CS.VANSICKLE@R20.UTEXAS.EDU (09/19/86)
Two simple things you CANNOT do in Turbo Prolog:
1. Compute lists containing elements of different basic types.
Turbo Prolog does not let you have goals such as
append([a,b,c],[1,2,3],L).
Turbo Prolog requires that the types of every predicate be
declared, but the typing system does not allow you to declare
types that mix basic types. Also lists like:
[1,a]
[2,[3,4]]
[5,a(6)]
cannot be created in Turbo Prolog. The syntax of types is:
a) name = basictype
where basictype is integer, char, real, string or symbol,
b) name = type*
where type is either a basic type or a user defined type,
the asterisk indicates a list,
c) name = f1(d11,...d1n1);f2(d21,...,d2n2);...fm(dm1,...d2nm)
where fi are functors and dij are types, called
"domains." The functors and their domains are
alternative structures allowed in the type being
defined.
The important thing to notice is that you cannot define a type
that has basic types as alternatives. You can only define
alternatives for types that contain functors. So you cannot
define types
mytype = integer;symbol
mylisttype = mytype*
which is what you would need to append a list of integers to a
list of symbols.
What the Turbo Prolog manual recommends for this case is to
define
mytype = s(symbol);i(integer)
mylisttype = mytype*
and declare append as
append(mylisttype,mylisttype,mylisttype)
which would allow us to state the goal
append([s(a),s(b),s(c)],[i(1),i(2),i(3)],L).
This is clumsy, kludgy, and ugly.
2. Compute expressions that contain different basic types or
mixtures of structures and basic types.
Simplifying arithmetic expressions that contain constants and
variables seems like it should be easy in a language designed
to do symbolic computation. In C & M Prolog some rules for
simplifying multiplication might be
simplify(0 * X,0).
simplify(X * 0,0).
simplify(1 * X,X).
simplify(X * 1,X).
In C & M Prolog you can enter goals such as
simplify(a - 1 * (b - c),X).
Now in Turbo Prolog, because of the limited typing, you cannot
have expressions that contain both symbols and integers. (You
also cannot have infix expressions, but that is another
issue). Instead, you would have to do something like this:
exprtype = i(integer);s(symbol);times(exprtype,exprtype)
and declare simplify as:
simplify(exprtype,exprtype)
and the clauses would be:
simplify(times(i(0),X),i(0)).
simplify(times(X,i(0)),i(0)).
simplify(times(i(1),X),X).
simplify(times(X,i(1)),X).
The goal would be:
simplify(minus(s(a),times(i(1),minus(s(b),s(c)))),X).
This should speak for itself, but I'll spell it out:
REAL Prolog can do symbolic computation involving mixtures of
symbols, numeric constants, and expressions; the programs are
simple and elegant; input and output are easy. In Turbo
Prolog you can't even create most of the expressions that real
Prolog can; the programs are long, opaque, and clumsy; you
have to write your own predicates to read and write
expressions in infix notation.
It is a shame that this product comes from a company with
a reputation for good software. If it came
from an unknown company people would be a lot more cautious
about buying it. Since it's from Borland, a lot of people
will assume it's good. They are going to be disappointed.
-- Larry Van Sickle
cs.vansickle@r20.utexas.edu 512-471-9589
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