Concepts+Techniques+and+Models+of+Computer+Programming_Part24

Concepts+Techniques+and+Models+of+Computer+Programming_Part24

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648 Relational Programming correct deductions from the axioms. That is, the results of all predicates are valid tuples in the predicates’ relations. We call this deterministic logic programming . Table 9.2 defines a translation scheme T which translates any statement s in the relational kernel language into a logical formula T ( s ). Procedure definitions are translated into predicate definitions. Note that exceptions are not translated. Raising an exception signals that the normal, logical execution is no longer valid. The logical sentence therefore does not hold in that case. Proving the correctness of this table is beyond the scope of this chapter. We leave it as an interesting exercise for mathematically-minded readers. A given logical semantics can correspond to many operational semantics. For example, the following three statements: 1. X=Y s 2. s X=Y 3. if X==Y then s else fail end all have the exactly same logical semantics, namely: x = y T ( s ) But their operational semantics are very different! The first statement binds X and Y and then executes s . The second statement executes s and then binds X and Y . The third statement waits until it can determine whether or not X and Y are equal. It then executes s , if it determines that they are equal. Writing a logic program consists of two parts: writing the logical semantics and then choosing an operational semantics for it. The art of logic program- ming consists in balancing two conflicting tensions: the logical semantics should be simple and the operational semantics should be efficient. All the declarative programs of Chapters 3 and 4 can be seen in this light. They are all logic pro- grams. In the Prolog language, this has given rise to a beautiful programming style [182, 21, 139]. Deterministic append Let us write a simple logic program to append two lists. We have already seen the Append function: fun {Append A B} case A of nil then B [] X|As then X|{Append As B} end end Let us expand it into a procedure: Copyright c 2001-3 by P. Van Roy and S. Haridi. All rights reserved.
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9.3 Relation to logic programming 649 Relational statement Logical formula skip true fail false s 1 s 2 T ( s 1 ) T ( s 2 ) local X in s end x.T ( s ) X=Y x = y X=f(l1:X1 ... ln:Xn) x = f ( l 1 : x 1 , ..., l n : x n ) if X then s 1 else s 2 end ( x = true T ( s 1 )) ( x = false T ( s 2 )) case X of f(l1:X1 ... ln:Xn) ( x 1 , ..., x n .x = f ( l 1 : x 1 , ..., l n : x n ) T ( s 1 )) then s 1 else s 2 end ( ¬∃ x 1 , ..., x n .x = f ( l 1 : x 1 , ..., l n : x n ) T ( s 2 )) proc {P X1 ... Xn} s end x 1 , ..., x n .p ( x 1 , ..., x n ) T ( s ) {P Y1 ... Yn} p ( y 1 , ..., y n ) choice s 1 [] ... [] s n end T ( s 1 ) ... T ( s n ) Table 9.2: Translating a relational program to logic proc {Append A B ?C} case A of nil then C=B [] X|As then Cs in C=X|Cs {Append As B Cs} end end According to Table 9.2, this procedure has the following logical semantics: a, b, c. append ( a, b, c ) ( a = nil c = b ) ( x, a , c .a = x | a c = x | c append ( a , b, c )) The procedure also has an operational semantics, given by the semantics of the declarative model. The call: {Append [1 2 3] [4 5] X} executes successfully and returns X=[1 2 3 4 5] . The call’s logical meaning is the tuple
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