CAS command question

[Han]

(01-04-2017 03:46 PM)DrD Wrote:  

(01-04-2017 03:06 PM)Han Wrote:  Regarding 2, this has always been the behavior. A literal (symbolic) expression requires quotation (single quotes). These are left unevaluated. Otherwise, it is "evaluated" as is normally done when entering commands on the command line. Simple example:

x:=t;
x+y // returns t+y
'x+y' // returns x+y

EDIT:


The second case uses evaluation. left(CAS(" 'x+y<4' ")) will return x+y

Whenever I hear something corporate like, "we've always done it this way," etc., it's often a good time to re-visit the issue. Is there a better way? Minimize cost, maximize profit are typical measures. In this case, Arno K's very efficient point makes a better case for the cost of evaluation when using either of these two commands.

The second case is a necessary step to reveal the enclosed expression. It runs through a program without any distinction regarding the relation's symbols. That is a distinction WITH a difference, as compared to what happens when the expression is, itself, evaluated, resulting in the left/right handedness being exchanged.

My point was simply that it should not come as a surprise that the argument to any function is evaluated unless specifically quoted because that was how the system was set up. (I am not making any subjective comment about which setup is better.) On the other hand, asking for a change in the behavior of specific functions would thereby cause a "surprise" in that it would no longer conform to the rest of the system. As for why it was originally set up that way, I can only guess that they probably tried it both ways (evaluation vs non-evaluation having priority) and made their decision based the pros and cons of each case.

Regarding your comment on the second case... I'm not sure I follow. The example I gave shows how to use strings and still not have it evaluate anything -- just add ' to the start and end of the string.

EDIT: I'm not sure to which of Arno K's comment you are referring, but if you are referring to his request to rename the commands, that would seem appropriate. The commands left and right behave more like "head" and "tail" (first and last arguments of an expression) for non-unary operators.