present state of unevaluated conditionals, was: Handling branch cuts ...

--- Stavros Macrakis  wrote:

> All you say is true.  But if nothing is known about the value of x,
> and you do ev(expr,nouns), instead of getting an unevaluated
> conditional back, you will get an error.  Not really satisfactory!

Well, the error can be turned off via the flag prederror.

(%i1) prederror: false$
(%i2) if a > b then a else b;
(%o2)               if unknown then a else b

Ugh! Here's a 1-line patch to preserve the unevaluated condition
(and also evaluate the alternatives). 
This is currently line 2554 in src/mlisp.lisp --

- (v (return (list* '(mcond) v (mapcar #'meval-atoms (cdr u))))))))
+ (v (return (list* '(mcond) (car u) (mapcar #'meval1 (cdr u))))))))

This is much nicer, I think --

(%i1) load("src/mlisp.lisp")$
(%i2) prederror: false$
(%i3) expr: if a > b then if a > c then a else c else if b > c then b
else c;
(%o3) if a > b then (if a > c then a else c)

                                    else (if b > c then b else c)
(%i4) assume (a > b)$
(%i5) expr, nouns;
(%o5)                if a > c then a else c
(%i6) forget (a > b)$
(%i7) assume (a > c)$
(%i8) expr, nouns;
(%o8)     if a > b then a else (if b > c then b else c)
(%i9) forget (a > c)$
(%i10) expr, nouns;
(%o10) if a > b then (if a > c then a else c)

                                    else (if b > c then b else c)
(%i11) assume (b < c)$
(%i12) expr, nouns;
(%o12)    if a > b then (if a > c then a else c) else c
(%i13) assume (a > c)$
(%i14) expr, nouns;
(%o14)                          a

All that seems as it should be. Also the following 
yields the expected plots --

(%i15) f(x) := if x < 0 then -x else sqrt(x);
(%o15)       f(x) := if x < 0 then - x else sqrt(x)
(%i16) plot2d (if x > 1 then log(x) else exp(x), [x, -1, 3])$
(%i17) plot2d (f(x), [x, -2, 2]);

run_testsuite() finds no unexpected errors, which 
doesn't prove much although it is evidence for the 
absence of unintended side-effects.

Well, that seems like a happy situation. Comments?

Robert Dodier


		
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