Re: assignment in conditional expression?
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- Subject: Re: assignment in conditional expression?
- From: David Jeske <jeske@...>
- Date: Tue, 8 Dec 1998 12:19:43 -0800
On Tue, Dec 08, 1998 at 03:30:07PM -0200, Roberto Ierusalimschy wrote:
> > [...] Is there some way to do assignment inside
> > an expression like this, and if not, why not?
>
> Why not?
> The main reason is syntactical. Assignments inside expressions would create
> many syntactical ambiguities in the language. For instance, the constructor
> {x=3, y=4} could be interpreted in the current way, or as a list of two
> expressions, x=3 (value 3 to be stored in index 1) and y=4 (value 4
> in index 2). As lhf said, you can do something "equivalent", but with
> another syntax (calling a function).
When I wrote the above, I was thinking about wanting to do nicer
iterators. However, I've decided that if there was a way to write a
'macro' which was bound into the callers scope, then we could do some
really nice things.
I propose the following idea for making a block of code which runs
inside the local-scope of the caller, but is otherwise similar to a
function and evaluates as such:
-- note that I don't necessarily like the syntactic use of "macro", I'm
-- just describing it this way for simplicity
macro (arg1, arg2, ...) -- arguments are optional
end
macro for(from,to,code)
local i = from
while (i<to) do
code(i);
i = i + 1;
end
end
for(1,2, macro (x)
print("iteration: ".. tostring(x));
end);
macro foreach(tbl,code)
local i,v = next(tbl,nil)
while (i) do
code(v,i,tbl); -- note the inverse (v,i)
i,v = next(tbl,i);
end
end
macro foreach_iter(tbl,code)
local iter_index_tbl = {}
local iter_value_tbl = {}
local iter_num = 1
local i,v = next(tbl,nil)
while(i) do
iter_index_tbl[iter_num] = i;
iter_value_tbl[iter_num] = v;
i,v = next(tbl,i);
iter_num = iter_num + 1;
end
local iter_var = 1;
while (iter_var < iter_num) do
code(iter_value_tbl[iter_var],iter_index_tbl[iter_var],tbl);
iter_var = iter_var + 1;
end
foreach({ x = 1, y = 2, z = 3}, macro (v,i,tbl)
print(format("[%s] = %d",i,v));
end);
-- output (might be in different order):
-- [x] = 1
-- [y] = 2
-- [z] = 3
local t = { x = 1, y = 2, z = 3};
foreach_iter(t, macro(v,i,tbl)
print(format("[%s] = %d",i,v));
t[i] = nil;
end);
-- output (might be in different order):
-- [x] = 1
-- [y] = 2
-- [z] = 3
foreach(t,macro(v,i,tbl)
print(format("[%s] = %d",i,v));
end);
-- no output, because the above 'foreach_iter' removed all the elements
--
David Jeske (N9LCA) + http://www.chat.net/~jeske/ + jeske@chat.net