Data Types

In LL, the following data types are supported:

• symbols
• numbers (integers and reals)
• strings
• lists and dotted pairs
• structures

Symbols, numbers, strings, and lists work just like their COMMON LISP counterparts. Because LL has to handle all data types that can be found on the WAM stack and heap, PROLOG structures had to be added. They are quite similar to (1-dimensional) COMMON LISP arrays in that they allow efficient random access to all of their elements in contrast to LISP/PROLOG lists which require linear access by applying a cdr operation repeatedly.

In LL, structures are enclosed in square brackets and use LISP's Cambridge Polish prefix notation:

LL structures evaluate to themselves, thus quoting is not necessary but - as with numbers and strings - allowed.

In analogy to constructing lists with list or cons, LL structures can be built up with struct ( denotes evaluation; expr = expr is equivalent to expr expr):

(struct 'f 'a 'b 'c) [f a b c] (struct 'f 'a (struct 'g 'b 'c)) [f a [g b c]]

Using struct is the only way to construct a structure which contains variables. The RELFUN/REL goal

X is a, Y is f[X,b] Y = f[a, b]

has, in LL, to be expressed as:

(let ((x 'a)) (struct 'f x 'b)) [f a b]

LL - just like COMMON LISP - does not support pattern matching or unification. Selecting the constituents of a structure has to be done with the following functions:

• (arity [f arg arg])
• (functor [f arg arg]) f
• (elt [f arg arg] ) arg with