POOLtype

We begin our study of DEK’s programs (leading up to TeX) with POOLtype.

It is a very simple (almost trivial) program, but reading it is useful to get a glimpse of WEB, of the programming style, etc.

Table of contents

Background: string pools

The year is 1980 or so. You’re having to write programs in PASCAL, trying to stick to the standard version, but coming up against its limitations. What do you do?

If you are Brian W. Kernighan, you’ll write an influential article called “Why Pascal is Not My Favorite Programming Language” (HTML, PDF). The top reason: without using one of the various diverging and nonstandard extensions to Pascal, if you wrote a function that takes a string as input parameter, it had to be a string of a fixed length — a string was an array of characters, and arrays of different lengths were different types. (If you wanted to handle strings of different lengths, you could create a separate function for each length.)

If you are Donald E. Knuth, and given to things like writing your own assembler as a matter of course, then you deal with these problems by ignoring the language’s insufficient solutions and building your own.

String pools were his solution to the string problem: in his program he simply declared a giant array of characters, and every string that was wished to be stored in the program would have its characters stored sequentially in that array, with a separate array for keeping track of where each string starts.

For example, suppose you have four strings “zeroth”, “first”, “2nd”, and “this is third”. They might go into the str_pool and str_start arrays as follows (I added the str_start_pointers array just for clarity in case it helps; it’s a version of str_start that uses pointers instead of indices… the TeX program does not use Pascal’s pointer types, and uses only integer indices but calls them pointers):

str_pool_via_pythontutor

For example, string 3 consists of the characters from position str_start[3] (= 14) of the str_pool array, to the position str_start[4] - 1 (= 26). The variables pool_ptr and str_ptr are the next indices that are going to be populated (for the next string we want to insert), in the str_pool and str_start arrays respectively.

Apart from strings that the program needs to construct at runtime, there are also strings that are part of the program itself: error messages and the like. These are populated into the string pool at the start of the program, via the mechanism of the WEB preprocessor (the TANGLE program) dumping them to a .pool file, and the program containing code to read from the .pool file at startup, to populate its string pool.

What the POOLtype program does

If you skipped the previous section, some background: WEB programs are processed by the program TANGLE, which (apart from the main .pas file) generates a .pool file. For example, tex.pool may look like this:

11buffer size
09pool size
17number of strings
03???
13m2d5c2l5x2v5i
28End of file on the terminal!
02! 
42(That makes 100 errors; please try again.)
02? 
22You want to edit file

… continued …

05mskip
02mu
00
08leaders 
04kern

… continued …

08whatsit?
04ext2
04ext3
08endwrite
24Unbalanced write command
59On this page there's a \write with fewer real {'s than }'s.
04ext4
16output file name
*117275187

So as you can see, it consists of a bunch of lines, on each of which the first two characters are ASCII digits ‘0’ to ‘9’ representing the length of the string, and the rest of the characters are the actual string. (If you’re wondering about lines 7 and 9 above, they are “02! ” and “02? ” – they end with a space.) The last line contains a kind of hash (not a cryptographic one, just a simple check).

All that the POOLtype program does is read this file, and print it again, in the following form:

0: "^^@"
1: "^^A"
2: "^^B"
3: "^^C"
...
65: "A"
66: "B"
67: "C"
68: "D"
...
252: "^^fc"
253: "^^fd"
254: "^^fe"
255: "^^ff"
256: "buffer size"
257: "pool size"
258: "number of strings"
259: "???"
260: "m2d5c2l5x2v5i"
261: "End of file on the terminal!"
262: "! "
263: "(That makes 100 errors; please try again.)"
264: "? "
265: "You want to edit file "
...
336: "mskip"
337: "mu"
338: ""
339: "leaders "
340: "kern"
...
1292: "whatsit?"
1293: "ext2"
1294: "ext3"
1295: "endwrite"
1296: "Unbalanced write command"
1297: "On this page there's a \write with fewer real {'s than }'s."
1298: "ext4"
1299: "output file name"
(23437 characters in all.)

So as you can see, it simply prints out the strings in the tex.pool file in a slightly more human-readable way, with a few changes:

Nevertheless, this trivial program provides some basic sanity testing to make sure that the TeX program would start up with the right tex.pool file and not something corrupted debugging help, e.g. in finding out what string 340 means.

A Python implementation

To make it clearer what POOLtype does, here is a Python implementation that has the same output as POOLtype, on a valid tex.pool file (it only tries to match the output, i.e. doesn’t do error checking):

from __future__ import print_function
import sys

def escape(c):
    # Printable characters don't need escaping.
    if ord(' ') <= ord(c) <= ord('~'): return c
    # Characters 0 to 31 are escaped as ^^ + one 64 ahead
    if ord(c) < 64:
        return '^^' + chr(ord(c) + 64)
    # 127 is escaped as ^^ + (127 - 64)
    if ord(c) == 127: return '^^?'
    # 128 to 255 are escaped as ^^ + two lowercase hex chars
    h = hex(ord(c))[2:]
    return '^^' + h

chars = 0
n = 0
while n < 256:
    out = escape(chr(n))
    chars += len(out)
    print('%d: "%s"' % (n, out.replace('"', '""')))
    n += 1
for line in open(sys.argv[1]).readlines():
    if line.startswith('*'): break
    length = int(line[:2])
    string = line[2:-1]
    out = ''.join(escape(c) for c in string)
    chars += len(out)
    print('%d: "%s"' % (n, out.replace('"', '""')))
    n += 1
print('(%d characters in all.)' % chars)

There’s a bit more that the real POOLtype program does, which we’ll see as we read it.

POOLtype

Finally, let’s look at POOLtype itself.

This program, like others written in WEB at the time TeX was written, was intended to be read on paper. (That’s just how Knuth programs.) It’s available in PDF form in any TeX distribution; e.g. you can see a PDF of the POOLtype program here.

To embed on this webpage, I’m instead going to treat each section separately.

There is a contents page:

The program is divided into 21 sections (followed by an index as section 22). The idea of literate programming with WEB is to write a program as a “web” of interconnected sections, such that the program can be understood by understanding the relationship between the sections and understanding each section in itself.

As we can see, these 21+1 sections are grouped into 4+1 parts:

The page numbers start at 102 because this (POOLtype) was part of a bigger “TeXware” report. (Actually though true, that’s not the reason; the starting page number of this contents page was somewhat arbitrarily chosen as 101; similarly the other programs in TeXware start at pages 201 (TFtoPL), 301 (PLtoTF) and 401 (DVItype) respectively.)

Anyway, let’s read each section in turn.

Part 1: Introduction

Every section of a WEB program consists of an optional (TeX) text part, followed by an optional (WEB macro) definitions part, followed by an optional (Pascal) code part. This one has only the text part.

How POOLtype could be useful when a program is being debugged: suppose you see that your program is trying to print string 456. Then you can use the output of POOLtype to see what that string is. (Of course, this helps only for the strings that are in the program itself, i.e. the ones the program loads from the pool file and starts up with – it’s no help for strings that were constructed later, at runtime.)

“It’s a pretty trivial routine” — it seems the words “program” and “routine” were used somewhat interchangeably; in some places DEK uses “the following program” to mean “the following bit of code”.

The “people may want to try transporting this program” refers to people who were (what we’d now call) sysadmins at other places / computer systems, installing TeX for their users — they’d have to adapt TeX with (hopefully minimal) “system-dependent changes” specific to their system, and the idea is that this program POOLtype is good practice for doing so.

Finally, note “routines copied from TeX82”. DEK is pretty free about copying code between different programs, where most of us programmers today would go to great lengths to avoid doing so.

Anyway, enough exegesis on three sentences; let’s move on to Section 2.

Here we see a text part followed by a code part.

I think “standard Pascal” here means the language as described in the “PASCAL User Manual and Report” (1975), by Kathleen Jensen and Niklaus Wirth, without any of the various extensions that had been added by various compilers in practice. (Being able to write anything substantial in only standard Pascal was a big deal; see Appendix.)

Note a few things here:

Moving on to section 3.

This section has text, followed by WEB macro definitions (no Pascal code). WEB has macros, with which DEK basically extends the programming language. Part of the popularity of C over Pascal must come from things like C having increment/decrement operators, and being able to write a semicolon to denote an empty statement. :-)

And with that, we’re already done with Part 1, introduction. The only code we saw so far was the outline in section 2 above.

Part 2: The character set

Code being copied verbatim between programs! The shock, the horror! It works out fine though.

Note it says “portable between a wide variety of computers”, where today we’d probably say something more specific, like “encodings”, or at most “operating systems”. The reason is that computers back then tended to come with their own operating systems, their own ecosystems of software, all expecting similar conventions for encoding, and so on.

And of course there was no Unicode then, and even ASCII was alien to some systems (that used EBCDIC for instance). Nevertheless, the approach of converting from the system’s characters to a uniform internal code immediately after input, and converting back just before output, is very much like the modern Unicode sandwich approach.

The internal code here includes standard ASCII, i.e. it has a few more characters (or in other words: what’s unprintable in ASCII is sometimes assigned here).

We’ll ignore the last two paragraphs for now as they’re more relevant to TeX than to POOLtype. Moving on to section 5.

This section fills out the “Types in the outer block” that was left unfilled in section 2 earlier. (Note the “This code is used in section 2” at the bottom, which sets the context for this section: I’ve found it useful when reading WEB programs to look for that first.) It turns out there’s only one global type declaration for this program. That defines the type ASCII_code to mean the range of integers 0..255. Note that a type in Pascal was more explicitly a set (or range) of values that could be stored in a variable, which (at least for integer types) is an improvement over the type system in C / C++ / Java / Python etc. In fact, it appears that in this respect, Pascal (and its descendant Ada) have the advantage over most programming languages.

Anyway, though it’s called ASCII_code, it’s really more like “TeX-internal character code” (the way Unicode is independent of encodings).

We get a glimpse of the lengths to which TeX went, to be portable across different Pascal implementations (and possibly some other languages too). The problem is this: suppose you read a character from a file. What is the name for the type of this character, in your programming language? It changed across Pascal implementations: in the one DEK was using, this type was called char and consisted of chr(0) to chr(255) (maybe not in 1982 though); in another, it may have had another name like extended_char or whatever, and consist of only chr(32) to chr(126). To make it easier for someone porting TeX, everywhere the program uses the type text_char, which is defined as char in the baseline TeX implementation but on another system may simply be redefined to whatever the name of the type is.

Note the usage of octal notation: '40 means 32, and '176 means 126. Better get used to reading octal if reading the TeX program; it comes up a lot.

The code part is not really related to the rest I guess… note that the “Local variables for initialization” are part of the procedure initialize, not of the “global” scope.

The code in this section replaces the “Globals in the outer block” that was left as a placeholder in section 2, but it’s only the first such section: as it says at the bottom, there are more globals, defined in sections 12, 13, and 18. With this mechanism, DEK solves one of the problems mentioned by Kernighan, that many variables need to be global and all of them have to be declared in one block, far away from where they are used.

Here the code defines two very important arrays:

This is like the “Unicode sandwich” approach in Python mentioned earlier, where we call decode(<encoding>) immediately after reading from a file (converting from the encoding-specific byte sequence to “universal” Unicode codepoints) and call encode(<encoding>) just before writing out (encoding-specific) byte sequences to a file.

Here’s an example with EBCDIC, in which ‘A’ is encoded as 193:

User's file on   --->   TeX reads               via xord
disk has 193             in 193            ---------------------->  65
                      (via Pascal runtime)      system-dependent   (type: ASCII_code = 0..65)
                      (type: text_char)

and conversely:

 65            via xchr
 ASCII_code  ----------------->   193                -----> Output byte 193 to disk
               system-dependent  type:text_char              (via Pascal runtime)

Note the Pascal feature, where array indices and values can be subrange types: for example on some system the type text_char might have been defined as range 32..126, and of course ASCII_code was defined in section 5 above as the range 0..255, so this is like saying:

xord: array[32..126] of 0..255;
xchr: array[0..255] of 32..126;

and such code is valid in Pascal.

You can go back and refer to section 2 to see how this section is used, to remind yourself that this section starts the body of the initialize procedure. (There are more things to be initialized; see the “see also sections 10, 11, and 14” below.)

There’s a trick here, to why this doesn’t need any system-dependent changes. For example, consider the assignment statement xchr['101] = 'A', or (using decimal instead of octal) xchr[65] = 'A'. Now, suppose this program was being compiled and run on a system that used EBCDIC encoding, where ‘A’ is encoded as byte 193. Then, this assignment statement would get interpreted and compiled as xchr[65] = 193. So that when TeX wants to translate from its internal code 65 (which always means ‘A’) to the system’s encoding (to print out ‘A’ on the terminal or write it to a file), it would look up xchr[65], find 193, and write out byte 193 to the file or terminal, and everything works as desired!

Changes are needed only if, for example, the character set (encoding) used does not contain, say |. Then the assignment xchr[124] = '|' should be changed to however it is proposed to print the character | (if it turns up) on that system.

(Actually, though DEK may not acknowledge it, the way this section is typeset already shows vestiges of the SAIL/SU-AI character set being different from present-day ASCII: whereas in ASCII the ' character is a straight single quote, in certain older character sets the same position was used for a slanted ’ character. That’s why we still see e.g. old GNU manuals and TeX code contain quotation like

``this''

which today just looks wrong. See Markus Kuhn’s page ASCII and Unicode quotation marks and the package upquote (doc) for more.)

These are decimal 0, 13 and 127 respectively, or hexadecimal 0, 0A, and 7F respectively, aka \0, \n and (say) \x7F or \0177.

Incidentally, null_code and carriage_return, though defined here as WEB macros, are not actually used in the program.

If some other character (0 to 31, or 127 to 255) gets into TeX, and is about to be printed, what should it be printed as? DEK thinks it’s safest to use a space by default (as these characters won’t get into TeX on “standard ASCII” systems), though any system may make its own choice. In fact, on his own local environment at Stanford at the time, there were characters like ≠ and the system-dependent changefile for that system made these changes.

Note we talking of TeX above, though this program we’re reading is actually POOLtype. Oops. Well, POOLtype must make the same decisions as TeX so that’s probably ok.

Having decided how TeX-internal characters will printed (i.e. having populated the xchr array), we now need to decide the inverse: what should characters from the text file be read as? We could write out a long list of assignments as in section 8 above, but we can also use the xchr array. The code starts with a conservative policy:

The order of these two loops ensures that if there are multiple internal character codes that get printed as the same external character (how likely is that?), then the order of preference is

128, 129, ..., 255, 0, 1, 2, ..., 32, 33, ..., 126

so that printable ASCII codes (32 to 126) are most preferred.

In other words:

Part 3: String handling

Finally, after half the program is done, we come to the main part of this program.

(Part of sequence: 7, 12, 13, 18.)

Global variables are always a pain to understand. By convention, single-letter variables are for unimportant things like loop indices or temporary variables.

One thing that helps understand these programs is think about what needs to be done, and how you would do it. In this case, we want to print "<count> characters in all" at the end, so we need to keep track of the count.

(Part of sequence: 8, 10, 11, 14.)

Note that with WEB, we’re able to keep the declaration and initialization close, even though they’re textually far apart in the actual program that Pascal sees.

Recall that section 2 only had an outline of the top of the program. Everything in it has now been filled (except some globals yet to be declared), and this is the rest. The “main” of the program is above, between begin and end, with certain placeholders that will be filled by sections 16 and 19. Note that initialize is a procedure call (and we’d probably write it on the line after begin). The macro abort is defined here (so that we can understand what the goto 9999 in it is referring to), but we’ll just keep it in mind for now; it’s used later.

The arithmetic in lc_hex (set l to the lowercase hex representation of a number from 0 to 15) is done by TANGLE; Pascal just sees, in place of lc_hex(n), the following:

l := n;
if l < 10 then l := l + 48 else l := l + 87

The code is reasonably straightforward, especially if you keep in mind what the goal is, but a few peculiarities (of Pascal and of WEB) are worth calling out:

The formatting is also a bit weird: blocks have begin and end at the same indent but within them the first statement after begin tends to start to the right of the begin, and in generation the statements are just lumped together on the line for no apparent reason. (This was considered desirable, it appears.)

One line of code, but a system-dependent one (and on certain systems could be less trivial). The text part documents the previous section, sort of. The default is that characters 32 to 126 are printable and the rest not, but these are the character codes that it says must be printable:

For fun, let’s verify the number 80:

def forced(k):
   return any(k in l for l in [range(33, 39), range(48, 58), range(97, 103), range(112, 122)])
tot = 0 
for i in range(64): 
   contrib = 2 if forced(i) and forced(i + 64) else 1 
   print('%d \t %s \t %d \t %s \t %s' % (i, forced(i), i + 64, forced(i + 64), contrib)) 
   tot += contrib 
print(tot)

prints 80.

Incidentally, though these sections are said to be copied verbatim from TeX, in section 49 there, there’s an additional character octal ‘136 (= decimal 94, character ^) that is required to be printable, and the count is given as 81. But changing our above code and running the program again, we still get 80 – that’s because it was already required that at least one of {30, 94} be printable, and forcing 94 to be printable doesn’t change the count. It appears this mistake has already been reported twice though (see this and reply).

These are the last globals to be declared. :-) BTW, another reason for so many global variables is that any variable needed by main, i.e. the begin .. end block that constitutes the main block of the program, needs to be global, I think. (Kernighan: “The result is that any variable that is to be initialized has global scope.”) So it would be useful to distinguish between variables that are global because they are needed by multiple procedures, and those that are global just because they are referenced by the main body of the function (what would in C be declared local to main).

And if it seems a bit strange to be explaining what the program does towards the end of the program: I’m afraid it’s not unusual for DEK, though in this case the explanation is simply changing “the INITEX program” in the TeX source to “the present program” here.

The only placeholder we haven’t filled so far. reset is the Pascal way of rewinding to the beginning of the file so it can be read from; basically fopen in this case.

Straightforward code, though it’s nice how the looping logic has been separated from a single iteration of the loop.

Note that though this is the last section with code, this is really the heart of the program.

Again does the obvious thing, except for this one which I’m not sure whether it’s trickery or normal for the time: what we’d write

l := (cm - '0') * 10 + (cn - '0')

is here written as (basically)

l := cm * 10 + cn - '0' * 11

with Pascal seeing actually

l := cm * 10 + cn - 48 * 11

as TANGLE translates "0" into 48.

Anyway, this is the end of the program, so I guess it doesn’t verify the hash code either. Oh well.

Part 4: System dependencies

If someone wants to add new sections to the program, best to add them at the end.

Part 5: Index

Apart from variables and macros, also topics and mentions of books/people are indexed, and “character set dependencies” and “system dependencies”. The underline means that the variable/macro was defined in that section.

When reading a WEB program, it is often necessary to flip back and forth to the index.

Index of module names.

Appendix

The output of tangle

This is the program as seen by Pascal (the pooltype.p file generated by TANGLE):

{2:}program POOLtype(poolfile,output);label 9999;
type{5:}ASCIIcode=0..255;{:5}var{7:}xord:array[char]of ASCIIcode;
xchr:array[ASCIIcode]of char;{:7}{12:}k,l:0..255;m,n:char;s:integer;
{:12}{13:}count:integer;{:13}{18:}poolfile:packed file of char;
xsum:boolean;{:18}procedure initialize;var{6:}i:integer;
{:6}begin{8:}xchr[32]:=' ';xchr[33]:='!';xchr[34]:='"';xchr[35]:='#';
xchr[36]:='$';xchr[37]:='%';xchr[38]:='&';xchr[39]:='''';xchr[40]:='(';
xchr[41]:=')';xchr[42]:='*';xchr[43]:='+';xchr[44]:=',';xchr[45]:='-';
xchr[46]:='.';xchr[47]:='/';xchr[48]:='0';xchr[49]:='1';xchr[50]:='2';
xchr[51]:='3';xchr[52]:='4';xchr[53]:='5';xchr[54]:='6';xchr[55]:='7';
xchr[56]:='8';xchr[57]:='9';xchr[58]:=':';xchr[59]:=';';xchr[60]:='<';
xchr[61]:='=';xchr[62]:='>';xchr[63]:='?';xchr[64]:='@';xchr[65]:='A';
xchr[66]:='B';xchr[67]:='C';xchr[68]:='D';xchr[69]:='E';xchr[70]:='F';
xchr[71]:='G';xchr[72]:='H';xchr[73]:='I';xchr[74]:='J';xchr[75]:='K';
xchr[76]:='L';xchr[77]:='M';xchr[78]:='N';xchr[79]:='O';xchr[80]:='P';
xchr[81]:='Q';xchr[82]:='R';xchr[83]:='S';xchr[84]:='T';xchr[85]:='U';
xchr[86]:='V';xchr[87]:='W';xchr[88]:='X';xchr[89]:='Y';xchr[90]:='Z';
xchr[91]:='[';xchr[92]:='\';xchr[93]:=']';xchr[94]:='^';xchr[95]:='_';
xchr[96]:='`';xchr[97]:='a';xchr[98]:='b';xchr[99]:='c';xchr[100]:='d';
xchr[101]:='e';xchr[102]:='f';xchr[103]:='g';xchr[104]:='h';
xchr[105]:='i';xchr[106]:='j';xchr[107]:='k';xchr[108]:='l';
xchr[109]:='m';xchr[110]:='n';xchr[111]:='o';xchr[112]:='p';
xchr[113]:='q';xchr[114]:='r';xchr[115]:='s';xchr[116]:='t';
xchr[117]:='u';xchr[118]:='v';xchr[119]:='w';xchr[120]:='x';
xchr[121]:='y';xchr[122]:='z';xchr[123]:='{';xchr[124]:='|';
xchr[125]:='}';xchr[126]:='~';{:8}{10:}for i:=0 to 31 do xchr[i]:=' ';
for i:=127 to 255 do xchr[i]:=' ';
{:10}{11:}for i:=0 to 255 do xord[chr(i)]:=127;
for i:=128 to 255 do xord[xchr[i]]:=i;
for i:=0 to 126 do xord[xchr[i]]:=i;{:11}{14:}count:=0;{:14}end;
{:2}{15:}begin initialize;
{16:}for k:=0 to 255 do begin write(k:3,': "');l:=k;
if({17:}(k<32)or(k>126){:17})then begin write(xchr[94],xchr[94]);
if k<64 then l:=k+64 else if k<128 then l:=k-64 else begin l:=k div 16;
if l<10 then l:=l+48 else l:=l+87;write(xchr[l]);l:=k mod 16;
if l<10 then l:=l+48 else l:=l+87;count:=count+1;end;count:=count+2;end;
if l=34 then write(xchr[l],xchr[l])else write(xchr[l]);count:=count+1;
writeln('"');end{:16};s:=256;{19:}reset(poolfile);xsum:=false;
if eof(poolfile)then begin writeln('! I can''t read the POOL file.');
goto 9999;end;repeat{20:}if eof(poolfile)then begin writeln(
'! POOL file contained no check sum');goto 9999;end;read(poolfile,m,n);
if m<>'*'then begin if(xord[m]<48)or(xord[m]>57)or(xord[n]<48)or(xord[n]
>57)then begin writeln('! POOL line doesn''t begin with two digits');
goto 9999;end;l:=xord[m]*10+xord[n]-48*11;write(s:3,': "');
count:=count+l;
for k:=1 to l do begin if eoln(poolfile)then begin writeln('"');
begin writeln('! That POOL line was too short');goto 9999;end;end;
read(poolfile,m);write(xchr[xord[m]]);
if xord[m]=34 then write(xchr[34]);end;writeln('"');s:=s+1;
end else xsum:=true;readln(poolfile){:20};until xsum;
if not eof(poolfile)then begin writeln(
'! There''s junk after the check sum');goto 9999;end{:19};
writeln('(',count:1,' characters in all.)');9999:end.{:15}

and this is the above with some formatting:

{2:}
program POOLtype(poolfile, output);
label 9999;
type
   {5:}
   ASCIIcode = 0..255;
   {:5}
var
   {7:}
   xord: array[char] of ASCIIcode;
   xchr: array[ASCIIcode] of char;
   {:7}
   {12:}
   k, l: 0..255;
   m, n: char;
   s: integer;
   {:12}
   {13:}
   count: integer;
   {:13}
   {18:}
   poolfile: packed file of char;
   xsum: boolean;
   {:18}
procedure initialize;
var
   {6:}
   i: integer;
   {:6}
begin
   {8:}
   xchr[32]:=' ';xchr[33]:='!';xchr[34]:='"';xchr[35]:='#';
   xchr[36]:='$';xchr[37]:='%';xchr[38]:='&';xchr[39]:='''';xchr[40]:='(';
   xchr[41]:=')';xchr[42]:='*';xchr[43]:='+';xchr[44]:=',';xchr[45]:='-';
   xchr[46]:='.';xchr[47]:='/';xchr[48]:='0';xchr[49]:='1';xchr[50]:='2';
   xchr[51]:='3';xchr[52]:='4';xchr[53]:='5';xchr[54]:='6';xchr[55]:='7';
   xchr[56]:='8';xchr[57]:='9';xchr[58]:=':';xchr[59]:=';';xchr[60]:='<';
   xchr[61]:='=';xchr[62]:='>';xchr[63]:='?';xchr[64]:='@';xchr[65]:='A';
   xchr[66]:='B';xchr[67]:='C';xchr[68]:='D';xchr[69]:='E';xchr[70]:='F';
   xchr[71]:='G';xchr[72]:='H';xchr[73]:='I';xchr[74]:='J';xchr[75]:='K';
   xchr[76]:='L';xchr[77]:='M';xchr[78]:='N';xchr[79]:='O';xchr[80]:='P';
   xchr[81]:='Q';xchr[82]:='R';xchr[83]:='S';xchr[84]:='T';xchr[85]:='U';
   xchr[86]:='V';xchr[87]:='W';xchr[88]:='X';xchr[89]:='Y';xchr[90]:='Z';
   xchr[91]:='[';xchr[92]:='\';xchr[93]:=']';xchr[94]:='^';xchr[95]:='_';
   xchr[96]:='`';xchr[97]:='a';xchr[98]:='b';xchr[99]:='c';xchr[100]:='d';
   xchr[101]:='e';xchr[102]:='f';xchr[103]:='g';xchr[104]:='h';
   xchr[105]:='i';xchr[106]:='j';xchr[107]:='k';xchr[108]:='l';
   xchr[109]:='m';xchr[110]:='n';xchr[111]:='o';xchr[112]:='p';
   xchr[113]:='q';xchr[114]:='r';xchr[115]:='s';xchr[116]:='t';
   xchr[117]:='u';xchr[118]:='v';xchr[119]:='w';xchr[120]:='x';
   xchr[121]:='y';xchr[122]:='z';xchr[123]:='{';xchr[124]:='|';
   xchr[125]:='}';xchr[126]:='~';
   {:8}
   {10:}
   for i:=0 to 31 do xchr[i] := ' ';
   for i:=127 to 255 do xchr[i] := ' ';
   {:10}
   {11:}
   for i:=0 to 255 do xord[chr(i)] := 127;
   for i:=128 to 255 do xord[xchr[i]] := i;
   for i:=0 to 126 do xord[xchr[i]] := i;
   {:11}
   {14:}
   count := 0;
   {:14}
end;
{:2}
{15:}
begin
   initialize;
   {16:}
   for k:=0 to 255 do
   begin
      write(k:3, ': "');
      l := k;
      if({17:}(k<32) or (k>126){:17}) then
      begin
         write(xchr[94], xchr[94]);
         if k<64 then
            l := k+64
         else if k<128 then
            l := k-64
         else
         begin
            l := k div 16;
            if l < 10 then l := l + 48 else l := l + 87;
            write(xchr[l]);
            l := k mod 16;
            if l < 10 then l := l + 48 else l := l + 87;
            count := count + 1;
         end;
         count := count + 2;
      end;
      if l = 34 then
         write(xchr[l], xchr[l])
      else
         write(xchr[l]);
      count := count + 1;
      writeln('"');
   end
   {:16};
   s := 256;
   {19:}
   reset(poolfile);
   xsum := false;
   if eof(poolfile) then
   begin
      writeln('! I can''t read the POOL file.');
      goto 9999;
   end;
   repeat
      {20:}
      if eof(poolfile) then
      begin
         writeln('! POOL file contained no check sum');
         goto 9999;
      end;
      read(poolfile, m, n);
      if m<>'*'then
      begin
         if(xord[m] < 48) or (xord[m] > 57) or (xord[n] < 48) or (xord[n] > 57) then
         begin
            writeln('! POOL line doesn''t begin with two digits');
            goto 9999;
         end;
         l := xord[m]*10 + xord[n] - 48*11;
         write(s:3, ': "');
         count := count + l;
         for k:=1 to l do
         begin
            if eoln(poolfile) then
            begin
               writeln('"');
               begin
                  writeln('! That POOL line was too short');
                  goto 9999;
               end;
            end;
            read(poolfile, m);
            write(xchr[xord[m]]);
            if xord[m] = 34 then
               write(xchr[34]);
         end;
         writeln('"');
         s := s+1;
      end
      else
         xsum := true;
      readln(poolfile)
      {:20};
   until xsum;
   if not eof(poolfile) then
   begin
      writeln('! There''s junk after the check sum');
      goto 9999;
   end
   {:19};
   writeln('(', count:1, ' characters in all.)');
9999:
end.
{:15}

– question: has WEB / literate programming helped?

Other problems with Pascal

There were other problems that Kernighan identified with standard Pascal as of 1981. Interestingly, Knuth must have run into many of these problems as well, but he came up with his “literate programming” (tangle + weave) pre-processing system as a way of solving them.

Overall, Kernighan concluded:

It is suitable only for small, self-contained programs that have only trivial interactions with their environment and that make no use of any software written by anyone else.

and Knuth (in his “WEB” language) wrote a big program with nontrivial interactions with the environment, but of course not using any software written by anyone else.

As an aside, consider Knuth’s comments (from a draft article surveying programming languages) written in 1966 but published in 2003 as Chapter 11 of Selected Papers on Computer Languages:

It is almost axiomatic that no programming language can ever be optimum. Hence every programming language has some features that can be made better. (Ad infinitum, possibly getting into a loop.) There also is ample data to support the hypothesis that no designer of programming languages will ever be pleased by all the features of a language that has been designed by somebody else. Thus it is wise for an author to avoid criticizing the existing programming languages too harshly.

It so happens (see 1, 2) that some of the diverging and nonstandard extensions to Pascal mentioned above have, over the passage of years, converged more-or-less, so what you may encounter under the name of “Pascal” today (2019) has fewer (though not none) of these problems. But in any case, that’s irrelevant to our story as of 1980.

The str_pool figure above

It was generated with the following program typed into Prof. Philip Guo’s fantastic pythontutor.com website:

char* str_start_pointers[5];
char str_pool[30];
int str_start[5];
int pool_ptr;
int str_ptr;

void add_string(char* s) {
  for (int i = 0; s[i] != '\0'; ++i) {
    str_pool[pool_ptr++] = s[i];
  }
  str_start[str_ptr] = pool_ptr;
  str_start_pointers[str_ptr] = &str_pool[pool_ptr];
  ++str_ptr;
}

int main() {
  str_start[str_ptr] = pool_ptr;
  str_start_pointers[str_ptr] = &str_pool[pool_ptr];
  ++str_ptr;
  add_string("zeroth");
  add_string("first");
  add_string("2nd");
  add_string("this is third");
}