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TEACH LISTSUMMARY                      Aaron Sloman, updated Oct 1986


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-- CREATING LISTS -----------------------------------------------------

>>>> [     ]

These "list constant brackets" may contain text items, i.e  words,
numbers and strings. E.g.:

       [ a b c d]            is a list of four words
       [1 cat 2 dog 3 pig ]  is a list of six items.
       [string 'a short string' 66]
          is a list with a word a string and a number.

[]  is the empty list. Can also be called NIL.

List brackets may also contain lists,  E.g.:

       [ [1 2]  [3] 4 ] is a list with two lists and one number.

It contains exactly three elements, of which the first contains two.

>>>>  [%    %]

These "decorated list brackets" are used to create lists whose contents
depend on the values of variables or the results of executing
procedures. For instance:

       [% 3, cat,  hd(l)  %]

is a list containing the number 3, the value of the variable CAT and the
first element of L, which must be a list, whereas

       [ 3, cat, hd(l) ]

contains the number 3, the comma, the word "CAT", a comma, the word
"HD", the word "(", etc.

        [% [% a %], [% b, c%], [% d %] %]

is a list of three lists, whose contents depend on the values of A B C
and D.

>>>>  ^   (the "up-arrow" or "hat" symbol)

Can be used in place of "%". For example

       [ ^x + ^y is ^(x + y) ]         is the same as
       [% x, "+", y, "is", x + y %]    and
       [ %x% + %y% is %x + y% ]

When followed by a variable, "^" means: "use the value of this
variable". When followed by a parenthesised expression, it means:

   "use the value of the parenthesised expression".

>>>>  ^^  (double "up-arrow" or "hat" symbol)

This is used to merge the contents of a list into an enclosing list.

       [ b c d ] -> list;
       [a ^list e ] =>
       ** [a [b c d] e]

       [a ^^list e ] =>
       ** [a b c d e]

The prefix "^^" can be thought of as "remove the list brackets".

>>>>  ::

This is an operation for joining an element onto an existing list, to
make a new list, e.g.

       3 :: [4 5 6]     is  a list of four numbers.
       "cat" :: []      is  the same as [cat]
       "cat" :: "dog"   will cause an error,

since :: must have a list as its second argument. Notice that

       x :: y

is equivalent to

       [^x ^^y]

The operation :: is easier for the machine, but otherwise has no


This is a procedure, which can be thought of as equivalent to :: .
That is

is the same as
        x :: l

The only difference is that the second argument of CONSPAIR need not be
a list. E.g.

       conspair(3, 4) -> l;
       l =>
       ** [3|4]    ;;; note the use of the vertical bar - indicating that
                   ;;; the second element is not a list
       tl(l) =>

This now produces an error, because TL insists on a proper list, and a
pair whose second element is not a pair is not acceptable. The procedure
BACK has no such qualms.

       back(l) =>
       ** 4

CONSPAIR is rarely needed in straightforward programs.

>>>>  <>

This is an operator which takes two lists and "concatenates" them to
produce a new list. The first list is copied, so that the original does
not have any extra elements added to its end. E.g.

       [cat dog ] <> [pig mouse]
is the same as:
       [cat dog pig mouse]

       lista <> listb

means: make a new list containing the elements of LISTA and the elements

       [^x] <> l -> l    is the same in effect as
       x :: l -> l;
       [^x ^^l] -> l;

But the former is much less efficient, since <> copies the first list.
Notice that when X and Y are lists:

       x <> y

   is equivalent to

       [^^x ^^y]

The former has no advantages except that it is easier for the machine.

The operator <> can also be used to join two words, two vectors, two

>>>>  REV

This is a procedure which takes a list as argument and creates a new one
containing the same elements in reverse order. E.g.

       rev([a b c])   is   [c b a]


This is a procedure which takes a list and a procedure and applies the
procedure to every element of the list, finally creating a new list
containing all the results. E.g.

       maplist(l, identfn)               makes a copy of the list l.

       maplist([cat dog 2 mouse 3 4 ], isword) =>
       ** [<true> <true> <false> <true> <false> <false> ]

It could have been defined thus, were it not in the system:

       define maplist (l,f);
         vars x;
         [% for x in l do f(x) endfor%]

Compare the procedure APPLIST.


This procedure can be applied to a word, string, or vector. It will
create a list containing all the elements of the word string or vector.
The elements of a word or string will be the numbers representing the
characters in it. (The code for lower case A is 97, for B 98, for Z
122.) So:

       datalist('abz') and datalist("abz")
          both give the list [97 98 122]

       maplist([cat dog mouse], datalist) =>
       ** [ [99 97 116] [100 111 103] [109 111 117 115 101] ]

This produces a list of lists of character codes. (See MAPLIST, above.)

NOTE: strictly, the above codes refer to the LOWER CASE characters.

-- ACCESSING COMPONENTS OF LISTS --------------------------------------

>>>>  HD

This procedure takes a list as argument and returns the first element of
the list. HD(LIST) is equivalent to LIST(1). (See below).

>>>>  TL

This procedure takes a list as argument and returns as its result the
sublist containing all but the first element. E.g. if L is a list then:

       hd(tl(l))            is the second element
       hd(tl(tl(l)))        is the third element = l(3)

>>>>  LAST

This library procedure, when applied to a list returns its last element:

       last([a b c d]) =>
       ** d

Notice that:


is equivalent to:


>>>>  Numerical subscripting of lists

In POP11 lists can be treated as procedures which take a number N, to
get the N'th element of a list. E.g.

       vars l;
       [a b c d] -> l;

       l(2) =>
       ** b

       l(3) =>
       ** c

The N'th element can also be altered by an assignment. Thus to replace
the third element with the list [THIRD ELEMENT], do:

       [third element] -> l(3);
       l =>
       ** [a b [third element] d]
       l(3) =>
       ** [third element]

-- MISCELLANEOUS PROCEDURES -------------------------------------------

>>>>  LENGTH

This procedure takes a list and counts the elements, returning a number
as its result. E.g

       length([ 99 a [a b c] d])  is 4
       length([])                 is 0

>>>>  ATOM

This is a type-testing procedure, which returns TRUE or FALSE as its
result. IF it is applied to a non-empty list (i.e. a list other than
[]), its result is FALSE. If it is applied to anything else, e.g. a
word, a procedure, a string, or [], the result is TRUE. So:

       atom([])        is true
       atom(99)        is true
       atom("cat")     is true
       atom(hd)        is true
       atom([cat dog]) is false
       atom(3 :: [])   is false


This procedure takes a list and a procedure and applies the procedure to
every element of the list. (Compare MAPLIST.) E.g.

       applist([ 99 ], isinteger)           yields <true>,
       applist([ 1 2 a b 3], isinteger)     puts on the stack:

       applist([], f)
does nothing, whatever procedure F is.

APPLIST could have been defined like MAPLIST (above), but without the
decorated list brackets.

>>>>  ISPAIR

This is TRUE if applied to a list (other than the empty list) FALSE

-- GLOBAL VARIABLE  (EMPTY LISTS) --------------------------------------

>>>>  NIL

This is a variable provided by the system whose value is the word "[]"
so that [], NIL, and VALOF("NIL") all refer to the same thing. [] is
conventionally used to represent the "empty" list, so that if you take a
list L and then assign TL(L) to L repeatedly, you'll end up with L
having the value []. Trying to apply HD or TL to [] will produce an
error ("non-list argument for list procedure, culprit []").

For more on lists see
    R.Barrett, A.Ramsay, A.Sloman,
         POP-11: A Practical language for Artificial Intelligence
         Ellis Horwood and John Wiley, 1985.

Also, try the following TEACH files.


-----<Copyright University of Sussex 1986.  All rights reserved.>-------