of (25+1)
  sqrt of (25+1) =  5.09902

There are functions for min/max, random numbers and extracting parts of a number, as well as special functions to get time (in 300ths of seconds since midnight), date, cpu/elapsed time and storage used.

* type the min of (1,2,3)
  the min of (1,2,3) =  1.0
* type the time, the date, the elapsed time, the cpu time
  the time =  1.963908E+07
  the date =  17133.0
  the elapsed time =  7.819461E+07
  the cpu time =  464.0
* type the total size, the size
  the total size =  188.0
  the size =  165.0

Boolean functions are similar to other languages, with # standing in for logical or.

* type 2 >= 3
  2 >= 3 = The False
* type 2 $lt 3
  2 $lt 3 = The True
* type the true # the false
  the true # the false = The True
* type the true & the false
  the true & the false = The False

Character expressions include length, case conversion, comparison and extraction.

* type the l of "abc"
  the l of "abc" =  3.0
* type the upper of "abC"
  the upper of "abC" = "ABC"
* type the first 3 characters of "abcde"
  the first 3 characters of "abcde" = "abc"
* type 2 $fc "abcde" + "X" + 1 $lc "abcde"
  2 $fc "abcde" + "X" + 1 $lc "abcde" = "abXe"
* type "aaa" > "AAA"
  "aaa" > "AAA" = The False

Finally, the type of an expression can be found and run time evaluation performed.

* type the mode of 42, the mode of the true, the mode of "abc"
  the mode of 42 =  1.0
  the mode of the true =  2.0
  the mode of "abc" =  3.0
* type the value of "2*21"
  the value of "2*21" =  42.0

Control flow

There's an IF statement with an optional ELSE clause. The IF and ELSE can be omitted but the punctuation is required.

* if 1 < 2, then type 'yes'; else type 'no'
  yes
* if 1 > 2, type 'yes'; type 'no'
  no

For loops have a range or an increment and an optional clause to terminate.

* for i = 1 to 5: type i ** 2
  i ** 2 =  1.0
  i ** 2 =  4.0
  i ** 2 =  9.0
  i ** 2 =  16.0
  i ** 2 =  25.0
* for i = 1 by 2 to 10: type i ** 2
  i ** 2 =  1.0
  i ** 2 =  9.0
  i ** 2 =  25.0
  i ** 2 =  49.0
  i ** 2 =  81.0
* for i = 1 by 2 while i < 20: type i
  i =  1.0
  i =  3.0
  i =  5.0
  i =  7.0
  i =  9.0
  i =  11.0
  i =  13.0
  i =  15.0
  i =  17.0
  i =  19.0

Indirect mode

Lines entered that start with a number are treated as stored instructions, broken down by part and step, that can be run later. Here we define a program in part 1 consisting of four steps.

* 1.0 i = 5
* 1.1 type i
* 1.2 i = i * i
* 1.3 type i

This can then be run with DO, which will execute all steps in a part.

* do part 1
  i =  5.0
  i =  25.0

Variables set in a program remain after execution is completed and it is possible to run a single step at a time.

* type i
  i =  25.0
* do step 1.2
* do step 1.3
  i =  625.0

Steps can call other parts with DO: execution will resume after the part is finished. It's also possible to transfer execution with TO which will not return. DONE will return from the current part.

* type part 2, part 3

  2.0    i = 3
  2.1    do step 3
  2.2    i = 5
  2.3    do step 3
  2.4    i = 7
  2.5    to step 3
  2.6    type 'not reached'


  3.0    type i
  3.1    done
  3.2    type 'also not reached'

* do part 2
  i =  3.0
  i =  5.0
  i =  7.0

The above also shows it is possible to list out programs with TYPE PART. You can see all entered parts with TYPE ALL PARTS. With TYPE ALL STUFF you will see all variables and parts defined.

DELETE can be used to remove a step or a variable definition.

I/O and system access

We've seen TYPE used to display variables. You can prompt for a value to be entered with DEMAND

* delete part 1
* 1.0 demand i
* 1.1 i = i * 10
* 1.2 type i
* do part 1
 i = ?_ 3
  i =  30.0

There is also a formatting facility for input and output that is covered in the manual.

Programs can be saved to disk for future list with SAVE. This takes a file name (which must already exist) and what to save. For example:

save as 'x.pil', all stuff  

will save all parts and variables to x.pil. File format is plain text so it can be edited outside of PIL if needed. LOAD 'x.pil' will then load the file back into PIL.

The implementation of PIL on MTS includes access to system facilities such as file creation and device access. For example, CREATE 'x.pil' will create a new file.

In the next post we'll see how to construct a larger PIL program.

Further information

MTS volume 12 has a complete reference to PIL as implemented on MTS.

If you can find a copy of the book 'History of Programming Languages', edited by Richard Wexelblat, this has a great section about JOSS talking about its design principles.

PIL - Introduction

Screenshot of a teletypewriter running PIL, from 2 4 Using the Michigan Terminal System 6 01.

In this series of posts we'll look at the Pittsburgh Interpretative Language, or PIL, a simple interpreted language that can be used to do calculations and build small programs interactively.

The Pittsburgh Interpretative Language

PIL was developed at the University of Pittsburgh for the System/360 in the late 60s. It was based on one of the first interpreted languages JOSS that originated at the RAND Corporation in 1963. PIL improves on JOSS by providing improved debugging capabilities and error reporting.

The design goals for PIL were, according to MTS Volume 12:

PIL is oriented toward problem-solving, with program development and debugging facilities having highest priority. For the beginning user, PIL was designed to be clear, unambiguous, and hence, easily learned. For the experienced programmer, the language offers increased flexibility with statement structure and expanded capabilities for the solution of non-numeric problems. For the researcher, PIL reduces the amount of time and effort that must be expended in problem solving.

PIL can be used as a simple desktop calculator with variables

* set a = 3
* set b = 4
* type the square root of (a*a + b*b)
the square root of (a*a + b*b) =  5.0  

It can also be used to build simple programs interactively that can then be run:

* 1.01 demand a
* 1.02 demand b
* 1.03 type the square root of (a*a + b*b)
* do part 1
 a = ?_ 3
 b = ?_ 4
  the square root of (a*a + b*b) =  5.0

PIL on MTS

PIL on MTS is based on the second version of PIL, PIL/2. It was modified to integrate well with MTS files and system services.

Prerequisites

No special installation instructions to get this language running - just do the standard D6.0 setup as described in this guide and then sign on as a regular user such as ST01.

Running a program using *PIL

Running the command *PIL on MTS will start the PIL interpreter. It is intended to be used in an interactive way where you enter commands and see the output directly. It can be used in batch mode by feeding commands into source but this is not the intended mode of operation. Inside the interpreter, programs and data can be loaded and saved with the load and save as statements.

Hello world

Let's see how to run a simple program to print 'Hello, world!' five times using PIL.

# $run *pil
# Execution begins   09:56:53 
  PIL/2: Ready
* for i = 1 to 5: type "Hello, world!"
  Hello, world!
  Hello, world!
  Hello, world!
  Hello, world!
  Hello, world!
* stop
# Execution terminated   09:59:06  T=0.004 

After starting PIL, enter the for statement at the * prompt. Output is shown immediately after the command is entered. Type stop to return to MTS.

In the next post we'll look at the language in more detail.

Further information

MTS volume 12 has a tutorial and reference for the PIL language and describes how it is integrated with MTS.

The video pictured at the top of this post, 2 4 Using the Michigan Terminal System 6 01, shows simple operation of PIL on a teletypewriter, starting from 21 minutes into the video.

The Pitt time-sharing system for the IBM system 360: Two year's experience. describes the operation of PIL at the University of Pittsburgh.

JOSS: Introduction To A Helpful Assistant is a very readable paper from the RAND Corporation on the language that inspired PIL.

The source code and implementation notes for PIL can be found in component 566 on tape 6.0T2 in the D6.0 distribution.

Source code for the hello world program can be found on github.

RATFOR & FLECS - Emirp primes

For the final post in this series, let's write a real program in RATFOR and FLECS and see how they compare with the original FORTRAN. We'll be implementing the reverse-primes emirp program we did before.

FLECS version

C     FLECS PROGRAM TO DISPLAY EMIRPS  
C  
C     *** TEST IF A NUMBER IS PRIME ***  
      LOGICAL FUNCTION PRIME(N)
      INTEGER N
C     DEAL WITH NUMBERS <= 3  
      IF (N .LE. 1) GOTO 200
      IF (N .EQ. 2 .OR. N .EQ. 3) GOTO 100
C     CHECK IF DIVISIBLE BY 2 OR 3  
      IF (MOD(N,2) .EQ. 0) GOTO 200
      IF (MOD(N,3) .EQ. 0) GOTO 200
C     SEE IF DIVISIBLE BY 5, 7, ..., UP TO APPROX SQRT(N)  
      DO (I=5,999999,2)
      IF (I*I .GT. N) GOTO 100
      IF (MOD(N,I) .EQ. 0) GOTO 200
      FIN
 100  PRIME = .TRUE.
      RETURN
 200  PRIME = .FALSE.
      RETURN
      END
C  
C     *** REVERSE AN INTEGER'S DIGITS ***  
      INTEGER FUNCTION REVRSE(N)
      INTEGER N
      INTEGER M,R
C     M IS COPY OF N FROM WHICH WE TAKE DIGITS  
C     R IS REVERSED DIGITS  
      M = N
      R = 0
C     LOOP UNTIL NO MORE DIGITS  
      UNTIL (M .LT. 1)
C     TAKE LAST DIGIT FROM M AND APPEND TO R  
      R = R * 10
      R = R + MOD(M, 10)
      M = M / 10
      FIN
      REVRSE = R
      RETURN
      END
C  
C     *** TEST IF AN INTEGER IS AN EMIRP ***  
      LOGICAL FUNCTION EMIRP(N)
      INTEGER N
C     EXTERNAL FUNCTIONS  
      INTEGER REVRSE
      LOGICAL PRIME
C     R CONTAINS REVERSED DIGITS OF N  
      INTEGER R
      R = REVRSE(N)
C     N AND R MUST BOTH BE PRIME AND NOT THE SAME VALUE  
      IF (N .NE. R)
      IF (PRIME(N))
      IF (PRIME(R))
      EMIRP = .TRUE.
      RETURN
      FIN
      FIN
      FIN
      EMIRP = .FALSE.
      RETURN
      END
C  
C     *** DISPLAY AN INTEGER ***  
      SUBROUTINE SHOW(N)
      INTEGER N
      WRITE(6,50) N
 50   FORMAT(I10)
      RETURN
      END
C  
C  
C     *** MAIN ENTRY POINT ***  
C     I IS COUNT OF EMIRPS FOUND  
C     N IS NUMBER TO TEST  
C     EXTERNAL FUNCTION  
      LOGICAL EMIRP
      INTEGER I,N
      TEST-1
      TEST-2
      TEST-3
      STOP
C  
C     *** SHOW FIRST 20 EMIRPS ***  
      TO TEST-1
      N = 0
      I = 0
      WHILE (I .LT. 20)
      N = N + 1
      IF (EMIRP(N))
      CALL SHOW(N)
      I = I + 1
      FIN
      FIN
      FIN
C  
C     *** SHOW EMIRPS BETWEEN 7,700 AND 8,000 ***  
      TO TEST-2
      DO (N=7700,8000)
      IF (EMIRP(N)) CALL SHOW(N)
      FIN
      FIN
C  
C     *** SHOW 10,000TH EMIRP ***  
      TO TEST-3
      N = 0
      DO (I=1,10000)
      REPEAT UNTIL (EMIRP(N)) N = N + 1
      FIN
      CALL SHOW(N)
      FIN
C  
      END

Apart from the FORMAT specification and the PRIME function we've eliminated all line numbers. PRIME could be written without line numbers but with the multiple paths out of the function that would need their own RETURN I think it's better this way.

The internal procedures come in handy, eliminating the need for subroutines for TEST1-3, though this does make N and I global which makes me a little uneasy if this was a larger program.

We use the block structure often, with UNTIL, WHILE and REPEAT ... UNTIL; this simplifies code, though without indentation it's a little hard to follow; the output of the preprocesor is useful here to show what it thinks the indentation should be, for example:

  86           TO TEST-1
  87           .  N = 0
  88           .  I = 0
  89           .  WHILE (I .LT. 20)
  90           .  .  N = N + 1
  91           .  .  IF (EMIRP(N))
  92           .  .  .  CALL SHOW(N)
  93           .  .  .  I = I + 1
  94           .  .  ...FIN
  95           .  ...FIN
  96           ...FIN

The compiler diagnostics also helped a lot with catching errors with missing FINs.

RATFOR

Now let's try writing the RATFOR version.

######################################################################
# Ratfor program to display emirps
######################################################################

######### Test if a number is prime #########
logical function prime(n)  
    integer n  # Number to test

    # Deal with numbers <= 3
    if (n < 1) goto 200
    if (n == 2 | n == 3) goto 100

    # Check if divisible by 2 or 3
    if (mod(n,2) == 0) goto 200
    if (mod(n,3) == 0) goto 200

    # See if divisible by 5, 7, ..., up to approx sqrt(n)
    for (i = 5; i < 1000000; i = i + 2) {
        if (I*I > n) goto 100
        if (mod(n,i) == 0) goto 200
    }

 100  prime = .true.
      return
 200  prime = .false.
      return
end

######### Reverse an integer's digits #########
integer function revrse(n)  
    integer n  # Number to reverse
    integer m  # Copy of n from which we take digits
    integer r  # Reversed digits
    m = n
    r = 0
    while (m >= 1) {
        # Take last digit from m and append to r
        r = r * 10
        r = r + mod(m, 10)
        m = m / 10
    }
    revrse = r
    return
end

######### Test if an integer is an emirp #########
logical function emirp(n)  
    integer n       # Number to test
    integer revrse  # External function
    logical prime   # External function
    integer r       # Reversed digits of n
    r = revrse(n)
    emirp = .false.
    # n and r must both be prime and not the same value
    if (n .ne. r & prime(n) & prime(r)) {
        emirp = .true.
    }
    return
end

######### Display an integer #########
subroutine show(n)  
    integer n
    write(6,50) n
50  format(i10)  
    return
end

######### Show first 20 emirps #########
subroutine test1  
    logical emirp   # External function
    integer i       # Count of emirps found
    integer n       # Number to test
    n = 0
    for (i = 1; i <= 20; i = i + 1) {
        repeat {
            n = n + 1
        } until (emirp(n))
        call show(n)
    }
    return
end

######### Show emirps between 7,700 and 8,000 #########
subroutine test2  
    logical emirp   # External function
    integer n       # Number to test
    for (n = 7700; n <= 8000; n = n + 1) {
        if (emirp(n)) {
            call show(n)
        }
    }
    return
end

######### Show 10,000th emirp #########
subroutine test3  
    logical emirp   # External function
    integer i       # Count of emirps found
    integer n       # Number to test
    n = 0
    for (i = 1; i <= 10000; i = i + 1) {
        repeat {
            n = n + 1
        } until (emirp(n))
    }
    call show(n)
    return
end

######### Main entry point #########
call test1  
call test2  
call test3  
stop  
end  

I feel right at home with the braces and the C style for loops, though I miss the increment operator ++. prime would be much better if I could just return (.true.) but that does not work on the version of RATFOR on MTS so we keep the line numbers and gotos.

With the above, plus the free form input (which was supported on MTS FORTRAN anyway) and the operators like < it was easy to write. However, I got precisely zero diagnostics from the RATFOR preprocessor, with all my typos caught by the FORTRAN compiler, from which I'd have to find the problem in the original source. Easy enough in a small program but would be painful in larger ones.

Final thoughts

RATFOR and FLECS both make writing FORTRAN easier and more pleasant at the cost of an extra step in the development process, and I found both succeed at that. RATFOR is clearer and easier to get started with (especially coming from a C background today); the implementation is almost aggressively simple, as the authors admit in their paper, and I wonder how well it would scale for writing larger programs. FLECS has a more robust implementation but a more diffuse design, such as two versions of switch; features like printing a neatly indented output would certainly help on MTS or its contemporaries but the language lacks the cosmetic features that make RATFOR easier to read.

Neither are much used today; FORTRAN 77 and beyond took some of these ideas and built them into the core language. The idea of translating a richer language into a widely used but less expressive language is still alive though: think of Coffeescript or Typescript producing Javascript.

Further information

Full source code for these programs can be found on github.

RATFOR & FLECS - Language Features

FORTRAN meeting From the UM Computing Center Newsletter, Volume 5 Number 14, 24 September 1975, via Google Books. Proposal 2) seems to indicate a different preprocessor was being considered for UM as well as FLECS, I wonder if this was RATFOR or something else?

Hi and welcome back. Today let's continue our exploration of RATFOR and FLECS by comparing the language features they add to vanilla FORTRAN. The quotes below are from the RATFOR paper and FLECS manual, links to which are provided at the end of this post. Code samples for FORTRAN and FLECS are shown in upper case, RATFOR in lower case.

Design

RATFOR attempts to retain the merits of FORTRAN (universality, portability, efficiency) while hiding the worst FORTRAN inadequacies. The language is FORTRAN except for two aspects - [control flow and syntactic sugar] ... Throughout, the design principle which has determined what should be in RATFOR and what should not has been RATFOR doesn’t know any FORTRAN.

RATFOR focuses on control flow - if statements, blocks, looping - and cosmetics such as free form input, comments and other features that make FORTRAN more pleasant to write. By not knowing any FORTRAN, the design limits what features can be made available but also keeps it simple to implement and reduces the temptation to change FORTRAN into a different language altogether.

FLECS is a language extension of FORTRAN which has additional control mechanisms . These mechanisms make it easier to write FORTRAN by eliminating much of the clerical detail associated with constructing FORTRAN programs. FLECS is also easier to read and comprehend than FORTRAN.

FLECS also tries ti improve FORTRAN's control statements, taking ideas from several different languages including Pascal and Lisp. It has less cosmetic additions than RATFOR but adds the concept of internal procedures and includes features in the translator that help the programmer see the structure of their program.

Structure

RATFOR allows blocks of statements to be introduced within braces where FORTRAN would only allow a single statement. The fixed column format in classic FORTRAN is relaxed so any indentation is allowed. Multiple statements can appear on the same line if they are separated by semicolons.

if (x > 100) {  
   call error(x)
   err = 1; return
}

FLECS also has blocks which extend from the start of a control statement to the keyword FIN. It retains the fixed formatting of FORTRAN but prints a nicely indented view of the program when translating. So the example above would be entered as this in FLECS:

      IF (X .GT. 100)
      CALL ERROR(X)
      ERR = 1
      FIN

and the translator would print

IF (X .GT. 100)  
.  CALL ERROR(X)
.  ERR = 1
...FIN

This is useful when entering programs via cards where it is difficult to get indentation right.

It's possible to have a single statement after a control structure in which case the FIN is not needed:

IF (X .GT. 100) CALL ERROR(X)  

RATFOR comments are introduced with # and apply from that point to the end of the line, less restrictive than C in FORTRAN and FLECS which must be in the first column.

% will stop RATFOR processing the rest of the line, passing it through to FORTRAN directly. FLECS will look for a FLECS statement in column 7 and if found will translate the line; if not found it will pass through the whole line to FORTRAN.

Textual substitution

RATFOR allows constants to be set with define SYMBOL VALUE; any use of SYMBOL in the RATFOR program will be replaced with VALUE in the generated FORTRAN program.

include FILE will insert a copy of FILE at that point in the program, just like C's #include.

Operators

RATFOR allows the now-familiar symbols <, <=, !=, | etc to be used instead of .LT., .LE., .NE., .OR. etc. FLECS retains the FORTRAN operators.

Strings

Text in RATFOR programs in single or double quotes is converted to FORTRAN nH strings. Backslash escapes the next character. FLECS keeps FORTRAN strings.

Conditionals

FORTRAN has a simple iF statement where only one statement can be executed if the condition is true. RATFOR extends this by allowing else and nested ifs. An else clause is attached to the nearest if.

if (x > 0) {  
  if (x > 10)
    write(6, 1) x
  else
    write(6, 2) x
else  
  weite(6, 3)

FLECS has IF and for negative tests UNLESS. It also has WHEN ... ELSE for a single positive and negative test.

The switch statement added in RATFOR looks like C but does not have break; the switch is exited after each case or default is executed. FLECS's equivalent is SELECT, so comparing the two:

switch (x) {  
  case 1: y=3
  case 2, 3: y=5
  default y=0
}
      SELECT (X)
      (1) Y=3
      (2) Y=5
      (3) Y=5
      (OTHERWISE) Y=0
      FIN

FLECS has CONDITIONAL which looks a lot like LISP's cond:

      CONDITIONAL
      (X.LT.-5.0)  U = U+W
      (X.LE.1.0)   U = U+W+Z
      (X.LE.10.5)  U = U-Z
      (OTHERWISE)  U = 0
      FIN

Looping

The FORTRAN DO loop has to have a line number marking the point where the loop will restart:

      DO 10 i = 1, n
      x(i) = 0.0
      y(i) = 0.0
      z(i) = 0.0
 10   CONTINUE

RATFOR replaces this with a block:

do i = 1, n {  
  x(i) = 0.0
  y(i) = 0.0
  z(i) = 0.0
}

It also allows break to exit a loop early and next to restart the loop like C's continue. It can be followed by an integer to say how many levels to apply, so break 2 would move out of a two level do statement immediately.

RATFOR also adds a while and for statement that look like C's - these allow immediate exit from the statement if the condition is true on entry, unlike in FORTRAN DO where the statement is always executed at least once (in the IBM implementation at least) and the conditional is tested at the end of the statement. A version of C's do ... while is provided as repeat ... until.

The FLECS equivalent for the above do loop would be:

      DO (I = 1, N)
      X(I) = 0.0
      Y(I) = 0.0
      Z(I) = 0.0
      FIN

FLEC's WHILE construct is similar to RATFOR's, with the conditional tested before the loop starts. By using REPEAT WHILE the body of the loop is executed at least once and the test made at the end of the loop. UNTIL can be used instead of WHILE in both cases to indicate that the loop ends when the conditional becomes true

      X = 0
      UNTIL (X.EQ.5)
      X = X + 1
      FIN

Return

To return a value from a function in FORTRAN and FLECS you must assign a value to the name of the function:

INTEGER FUNCTION DECREMENT(I)  
INTEGER I  
DECREMENT = I - 1  
RETURN  
END  

In the RATFOR paper it sayd you can give return a value:

integer function decrement(i)  
integer i  
return (i-1)  
end  

However, note this is not supported in the version supplied with MTS - it will just pass through such a return statement causing an error from the FORTRAN compiler.

Internal procedures

FLECS allows a group of statements to be defined as a procedure with TO which can then be called by giving its name. No parameters are passed - it uses global variables to communicate. The below example will print 5.

      INTEGER X
      X = 1
      INCREMENT-IT
      DOUBLE-AND-INCREMENT
      WRITE(6,50) X
      STOP
 50   FORMAT(I10) 
      TO INCREMENT-IT X = X + 1
      TO DOUBLE-AND-INCREMENT
      X = X * 2
      INCREMENT
      FIN
      END

Procedure names must include at least one hyphen and recursion is not allowed.

Operation

RATFOR runs as a simple translator, taking a RATFOR input file and producing a FORTRAN output file that must then be fed to the FORTRAN compiler. FLECS, as modified at UM, will both translate and call the FORTRAN compiler, producing machine code output that can be run directly.

Error handling

RATFOR will catch some errors, such as missing closing braces, but will otherwise delegate problems with the program to the FORTRAN compiler to catch, as it does not understand FORTRAN syntax. This could be difficult to trace back to the source of the error as the FORTRAN compiler would show the error in the generated FORTRAN, not the RATFOR original.

FLECS will find syntax errors and remove them from the program, allowing translation to continue at the cost of possibly causing further errors; it will not move on to compilation in this case.

Implementation

Not surprisingly given its authors' roots, RATFOR was originally written in around 1000 lines of C using yacc. The authors say it took less than a week to implement. As C was not widely available in the mid 70's, a version of RATFOR in RATFOR was produced that would generate around 2500 lines of basic FORTRAN so it could be used anywhere.

The FLECS implementation comes in at around 2200 lines of FLECS and took around six months to develop according to comments in the source code.

Further information

See Kernighan's RATFOR paper or the FLECSUser's Manual (in component 673/22; I've uploaded a copy here) for more information on the languages.

RATFOR & FLECS - Introduction

Most programmers will agree that FORTRAN is an unpleasant language to program in, yet there are many occasions when they are forced to use it.

From the introduction to 'RATFOR — A Preprocessor for a Rational FORTRAN' by Brian W. Kernighan

FORTRAN was the lingua franca for mainframe programmers in the 1960s and 1970s, but as Kernighan states it's not always easy to program in - the main reasons are lack of good control structures and the fixed line format. As a result, a number of preprocessors were developed that translated enhanced code down to plain FORTRAN that could then be compiled anywhere a compiler was available.

In this series of posts, we'll look at two preprocessors available on MTS: RATFOR and FLECS. MTS also had OVERDRIVE, but this is not available on D6.0 due to copyright reasons.

Prerequisites

No special installation instructions to get these preprocessors running - just do the standard D6.0 setup as described in this guide and then sign on as a regular user such as ST01.

RATFOR

RATFOR was developed by Brian Kernighan at Bell Telephone Labs in 1974; its syntax was (not surprisingly) inspired by the C programming language, with keywords like for, while and until. It was used as the language for examples in Software Tools and became one of the most popular preprocessors in use. Versions are still available today that run on Unix systems.

Preprocessing using *RATFOR

The version on MTS is called *RATFOR and takes a RATFOR program as input on scards and writes FORTRAN source to spunch. The generated file can then be compiled with *FTN.

Hello world

Here's a terminal log of how to compile and run a simple hello world program in RATFOR. This assumes the source code is in file hello.r.

# $list hello.r

      1     # *** Simple hello world program ***
      2     #
      3     integer i
      4     for (i = 0; i < 5; i = i + 1)
      5     {
      6        write(6, 200)
      7     }
      8     stop
      9     200 format("Hello, world!")
     10     end

# $run *ratfor scards=hello.r spunch=-hello.f
Execution begins   21:56:08  
Execution terminated   21:56:08  T=0.004

# #list -hello.f

      1           INTEGERI
      2           CONTINUE
      3           I=0
      4     23000 IF(.NOT.(I.LT.5))GOTO 23002
      5           WRITE(6,200)
      6     23001 I=I+1
      7           GOTO 23000
      8     23002 CONTINUE
      9           STOP
     10     200   FORMAT(13HHello, world!)
     11           END

# $run *ftn scards=-hello.f spunch=-load
Execution begins   21:56:36  
 No errors in MAIN
Execution terminated   21:56:36  T=0.008

# #run -load
Execution begins   21:56:39  
 Hello, world!
 Hello, world!
 Hello, world!
 Hello, world!
 Hello, world!
Execution terminated   21:56:39  T=0.001  

FLECS

FLECS was written in the early 1970s by Terry Beyer at the University of Oregon. It provides a smaller set of control structures that RATFOR but the syntax is closer to FORTRAN. Keywords include IF...THEN...ELSE and CONDITIONAL and multi-line statements are supported. It does not appear to have been used much past the introduction of FORTRAN77, but a a version is still available today for HPUX.

Compiling using UNSP:FLX

At the time D6.0 was released, FLECS was unsupported at UM so is available as the file FLX in UNSP:. The preprocessor does not used scards and spunch; instead, all parameters need to be passed in to par. Unlike RATFOR, FLECS can call the FORTRAN compiler directly to generate object code. In the listing below, PAR=SOURCE=hello.fl,P=*SINK*,FTNSOURCE,LOAD=-load would read source from hello.fl, print diagnostics to *SINK including the FORTRAN source generated, and write compiled output to -load.

Hello world

Here's a terminal log of how to compile and run a simple hello world program in FLECS. This assumes the source code is in file hello.fl.

# $list hello.fl

      1     C *** SIMPLE HELLO WORLD PROGRAM ***
      2     C
      3           DO (I = 1,5)
      4           WRITE (6,20)
      5           FIN
      6           STOP
      7        20 FORMAT(13H HELLO, WORLD)
      8           END

# $run UNSP:FLX PAR=SOURCE=hello.fl,P=*SINK*,FTNSOURCE,LOAD=-load
Execution begins   21:47:21  
 FFI(CT206)


 (FLECS VERSION 22.38)  MTS Version CT155 21:47:21    JAN 21, 1916    Page   1

 MTS Line#        Indented Source Listing...

      1     C *** SIMPLE HELLO WORLD PROGRAM ***
      2     C
      3           DO (I = 1,5)
      4           .  WRITE (6,20)
      5           ...FIN
      6           STOP
      7        20 FORMAT(13H HELLO, WORLD)
      8           END

   0.001 seconds CPU time used.  Translation rate is 480000 lines per CPU minute.

 There were   NO MAJOR ERRORS and   NO MINOR ERRORS in the above module.
 No preprocessor errors in module  1.


  MICHIGAN TERMINAL SYSTEM FORTRAN G(21.8) MAIN 01-21-16 21:47:21 PAGE P001

    0001              DO 99998 I = 1,5             3.000
    0002              WRITE (6,20)                 4.000
    0003        99998 CONTINUE                     5.000
    0004              STOP                         6.000
    0005           20 FORMAT(13H HELLO, WORLD)     7.000
    0006              END                          8.000
     *OPTIONS IN EFFECT*  ID,EBCDIC,SOURCE,NOLIST,NODECK,LOAD,NOMAP
     *OPTIONS IN EFFECT*  NAME = MAIN    , LINECNT =       57
     *STATISTICS*    SOURCE STATEMENTS =        6,PROGRAM SIZE =      344
     *STATISTICS*  NO DIAGNOSTICS GENERATED
 No errors in MAIN

 NO STATEMENTS FLAGGED IN THE ABOVE COMPILATIONS.
Execution terminated   21:47:21  T=0.018

# $run -load
Execution begins   21:47:31  
 HELLO, WORLD
 HELLO, WORLD
 HELLO, WORLD
 HELLO, WORLD
 HELLO, WORLD
Execution terminated   21:47:31  T=0.001  

Further information

The Wikipedia article on RATFOR has a basic introduction to language features and the history of its development. Kernighan's paper on RATFOR goes into more detail on the language.

Not much appears to exist on the Internet describing FLECS, but the D6.0 MTS tapes does include the complete User's Manual (in component 673/22) and the interface to MTS.

MTS Volume 6 describes the FORTRAN compilers on MTS, which are needed to compile the RATFOR preprocessor's output.

SNOBOL - Date formats

Let's implement a simple program in SNOBOL on MTS to print today's date in different formats.

The problem

The problem is quite simple: take today's date and display it in ISO format (eg 2015-10-11) and a human readable format (eg Sunday, October 11, 2015). Further details and implementations in other languages can be found on Rosetta Code.

Getting today's date

There's a built in function in SNOBOL to return the date as an eight character string. Interestingly, the SNOBOL 4 Programming Language says this returns 'MM/DD/YY' but on MTS it returns 'MM-DD-YY'. Let's get this and store in a variable.

        NOW = DATE()

Breaking the date into components

We take the date and extract the month, day and year using SNOBOL's pattern matching facility and assign it to variables DAY, MONTH, and YEAR.

        PART = SPAN("0123456789")
        SEP  = "-"
        NOW (PART . MONTH) SEP (PART . DAY) SEP (PART . YEAR)

Y2K strikes again

The value returned from DATE() has a two digit year, so let's assume we are running this in the 21st century.

        CENTURY = 2000
        CYEAR = YEAR + CENTURY

Displaying in ISO format

So displaying the date in ISO format is now simply a case of concatenating the day, month and four digit year and then outputting it.

        ISO = CYEAR SEP MONTH SEP DAY
        OUTPUT = ISO

Day of the week

We now turn to the human readable form but there is a slight problem - we need to know which day of the week it is, eg Monday, and there is no facility in SNOBOL to calculate this. There may be a MTS external library we could call to get this, but instead we will use Gauss' algorithm to derive the day as a number from 0 (Sunday) to 6 (Saturday).

* GYEAR is the 4 digit year, unless Jan or Feb then subtract 2
* GMONTH is MONTH-2 modulus 12, Jan is 11, Feb is 12
        GT(MONTH, 2)               :S(G1)F(G2)
G1      GYEAR = CYEAR  
        GMONTH = MONTH - 2         :(GX)
G2      GYEAR = CYEAR - 1  
        EQ(MONTH, 1)               :S(G3)F(G4)
G3      GMONTH = 11                :(GX)  
G4      GMONTH = 12                :(GX)  
GX      WDAY = REMDR(DAY, 7)  
* Calculate the month term 
        MT = (2.6 * GMONTH) - 0.2
* Add the month term - the 0.00005 is needed due to lack of FP precision
        WDAY = WDAY + REMDR(CONVERT(MT + 0.00005, 'INTEGER'), 7)
        WDAY = WDAY + 5 * REMDR(REMDR(GYEAR, 4), 7)
        WDAY = WDAY + 4 * REMDR(REMDR(GYEAR, 100), 7)
        WDAY = WDAY + 6 * REMDR(REMDR(GYEAR, 400), 7)
        WDAY = REMDR(WDAY, 7)

Month and day names

We will need a way of translating a month and day number into a name, eg January or Monday. SNOBOL's arrays can be used for this. Note that the DAYS array is indexed from 0 to 6 instead of 1 to 7.

        MONTHS = ARRAY("12")
        MONTHS<1> = "January"
        MONTHS<2> = "February"
* ...
        MONTHS<11> = "November"
        MONTHS<12> = "December"


        DAYS = ARRAY("0:6")
        DAYS<0> = "Sunday"
        DAYS<1> = "Monday"
* ...
        DAYS<5> = "Friday"
        DAYS<6> = "Saturday"

Displaying in readable format

We now have all the components to display the date in readable format.

        READABLE = DAYS<WDAY> ", " MONTHS<MONTH> " " DAY ", " CYEAR
        OUTPUT = READABLE

Running the program

Here's what the output of the program looks like.

# $run *snobol4 5=date.sn
# Execution begins   21:09:05

 SNOBOL4 (VERSION 3.10, APRIL 1, 1973)
 (MTS IMPLEMENTATION MAY 1, 1975)

 0 SYNTACTIC ERROR(S) IN SOURCE PROGRAM


 2015-10-11
 Sunday, October 11, 2015


 NORMAL TERMINATION AT LEVEL  0
 LAST STATEMENT EXECUTED WAS   53


 SNOBOL4 STATISTICS SUMMARY
              38 MS. COMPILATION TIME
               1 MS. EXECUTION TIME
              41 STATEMENTS EXECUTED,       0 FAILED
              21 ARITHMETIC OPERATIONS PERFORMED
               1 PATTERN MATCHES PERFORMED
               0 REGENERATIONS OF DYNAMIC STORAGE
               0 READS PERFORMED
               2 WRITES PERFORMED
            0.02 MS. AVERAGE PER STATEMENT EXECUTED

# Execution terminated   21:09:05  T=0.045

Final thoughts on SNOBOL

Using SNOBOL feels close to modern scripting languages such as Perl, Python or Ruby. I really like the pattern matching facilities where you can do things like EXPR = TERM | *EXPR OP TERM which is much more powerful than regular expressions; I don't think there is any modern language that has this built in. The lack of control flow processing apart from GOTO is annoying; later versions of the language such as SPITBOL addressed this. I imagine that it was also rather slow when running on a mainframe, especially as it had to be compiled each time it was run.

I don't think SNOBOL is much in use today, but the maintainer of SPITBOL is still active.

Further information

Full source code for this program is on github.