AT&T Bell Laboratories Murray Hill, New Jersey 07974
Computing Science Technical Report No. 100
Why Pascal is Not My Favorite Programming Language
Brian W. Kernighan
April 2, 1981
Why Pascal is Not My Favorite Programming Language
Brian W. Kernighan
AT&T Bell Laboratories Murray Hill, New Jersey 07974
ABSTRACT
The programming language Pascal has become the dominant language of
instruction in computer science education. It has also strongly influenced
lan guages developed subsequently, in particular Ada.
Pascal was originally intended primarily as a teaching language, but it has
been more and more often recommended as a language for serious programming as
well, for example, for system programming tasks and even operating systems.
Pascal, at least in its standard form, is just plain not suitable for serious
pro gramming. This paper discusses my personal discovery of some of the
reasons why.
April 2, 1981
Why Pascal is Not My Favorite Programming Language
Brian W. Kernighan
AT&T Bell Laboratories Murray Hill, New Jersey 07974
1. Genesis
This paper has its origins in two events -- a spate of papers that compare C
and Pas cal 1, 2, 3, 4 and a personal attempt to rewrite Software Tools 5 in
Pascal.
Comparing C and Pascal is rather like comparing a Learjet to a Piper Cub --
one is meant for getting something done while the other is meant for learning
-- so such comparisons tend to be somewhat farfetched. But the revision of
Software Tools seems a more relevant comparison. The programs therein were
originally written in Ratfor, a ``structured'' dialect of Fortran imple mented
by a preprocessor. Since Ratfor is really Fortran in disguise, it has few of
the assets that Pascal brings -- data types more suited to character
processing, data structuring capabilities for better defining the organization
of one's data, and strong typing to enforce telling the truth about the data.
It turned out to be harder than I had expected to rewrite the programs in
Pascal. This paper is an attempt to distill out of the experience some
lessons about Pascal's suitability for program ming (as distinguished from
learning about programming). It is not a comparison of Pascal with C or
Ratfor.
The programs were first written in that dialect of Pascal supported by the
Pascal interpreter pi provided by the University of California at Berkeley.
The language is close to the nominal standard of Jensen and Wirth, 6 with good
diagnostics and careful run-time checking. Since then, the programs have also
been run, unchanged except for new libraries of primitives, on four other
systems: an interpreter from the Free University of Amsterdam (hereinafter
referred to as VU, for Vrije Universiteit), a VAX version of the Berkeley
system (a true compiler), a compiler purveyed by Whitesmiths, Ltd., and UCSD
Pascal on a Z80. All but the last of these Pascal systems are written in C.
Pascal is a much-discussed language. A recent bibliography 7 lists 175 items
under the heading of ``discussion, analysis and debate.'' The most often
cited papers (well worth reading) are a strong critique by Habermann 8 and an
equally strong rejoinder by Lecarme and Des jardins. 9 The paper by Boom and
DeJong 10 is also good reading. Wirth's own assessment of Pas cal is found in
[11]. I have no desire or ability to summarize the literature; this paper
represents my personal observations and most of it necessarily duplicates
points made by others. I have tried to organize the rest of the material
around the issues of
types and scope control flow environment cosmetics
and within each area more or less in decreasing order of significance.
To state my conclusions at the outset: Pascal may be an admirable language for
teaching beginners how to program; I have no first-hand experience with that.
It was a considerable achievement for 1968. It has certainly influenced the
design of recent languages, of which Ada is likely to be the most important.
But in its standard form (both current and proposed), Pascal is not adequate
for writing real programs. 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.
2. Types and Scopes
Pascal is (almost) a strongly typed language. Roughly speaking, that means
that each object in a program has a well-defined type which implicitly defines
the legal values of and operations on the object. The language guarantees
that it will prohibit illegal values and operations, by some mixture of
compile- and run-time checking. Of course compilers may not actually do all
the checking implied in the language definition. Furthermore, strong typing
is not to be confused with dimensional analysis. If one defines types apple
and orange with
type apple = integer; orange = integer;
then any arbitrary arithmetic expression involving apples and oranges is
perfectly legal.
Strong typing shows up in a variety of ways. For instance, arguments to
functions and pro cedures are checked for proper type matching. Gone is the
Fortran freedom to pass a floating point number into a subroutine that expects
an integer; this I deem a desirable attribute of Pascal, since it warns of a
construction that will certainly cause an error.
Integer variables may be declared to have an associated range of legal values,
and the com piler and run-time support ensure that one does not put large
integers into variables that only hold small ones. This too seems like a
service, although of course run-time checking does exact a penalty.
Let us move on to some problems of type and scope.
2.1. The size of an array is part of its type
If one declares
var arr10 : array [1..10] of integer; arr20 : array [1..20] of integer;
then arr10 and arr20 are arrays of 10 and 20 integers respectively. Suppose
we want to write a procedure sort to sort an integer array. Because arr10 and
arr20 have different types, it is not possible to write a single procedure
that will sort them both.
The place where this affects Software Tools particularly, and I think programs
in general, is that it makes it difficult indeed to create a library of
routines for doing common, general-purpose operations like sorting.
The particular data type most often affected is array of char, for in Pascal a
string is an array of characters. Consider writing a function index(s,c) that
will return the position in the string s where the character c first occurs,
or zero if it does not. The problem is how to handle the string argument of
index. The calls index('hello',c) and index('goodbye',c) can not both be
legal, since the strings have different lengths. (I pass over the question of
how the end of a constant string like 'hello' can be detected, because it
can't.)
The next try is
var temp : array [1..10] of char;
temp := 'hello';
n := index(temp,c);
but the assignment to temp is illegal because 'hello' and temp are of
different lengths.
The only escape from this infinite regress is to define a family of routines
with a member for each possible string size, or to make all strings (including
constant strings like 'define') of the same length.
The latter approach is the lesser of two great evils. In Tools, a type called
string is declared as
type string = array [1..MAXSTR] of char;
where the constant MAXSTR is ``big enough,'' and all strings in all programs
are exactly this size. This is far from ideal, although it made it possible to
get the programs running. It does not solve the problem of creating true
libraries of useful routines.
There are some situations where it is simply not acceptable to use the
fixed-size array repre sentation. For example, the Tools program to sort
lines of text operates by filling up memory with as many lines as will fit;
its running time depends strongly on how full the memory can be packed. Thus
for sort, another representation is used, a long array of characters and a set
of indices into this array:
type charbuf = array [1..MAXBUF] of char; charindex = array [1..MAXINDEX] of
0..MAXBUF;
But the procedures and functions written to process the fixed-length
representation cannot be used with the variable-length form; an entirely new
set of routines is needed to copy and com pare strings in this representation.
In Fortran or C the same functions could be used for both.
As suggested above, a constant string is written as
'this is a string'
and has the type packed array [1..n] of char, where n is the length. Thus
each string literal of different length has a different type. The only way to
write a routine that will print a message and clean up is to pad all messages
out to the same maximum length:
error('short message '); error('this is a somewhat longer message');
Many commercial Pascal compilers provide a string data type that explicitly
avoids the problem; string's are all taken to be the same type regardless of
size. This solves the problem for this single data type, but no other. It
also fails to solve secondary problems like computing the length of a constant
string; another built-in function is the usual solution.
Pascal enthusiasts often claim that to cope with the array-size problem one
merely has to copy some library routine and fill in the parameters for the
program at hand, but the defense sounds weak at best: 12
``Since the bounds of an array are part of its type (or, more exactly, of the
type of its indexes), it is impossible to define a procedure or function which
applies to arrays with differing bounds. Although this restriction may appear
to be a severe one, the experiences we have had with Pascal tend to show that
it tends to occur very infre quently. [...] However, the need to bind the
size of parametric arrays is a serious defect in connection with the use of
program libraries.''
This botch is the biggest single problem with Pascal. I believe that if it
could be fixed, the language would be an order of magnitude more usable. The
proposed ISO standard for Pascal 13
provides such a fix (``conformant array schemas''), but the acceptance of this
part of the standard is apparently still in doubt.
2.2. There are no static variables and no initialization
A static variable (often called an own variable in Algol-speaking countries)
is one that is pri vate to some routine and retains its value from one call of
the routine to the next. De facto, For tran variables are internal static,
except for COMMON;# in C there is a static declaration that can be applied to
local variables. __________________
# Strictly speaking, in Fortran 77 one must use SAVE to force the static
attribute.
Pascal has no such storage class. This means that if a Pascal function or
procedure intends to remember a value from one call to another, the variable
used must be external to the function or procedure. Thus it must be visible
to other procedures, and its name must be unique in the larger scope. A
simple example of the problem is a random number generator: the value used to
compute the current output must be saved to compute the next one, so it must
be stored in a vari able whose lifetime includes all calls of the random
number generator. In practice, this is typi cally the outermost block of the
program. Thus the declaration of such a variable is far removed from the
place where it is actually used.
One example comes from the text formatter described in Chapter 7 of Tools. The
variable dir controls the direction from which excess blanks are inserted
during line justification, to obtain left and right alternately. In Pascal,
the code looks like this:
program formatter (...);
var dir : 0..1; { direction to add extra spaces } . . . procedure justify
(...); begin dir := 1 - dir; { opposite direction from last time } ... end;
...
begin { main routine of formatter } dir := 0; ... end;
The declaration, initialization and use of the variable dir are scattered all
over the program, liter ally hundreds of lines apart. In C or Fortran, dir
can be made private to the only routine that needs to know about it:
... main() { ... }
...
justify() { static int dir = 0;
dir = 1 - dir; ... }
There are of course many other examples of the same problem on a larger scale;
functions for buffered I/O, storage management, and symbol tables all spring
to mind.
There are at least two related problems. Pascal provides no way to initialize
variables stati cally (i.e., at compile time); there is nothing analogous to
Fortran's DATA statement or initializers like
int dir = 0;
in C. This means that a Pascal program must contain explicit assignment
statements to initialize variables (like the
dir := 0;
above). This code makes the program source text bigger, and the program
itself bigger at run time.
Furthermore, the lack of initializers exacerbates the problem of too-large
scope caused by the lack of a static storage class. The time to initialize
things is at the beginning, so either the main routine itself begins with a
lot of initialization code, or it calls one or more routines to do the
initializations. In either case, variables to be initialized must be visible,
which means in effect at the highest level of the hierarchy. The result is
that any variable that is to be initialized has glo bal scope.
The third difficulty is that there is no way for two routines to share a
variable unless it is declared at or above their least common ancestor.
Fortran COMMON and C's external static stor age class both provide a way for
two routines to cooperate privately, without sharing informa tion with their
ancestors.
The new standard does not offer static variables, initialization or
non-hierarchical commu nication.
2.3. Related program components must be kept separate
Since the original Pascal was implemented with a one-pass compiler, the
language believes strongly in declaration before use. In particular,
procedures and functions must be declared (body and all) before they are used.
The result is that a typical Pascal program reads from the bottom up -- all
the procedures and functions are displayed before any of the code that calls
them, at all levels. This is essentially opposite to the order in which the
functions are designed and used.
To some extent this can be mitigated by a mechanism like the #include facility
of C and Ratfor: source files can be included where needed without cluttering
up the program. #include is not part of standard Pascal, although the UCB, VU
and Whitesmiths compilers all provide it.
There is also a forward declaration in Pascal that permits separating the
declaration of the function or procedure header from the body; it is intended
for defining mutually recursive proce dures. When the body is declared later
on, the header on that declaration may contain only the function name, and
must not repeat the information from the first instance.
A related problem is that Pascal has a strict order in which it is willing to
accept declara tions. Each procedure or function consists of
label label declarations, if any const constant declarations, if any type type
declarations, if any var variable declarations, if any procedure and function
declarations, if any begin body of function or procedure end
This means that all declarations of one kind (types, for instance) must be
grouped together for the convenience of the compiler, even when the programmer
would like to keep together things that are logically related so as to
understand the program better. Since a program has to be presented to the
compiler all at once, it is rarely possible to keep the declaration,
initialization and use of types and variables close together. Even some of
the most dedicated Pascal supporters agree: 14
``The inability to make such groupings in structuring large programs is one of
Pascal's most frustrating limitations.''
A file inclusion facility helps only a little here.
The new standard does not relax the requirements on the order of declarations.
2.4. There is no separate compilation
The ``official'' Pascal language does not provide separate compilation, and so
each imple mentation decides on its own what to do. Some (the Berkeley
interpreter, for instance) disallow it entirely; this is closest to the spirit
of the language and matches the letter exactly. Many others provide a
declaration that specifies that the body of a function is externally defined.
In any case, all such mechanisms are non-standard, and thus done differently
by different systems.
Theoretically, there is no need for separate compilation -- if one's compiler
is very fast (and if the source for all routines is always available and if
one's compiler has a file inclusion facility so that multiple copies of source
are not needed), recompiling everything is equivalent. In practice, of
course, compilers are never fast enough and source is often hidden and file
inclusion is not part of the language, so changes are time-consuming.
Some systems permit separate compilation but do not validate consistency of
types across the boundary. This creates a giant hole in the strong typing.
(Most other languages do no cross compilation checking either, so Pascal is
not inferior in this respect.) I have seen at least one paper (mercifully
unpublished) that on page n castigates C for failing to check types across
sepa rate compilation boundaries while suggesting on page n+1 that the way to
cope with Pascal is to compile procedures separately to avoid type checking.
The new standard does not offer separate compilation.
2.5. Some miscellaneous problems of type and scope
Most of the following points are minor irritations, but I have to stick them
in somewhere.
It is not legal to name a non-basic type as the literal formal parameter of a
procedure; the following is not allowed:
procedure add10 (var a : array [1..10] of integer);
Rather, one must invent a type name, make a type declaration, and declare the
formal parameter to be an instance of that type:
type a10 = array [1..10] of integer; ... procedure add10 (var a : a10);
Naturally the type declaration is physically separated from the procedure that
uses it. The disci pline of inventing type names is helpful for types that
are used often, but it is a distraction for things used only once.
It is nice to have the declaration var for formal parameters of functions and
procedures; the procedure clearly states that it intends to modify the
argument. But the calling program has no way to declare that a variable is to
be modified -- the information is only in one place, while two places would be
better. (Half a loaf is better than none, though -- Fortran tells the user
nothing about who will do what to variables.)
It is also a minor bother that arrays are passed by value by default -- the
net effect is that every array parameter is declared var by the programmer
more or less without thinking. If the var declaration is inadvertently
omitted, the resulting bug is subtle.
Pascal's set construct seems like a good idea, providing notational
convenience and some free type checking. For example, a set of tests like
if (c = blank) or (c = tab) or (c = newline) then ...
can be written rather more clearly and perhaps more efficiently as
if c in [blank, tab, newline] then ...
But in practice, set types are not useful for much more than this, because the
size of a set is strongly implementation dependent (probably because it was so
in the original CDC implementa tion: 59 bits). For example, it is natural to
attempt to write the function isalphanum(c) (``is c alphanumeric?'') as
{ isalphanum(c) -- true if c is letter or digit } function isalphanum (c :
char) : boolean; begin isalphanum := c in ['a'..'z', 'A'..'Z', '0'..'9'] end;
But in many implementations of Pascal (including the original) this code fails
because sets are just too small. Accordingly, sets are generally best left
unused if one intends to write portable pro grams. (This specific routine
also runs an order of magnitude slower with sets than with a range test or
array reference.)
2.6. There is no escape
There is no way to override the type mechanism when necessary, nothing
analogous to the ``cast'' mechanism in C. This means that it is not possible
to write programs like storage alloca tors or I/O systems in Pascal, because
there is no way to talk about the type of object that they return, and no way
to force such objects into an arbitrary type for another use. (Strictly
speaking, there is a large hole in the type-checking near variant records,
through which some otherwise illegal type mismatches can be obtained.)
3. Control Flow
The control flow deficiencies of Pascal are minor but numerous -- the death of
a thousand cuts, rather than a single blow to a vital spot.
There is no guaranteed order of evaluation of the logical operators and and or
-- nothing like && and || in C. This failing, which is shared with most other
languages, hurts most often in loop control:
while (i <= XMAX) and (x[i] > 0) do ...
is extremely unwise Pascal usage, since there is no way to ensure that i is
tested before x[i] is.
By the way, the parentheses in this code are mandatory -- the language has
only four levels of operator precedence, with relationals at the bottom.
There is no break statement for exiting loops. This is consistent with the
one entry-one exit philosophy espoused by proponents of structured
programming, but it does lead to nasty cir cumlocutions or duplicated code,
particularly when coupled with the inability to control the order in which
logical expressions are evaluated. Consider this common situation, expressed
in C or Ratfor:
while (getnext(...)) { if (something) break rest of loop }
With no break statement, the first attempt in Pascal is
done := false; while (not done) and (getnext(...)) do if something then done
:= true else begin rest of loop end
But this doesn't work, because there is no way to force the ``not done'' to be
evaluated before the next call of getnext. This leads, after several false
starts, to
done := false; while not done do begin done := getnext(...); if something then
done := true else if not done then begin rest of loop end end
Of course recidivists can use a goto and a label (numeric only and it has to
be declared) to exit a loop. Otherwise, early exits are a pain, almost always
requiring the invention of a boolean vari able and a certain amount of
cunning. Compare finding the last non-blank in an array in Ratfor:
for (i = max; i > 0; i = i - 1) if (arr(i) != ' ') break
with Pascal:
done := false; i := max; while (i > 0) and (not done) do if arr[i] = ' ' then
i := i - 1 else done := true;
The index of a for loop is undefined outside the loop, so it is not possible
to figure out whether one went to the end or not. The increment of a for loop
can only be +1 or -1, a minor restriction.
There is no return statement, again for one in-one out reasons. A function
value is returned by setting the value of a pseudo-variable (as in Fortran),
then falling off the end of the function. This sometimes leads to contortions
to make sure that all paths actually get to the end of the function with the
proper value. There is also no standard way to terminate execution except by
reaching the end of the outermost block, although many implementations provide
a halt that causes immediate termination.
The case statement is better designed than in C, except that there is no
default clause and the behavior is undefined if the input expression does not
match any of the cases. This crucial omission renders the case construct
almost worthless. In over 6000 lines of Pascal in Software Tools in Pascal, I
used it only four times, although if there had been a default, a case would
have served in at least a dozen places.
The new standard offers no relief on any of these points.
4. The Environment
The Pascal run-time environment is relatively sparse, and there is no
extension mechanism except perhaps source-level libraries in the ``official''
language.
Pascal's built-in I/O has a deservedly bad reputation. It believes strongly
in record oriented input and output. It also has a look-ahead convention that
is hard to implement prop erly in an interactive environment. Basically, the
problem is that the I/O system believes that it must read one record ahead of
the record that is being processed. In an interactive system, this means that
when a program is started, its first operation is to try to read the terminal
for the first line of input, before any of the program itself has been
executed. But in the program
write('Please enter your name: '); read(name); ...
read-ahead causes the program to hang, waiting for input before printing the
prompt that asks for it.
It is possible to escape most of the evil effects of this I/O design by very
careful implemen tation, but not all Pascal systems do so, and in any case it
is relatively costly.
The I/O design reflects the original operating system upon which Pascal was
designed; even Wirth acknowledges that bias, though not its defects. 15 It is
assumed that text files consist of records, that is, lines of text. When the
last character of a line is read, the built-in function eoln becomes true; at
that point, one must call readln to initiate reading a new line and reset
eoln. Similarly, when the last character of the file is read, the built-in eof
becomes true. In both cases, eoln and eof must be tested before each read
rather than after.
Given this, considerable pains must be taken to simulate sensible input. This
implementa tion of getc works for Berkeley and VU I/O systems, but may not
necessarily work for anything else:
{ getc -- read character from standard input } function getc (var c :
character) : character; var ch : char; begin if eof then c := ENDFILE else if
eoln then begin readln; c := NEWLINE end else begin read(ch); c := ord(ch)
end; getc := c end;
The type character is not the same as char, since ENDFILE and perhaps NEWLINE
are not legal values for a char variable.
There is no notion at all of access to a file system except for predefined
files named by (in effect) logical unit number in the program statement that
begins each program. This apparently reflects the CDC batch system in which
Pascal was originally developed. A file variable
var fv : file of type
is a very special kind of object -- it cannot be assigned to, nor used except
by calls to built-in pro cedures like eof, eoln, read, write, reset and
rewrite. (reset rewinds a file and makes it ready for re-reading; rewrite
makes a file ready for writing.)
Most implementations of Pascal provide an escape hatch to allow access to
files by name from the outside environment, but not conveniently and not
standardly. For example, many sys tems permit a filename argument in calls to
reset and rewrite:
reset(fv, filename);
But reset and rewrite are procedures, not functions -- there is no status
return and no way to regain control if for some reason the attempted access
fails. (UCSD provides a compile-time flag that disables the normal abort.)
And since fv's cannot appear in expressions like
reset(fv, filename); if fv = failure then ...
there is no escape in that direction either. This straitjacket makes it
essentially impossible to write programs that recover from mis-spelled file
names, etc. I never solved it adequately in the Tools revision.
There is no notion of access to command-line arguments, again probably
reflecting Pascal's batch-processing origins. Local routines may allow it by
adding non-standard procedures to the environment.
Since it is not possible to write a general-purpose storage allocator in
Pascal (there being no way to talk about the types that such a function would
return), the language has a built-in proce dure called new that allocates
space from a heap. Only defined types may be allocated, so it is not possible
to allocate, for example, arrays of arbitrary size to hold character strings.
The point ers returned by new may be passed around but not manipulated: there
is no pointer arithmetic. There is no way to regain control if storage runs
out.
The new standard offers no change in any of these areas.
5. Cosmetic Issues
Most of these issues are irksome to an experienced programmer, and some are
probably a nuisance even to beginners. All can be lived with.
Pascal, in common with most other Algol-inspired languages, uses the semicolon
as a state ment separator rather than a terminator (as it is in PL/I and C).
As a result one must have a rea sonably sophisticated notion of what a
statement is to put semicolons in properly. Perhaps more important, if one is
serious about using them in the proper places, a fair amount of nuisance edit
ing is needed. Consider the first cut at a program:
if a then b; c;
But if something must be inserted before b, it no longer needs a semicolon,
because it now pre cedes an end:
if a then begin b0; b end; c;
Now if we add an else, we must remove the semicolon on the end:
if a then begin b0; b end else d; c;
And so on and so on, with semicolons rippling up and down the program as it
evolves.
One generally accepted experimental result in programmer psychology is that
semicolon as separator is about ten times more prone to error than semicolon
as terminator. 16 (In Ada, 17 the most significant language based on Pascal,
semicolon is a terminator.) Fortunately, in Pascal one can almost always
close one's eyes and get away with a semicolon as a terminator. The excep
tions are in places like declarations, where the separator vs. terminator
problem doesn't seem as serious anyway, and just before else, which is easy to
remember.
C and Ratfor programmers find begin and end bulky compared to { and }.
A function name by itself is a call of that function; there is no way to
distinguish such a function call from a simple variable except by knowing the
names of the functions. Pascal uses the Fortran trick of having the function
name act like a variable within the function, except that where in Fortran the
function name really is a variable, and can appear in expressions, in Pascal,
its appearance in an expression is a recursive invocation: if f is a
zero-argument function, f:=f+1 is a recursive call of f.
There is a paucity of operators (probably related to the paucity of precedence
levels). In particular, there are no bit-manipulation operators (AND, OR,
XOR, etc.). I simply gave up trying to write the following trivial encryption
program in Pascal:
i := 1; while getc(c) <> ENDFILE do begin putc(xor(c, key[i])); i := i mod
keylen + 1 end
because I couldn't write a sensible xor function. The set types help a bit
here (so to speak), but not enough; people who claim that Pascal is a system
programming language have generally overlooked this point. For example, [18,
p. 685]
``Pascal is at the present time [1977] the best language in the public domain
for pur poses of system programming and software implementation.''
seems a bit naive.
There is no null string, perhaps because Pascal uses the doubled quote
notation to indicate a quote embedded in a string:
'This is a '' character'
There is no way to put non-graphic symbols into strings. In fact, non-graphic
characters are unpersons in a stronger sense, since they are not mentioned in
any part of the standard language. Concepts like newlines, tabs, and so on are
handled on each system in an ad hoc manner, usually by knowing something about
the character set (e.g., ASCII newline has decimal value 10).
There is no macro processor. The const mechanism for defining manifest
constants takes care of about 95 percent of the uses of simple #define
statements in C, but more involved ones are hopeless. It is certainly
possible to put a macro preprocessor on a Pascal compiler. This allowed me to
simulate a sensible error procedure as
#define error(s) begin writeln(s); halt end
(halt in turn might be defined as a branch to the end of the outermost block.)
Then calls like
error('little string'); error('much bigger string');
work since writeln (as part of the standard Pascal environment) can handle
strings of any size. It is unfortunate that there is no way to make this
convenience available to routines in general.
The language prohibits expressions in declarations, so it is not possible to
write things like
const SIZE = 10; type arr = array [1..SIZE+1] of integer;
or even simpler ones like
const SIZE = 10; SIZE1 = SIZE + 1;
6. Perspective
The effort to rewrite the programs in Software Tools started in March, 1980,
and, in fits and starts, lasted until January, 1981. The final product 19 was
published in June, 1981. During that time I gradually adapted to most of the
superficial problems with Pascal (cosmetics, the inade quacies of control
flow), and developed imperfect solutions to the significant ones (array sizes,
run-time environment).
The programs in the book are meant to be complete, well-engineered programs
that do non-trivial tasks. But they do not have to be efficient, nor are
their interactions with the operat ing system very complicated, so I was able
to get by with some pretty kludgy solutions, ones that simply wouldn't work
for real programs.
There is no significant way in which I found Pascal superior to C, but there
are several places where it is a clear improvement over Ratfor. Most obvious
by far is recursion: several pro grams are much cleaner when written
recursively, notably the pattern-search, quicksort, and expression evaluation.
Enumeration data types are a good idea. They simultaneously delimit the range
of legal values and document them. Records help to group related variables.
I found relatively little use for pointers.
Boolean variables are nicer than integers for Boolean conditions; the original
Ratfor pro grams contained some unnatural constructions because Fortran's
logical variables are badly designed.
Occasionally Pascal's type checking would warn of a slip of the hand in
writing a program; the run-time checking of values also indicated errors from
time to time, particularly subscript range violations.
Turning to the negative side, recompiling a large program from scratch to
change a single line of source is extremely tiresome; separate compilation,
with or without type checking, is mandatory for large programs.
I derived little benefit from the fact that characters are part of Pascal and
not part of For tran, because the Pascal treatment of strings and non-graphics
is so inadequate. In both lan guages, it is appallingly clumsy to initialize
literal strings for tables of keywords, error messages, and the like.
The finished programs are in general about the same number of source lines as
their Ratfor equivalents. At first this surprised me, since my preconception
was that Pascal is a wordier and less expressive language. The real reason
seems to be that Pascal permits arbitrary expressions in places like loop
limits and subscripts where Fortran (that is, portable Fortran 66) does not,
so some useless assignments can be eliminated; furthermore, the Ratfor
programs declare functions while Pascal ones do not.
To close, let me summarize the main points in the case against Pascal.
1. Since the size of an array is part of its type, it is not possible to
write general-purpose rou tines, that is, to deal with arrays of different
sizes. In particular, string handling is very dif ficult.
2. The lack of static variables, initialization and a way to communicate
non-hierarchically combine to destroy the ``locality'' of a program --
variables require much more scope than they ought to.
3. The one-pass nature of the language forces procedures and functions to be
presented in an unnatural order; the enforced separation of various
declarations scatters program compo nents that logically belong together.
4. The lack of separate compilation impedes the development of large programs
and makes the use of libraries impossible.
5. The order of logical expression evaluation cannot be controlled, which
leads to convoluted code and extraneous variables.
6. The case statement is emasculated because there is no default clause.
7. The standard I/O is defective. There is no sensible provision for dealing
with files or pro gram arguments as part of the standard language, and no
extension mechanism.
8. The language lacks most of the tools needed for assembling large programs,
most notably file inclusion.
9. There is no escape.
This last point is perhaps the most important. The language is inadequate but
circum scribed, because there is no way to escape its limitations. There are
no casts to disable the type checking when necessary. There is no way to
replace the defective run-time environment with a sensible one, unless one
controls the compiler that defines the ``standard procedures.'' The lan guage
is closed.
People who use Pascal for serious programming fall into a fatal trap. Because
the language is so impotent, it must be extended. But each group extends
Pascal in its own direction, to make it look like whatever language they
really want. Extensions for separate compilation, Fortran like COMMON, string
data types, internal static variables, initialization, octal numbers, bit
opera tors, etc., all add to the utility of the language for one group, but
destroy its portability to others.
I feel that it is a mistake to use Pascal for anything much beyond its
original target. In its pure form, Pascal is a toy language, suitable for
teaching but not for real programming.
Acknowledgments
I am grateful to Al Aho, Al Feuer, Narain Gehani, Bob Martin, Doug McIlroy,
Rob Pike, Dennis Ritchie, Chris Van Wyk and Charles Wetherell for helpful
criticisms of earlier versions of this paper.
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