ShenDoc 30


 

 

Prolog

Shen contains a Prolog, just as Qi II, but the m-prolog syntax has been dropped. The main reason for this was that embedding executable code in a string (to preserve conformancy with Edinburgh syntax ) generated awkward anomalies with respect of the rest of the system. For example a special string searching routine had to be developed for m-prolog declarations embedded in a package; symbol overloading had to be used because Edinburgh Prolog uses '=' to mean something different from simple equality; you cannot insert comments inside an m-prolog program and searching in an m-prolog program is more difficult since the structure is in a string not a list. To compensate Qi developed a low level s-prolog convention in which Prolog programs were s-exprs.

In place of the awkward dual convention, Shen has one Prolog notation consistent with the rest of Shen which uses 'defprolog'. Here are the member, reverse and append functions in Shen Prolog.

(defprolog member
  X [X | _] <--;
  X [_ | Y] <-- (member X Y);)

(defprolog rev
  [] [] <--;
  [X | Y] Z <-- (rev Y W) (conc W [X] Z);)

(defprolog conc
  [] X X <--;
  [X | Y] Z [X | W] <-- (conc Y Z W);)

The following functions are found in Shen Prolog

predicate arity description
var? 1 succeeds if the term is a Prolog variable
is 2 unifies the first term to the second without an occurs check
is! 2 unifies the first term to the second with an occurs check
findall 3 takes a variable X , a literal L and a variable Y and finds all values for X in L for which L is provable and binds the list of these values to Y. (findall X (woman X) Y) will find all values for X for which (woman X) is provable and bind the result to Y.
when 1 the term is evaluated to true or false and the call succeeds if it is true. All variables in the term are completely dereferenced.
call 1 apply the predicate at the head of the list to the terms of the tail. (Prolog apply)
return 1 terminate Prolog returning the dereferenced value of the term.
! 0 Prolog cut
fork 1 fork does the job of the semi-colon in Edinburgh Prolog; it succeeds if one of the literals in the following list succeeds.
receive 1 bind a variable to a value outside of Prolog

Both 'findall' and 'call' were revised in the S series kernels to take applications and not lists.

(41-) (defprolog mymap
        F [] [] <--;
        F [X | Y] [W | Z] <-- (call (F X W)) (mymap F Y Z);)
(fn mymap)

(42-) (defprolog ab
        a b <--;
        X X <--;)
(fn ab)

(43-) (prolog? (mymap (fn ab) [a a a] X) (return X))
[b b b]

'fork' was introduced in the S series kernels.

(defprolog f
   X <-- (g X) (h X);
   X <-- (g X) (i X);
   X <-- (g X) (j X);)

can be coded as

(defprolog f
   X <-- (g X) (fork (h X) (i X) (j X));)

'receive' was introduced in version 14 to allow Prolog to be called with a variable whose binding is made outside Prolog. Thus '(foo a)' in

(define foo
X -> (prolog? (bar (receive X) (receive X))))

will call '(bar a a)' inside Prolog.

Mode declarations suspend unification in favour of pattern-matching and are used only in the head of a clause. Thus

(defprolog f
  (- a) <--;)

enforces a strict match between the argument a and the input to f. The default mode is unification signalled by +. Mode declarations can be nested as in (- [f a (+ b)]). In this event the innermost declaration has priority. The above expression would thus perform pattern-matching with respect to all its elements save b which would receive unification. The abbreviated notation was introduced in the S series kernels; '(mode a -)' will still compile.

Acknowledgements

History
Basic Types in Shen and Kλ
The Primitive Functions of Kλ
The Syntax of Kλ
Notes on the Implementation of Kλ
Boolean Operators
The Syntax of Symbols
The Semantics of Symbols in Shen and Kλ
Packages
Prolog
Shen-YACC
Strings
Strings and Pattern Matching
Lists
Streams
Character Streams and Byte Streams
Bytes and Unicode
Reader Macros
Vectors
Standard Vectors and Pattern Matching
Non-standard Vectors and Tuples
Equality
I/O
Generic Functions
Eval
Type Declarations
External Global Variables
Property Lists and Hashing
Error Handling
Numbers
Floats and Integers
The Timer
Comments
Special Forms

Built by Shen Technology (c) Mark Tarver, September 2021