We need a short, official name for "list of values"
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- Subject: We need a short, official name for "list of values"
- From: Eduardo Ochs <eduardoochs@...>
- Date: 2011年2月25日 23:26:38 -0300
I started to write this as an answer to the thread called "Surprising
behavior of table.insert()", but it quickly became something else -
so I am posting it as a new thread.
Please look at the example below. The function "id" is an easy way to
produce a "nullary list", as well as lists of any length.
function id (...) return ... end
print(id()) -->
print(id(nil)) --> nil
print(id(nil, nil)) --> nil nil
T = {}
table.insert(T, id()) --> "wrong number of arguments"
table.insert(T, id(nil)) --> ok
table.insert(T, id(1)) --> ok
table.insert(T, id(1, 2)) --> ok
table.insert(T, id(1, 2, 3)) --> "wrong number of arguments"
assert(id()) --> "bad argument... value expected"
assert(id(nil)) --> "assertion failed"
assert(id(nil, nil)) --> "assertion failed"
assert(id(nil, nil, nil)) --> "assertion failed"
assert(id(1)) --> ok
assert(id(1, 2)) --> ok
-- And now for the grand finale...
print((id())) --> nil
print((id(nil))) --> nil
print((id(nil, nil))) --> nil
print((id())) --> nil
print((id(1))) --> 1
print((id(1, 2))) --> 1
That code was intented as an answer to something in the "Surprising
behavior..." thread - but then I realized that I have a question. In:
id(1, 2, 3)
the "1, 2, 3" is _syntactically_ an "explist" - that terminology
appears at lots of places in the manual - but what is the term for the
corresponding thing in the _semantics_? Let me suppose, for a moment,
that it is "vlist" (for "list of values"); then functions always
receive vlists and returns vlists, and the function "id" above returns
exactly the same vlist that it received, even if it had length 0...
By the way, I just did a quick check on manual for lua-5.2.0-alpha -
just grepping for all occurrenced of "return" there and reading the
text around each one - and I got the impression that indeed there
isn't any short, official name for what I am calling a "vlist"; only
expressions like:
values
results
return list
returned values
returns (as in: the returns of the called function)
and that lack of a distinctive name may be one of the reasons why so
many people fail to realize that vlists are practically first-class
values... the "practically" is because to store vlists into variables
we need to pack them, creating tables with an "n" field:
function mypack (...) return {n=select("#", ...), ...} end
function myunpack (T) return unpack(T, 1, T.n) end -- 5.1
function myunpack (T) return table.unpack(T, 1, T.n) end -- 5.2
print(myunpack(mypack())) -->
print(myunpack(mypack(nil))) --> nil
print(myunpack(mypack(nil, nil))) --> nil nil
print(myunpack(mypack(nil, nil, nil))) --> nil nil nil
That's it for the moment.
Cheers,
Eduardo Ochs
eduardoochs@gmail.com
http://angg.twu.net/
P.S.: I know that some people hate the idea of using tables with an
"n" field, and are more than willing to change the language completely
just to avoid it... But I think that _this particular discussion_ will
be more productive if I ask politely to them to not participate... So:
please? 8-)