🚀 go-pugleaf

In article <1992Aug25.074623.25450@greco-prog.fr>, billaud@greco-prog.fr (Michel BILLAUD) writes:
> In article <meskes.713709771@ulysses> meskes@ulysses.informatik.rwth-aachen.de (Michael Meskes) writes:
> 
> >Can you program a IF...THEN...ELSE in Prolog without using the cut?
> 
> No. a proof by contradiction:

Well, a lot depends on what goes in the gaps.  If the stuff to go in the
gaps is logical formulas, the proof goes through.  But with suitable
restrictions it is possible.  For example, define
	<test> ::= <expression> <a.relop> <expression>
	        |  <term> <t.relop> <term>
	        |  NOT <test>
	        |  <test> AND <test>
	        |  <test> OR <test>
where the understanding is that the arithmetic expressions whose values
are to be compared and the terms which are to be compared must be ground.
(NU-Prolog-style 'when' declarations for comparison predicates can do this.)
Then we can negate a formula:
	NOT(E1 a.R E2) => E1 NOT(a.R) E2		e.g. NOT(<) = >=
	NOT(T1 t.R T2) => T1 NOT(t.R) T2		e.g. NOT(@<) = @>=
	NOT(NOT(T))    => T
	NOT(X1 AND X2) => NOT(X1) OR NOT(X2)
	NOT(X1 OR  X2) => NOT(X1) AND NOT(X2)
and something of the form
	IF <test> THEN <true> ELSE <false>
can be converted to
	( <test>, <true>
	; NOT(<test>), <false>
	)
Note that SB-Prolog already recognises some special cases of this pattern,
so that ( X < 0, a ; X >= 0, b ) will be compiled as an if-then-else.

If you are writing an interpreter for some other language (e.g. a pure
functional programming language) you can _interpret_ an if-then-else in
that language without a Prolog cut.

-- 
You can lie with statistics ... but not to a statistician.