TEACH BRACKETS                                           A.Sloman Feb 82

=== BRACKETS IN POP11 ==================================================

POP11 makes use of several different  kinds of brackets to indicate  the
beginnings, and ends,  and sometimes  middles, of bits  of program.  For
instance here are some opening and closing brackets, arranged as matched
pairs:

        (       )       (see HELP * ROUNDBRA)
        [       ]       (see HELP * SQUAREBRA)
        {       }       (see HELP * SQUAREBRA)

If you are using a terminal with upper case only, some of these will not
print properly.

If you don't balance brackets properly, POP11 complains. E.g. Try typing
the following unmatched pair of brackets to POP11:

        (       ]

You'll get a mishap  message: POP11 complains  about a MISPLACED  SYNTAX
WORD. Similarly if you give it a closing bracket without an opener, e.g.

        ) or
        }

Try the following too:

        [       }
        {       ]

Here the opening brackets indicate that a structure is to be built,  ie.
a list (indicated by  "[") or a  vector indicated by  "{". If the  wrong
sort of closing bracket is found you'll get a 'MISSING CLOSING  BRACKET'
error.

Compare

    [a list with {a vector} in it] =>
    {a vector with [a list] in it } =>

The situation is further complicated by the fact that the list  brackets
[ ..... ] can enclose words, including some of the other brackets,  e.g.
"(" and ")". So the following will not give an error:

    [ ) a list of seven items ( ] =>

Though this will:

    ( ] not legal POP11 [ ) =>

Try that.

If you want  to have  a list  or vector bracket  included in  a list  or
vector, you'll have to 'unquote'  it with '% ....  %' and then quote  it
with "..", e.g.

    [a bracket % "}" % and another % "[" % ]

which creates a list of six elements.

Besides the 'obvious'  brackets, there are  several unobvious ones.  For
instance, the word "define" is a sort of opening bracket. That is,  when
POP11 reads it  in, it has  definite expectations about  what is  coming
next, namely a procedure heading, terminated by a semi-colon. e.g.

    define double(x) -> result;

But if you leave out the "(", you'll get a complaint. Try:

    define double x)

This is not a case of unexpected closing bracket. Rather, POP11  expects
a list of  'input variables'  to follow the  procedure name,  and if  it
meets ")" without its opener, it  gets confused. If everything is OK  up
to  the  semi-colon,  then  POP11,  expects  the  closing  bracket  of a
procedure definition, and if it  first meets another closer, before  its
opener, then there's trouble. E.g. try

    define double(x);
        ]

Compare

    define double(x);
    enddefine;

    define double(x);
    endif

ENDIF is  the final  closing bracket  for IF,  just as  ENDUNTIL is  the
closing bracket for  UNTIL, ENDREPEAT  the closing  bracket for  REPEAT,
etc.

POP11 doesn't like to  find 'ENDIF' when  it's looking for  'ENDDEFINE'.
Similarly, it  doesn't  like to  find  ENDDEFINE when  its  looking  for
something else. Try the following:

    if enddefine
    if )

The first 'closing bracket' expected by IF is 'THEN'. It doesn't like
finding a different closing bracket, or even a semi-colon before then
try:

    if ;
    if 3 > 2 ;

After IF  (and several  other 'openers',  like WHILE  and UNTIL),  POP11
treats ";" as an  unacceptable closing bracket.  After THEN, things  are
different. What  do  you  think  will be  'expected'  in  the  following
mishaps:

    if then )
    if then enddefine

What about

    if then else enddefine
    if then elseif ;

Getting used to all  these different cases  where mishaps occur  takes a
little time. If you have a program in a file and you try to load it  the
mishap message will include a line  number telling you how far the  file
had been read  in before the  mishap occurred. If  your mishap  involves
unmatched brackets, the actual cause of  the mishap could be a long  way
further back in the  file. Try putting the  following in a file,  making
sure you put in an exact copy:

    define silly(x);
        if islist(x) then
            length(([this is a long and silly list of words])
        endif
    enddefine;

Note the unmatched "(".

When you load the  file, you'll get  a line number  in the mishap.  This
does not  correspond to  the actual  line  in which  the extra  '('  was
inserted, but to the last line  read in before the mishap was  detected.
The 'doing'  line of  the mishap  message also  helps you  to see  which
'opening brackets' had not  yet met their  final closing brackets,  e.g.
'DEFINE', 'IF', and '('

The looping brackets 'UNTIL'  and 'WHILE' are a  bit like 'IF', in  that
you have a middle bracket 'DO', like 'THEN', and a final closing bracket
e.g. 'ENDUNTIL'. At present the  POP11 system accepts 'THEN' instead  of
'DO', and 'ENDDO' instead of 'ENDUNTIL'.  For more on POP11 syntax,  see
TEACH * POPSUMMARY

UNLESS is much like IF,  except that it does  not permit ELSE or  ELSEIF
clauses. Try:

    unless x > 3 then
        x=>
    else
        3 =>
    endunless;

--- C.all/teach/brackets -----------------------------------------------
--- Copyright University of Sussex 1988. All rights reserved. ----------