Lua学习笔记之Expressions

Both function calls and vararg expressions can result in multiple values. If an expression is used as a statement (only possible for function calls (see §2.4.6)),
then its return list is adjusted to zero elements, thus discarding all returned values. If an expression is used as the last (or the only) element of a list of expressions, then no adjustment is made (unless the call is enclosed in parentheses). In all other
contexts, Lua adjusts the result list to one element, discarding all values except the first one.

Here are some examples:

f()                -- adjusted to 0 results
g(f(), x)          -- f() is adjusted to 1 result
g(x, f())          -- g gets x plus all results from f()
a,b,c = f(), x     -- f() is adjusted to 1 result (c gets nil)
a,b = ...          -- a gets the first vararg parameter, b gets
-- the second (both a and b can get nil if there
-- is no corresponding vararg parameter)

a,b,c = x, f()     -- f() is adjusted to 2 results
a,b,c = f()        -- f() is adjusted to 3 results
return f()         -- returns all results from f()
return ...         -- returns all received vararg parameters
return x,y,f()     -- returns x, y, and all results from f()
{f()}              -- creates a list with all results from f()
{...}              -- creates a list with all vararg parameters
{f(), nil}         -- f() is adjusted to 1 result

Any expression enclosed in parentheses always results in only one value. Thus, 

(f(x,y,z))

 is always a single value, even if 

f

 returns several values. (The value of 

(f(x,y,z))

 is
the first value returned by 

f

 or nil if 

f

 does not return any values.)