Re: Multi-assign
- Posted by jimcbrown (admin) Dec 05, 2011
- 1686 views
I have viewed this thread with dismay.
A function (presumably) returns one value (sequence or atom).
Why?
What's wrong with having a function return multiple values?
Everyone seems to see only the positive (multi-assignment is good!). Yes it is useful (marginally).
What are the negative effects?
If you are able to recover a, b & c then the {} are not acting as sequence forming operators in (1). (Otherwise: there would be a complaint about them being used before being initialised.)
Agreed. This is not the creation-of-sequence operator. This is in fact the inverse, it is desequencing.
The assumption is that the function will return values appropiate to the types of the LHS. What should those types be? The library functions often return different types (eg gets() returns -1 or a sequence).
Desequencing implies that the return type has to be a sequence (before the sequence can be desequenced) and to have a different type is to cause an error.
{a,?,c} cannot be a sequence a slice [1..2] - what is its value?, its length?
Well, you have a return value that is a sequence s that is desequenced such that a is assigned to s[1] and c is assigned to s[3] and s[2] is ignored.
f(x, ?, y): by the reasoning of {a, ?, c} the ? means 'disregard this value' not 'use the default'.
Disregard this value makes no sense applied to a function parameter.
So that symbol has at least two meanings. What of it?
I see these two technical meanings as very closely related. In both usages, the user is telling the language that the user does not care about the value.
{a,b,c} = {1,2,3} -- either a:=1, b:=2, c:=3 or {a=1,b=2,c=3} {1,2,3} = {a,b,c} -- either an error or {a=1,b=2,c=3}
The answer:
{a,b,c} = {1,2,3} -- a:=1, b:=2, c:=3 -- conceptually shorthand for sequence temp_s = {1,2,3} a = s[1] b = s[2] c = s[3] delete temp_s {1,2,3} = {a,b,c} -- an error
The problem is not that you can't make this work. The problem is that syntactically they are identical. Therefore you cannot distinguish between an erroneous expression and a (correct) assignment and vice versa.
I'm not clear on what you mean here.
It is technically possible to distinguish between the two. I know this because the code to do so has already been written and tested. The interpreter/translator/parser is able to do this.
Perhaps you mean that you think the concept is too confusing for a user?
Instead of using {a,b,c} why not use something like < a, b,c > or | a, b, c | for multi-assign? That way it is clear this is not a sequence, but a list of some kind.
I don't care much one way or the other on what symbols or characters are used, but changing that would not change the fundamental nature of desequencing.
A function would still return a sequence, and the operators would still desequence it.
