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Calls cannot be tail in the scope of a to-be-closed variable

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A to-be-closed variable must be closed when a block ends, so even
a 'return foo()' cannot directly returns the results of 'foo'; the
function must close the scope before returning.
This commit is contained in:
Roberto Ierusalimschy
2018-12-04 15:01:42 -02:00
parent 6d04537ea6
commit 28d829c867
4 changed files with 21 additions and 16 deletions
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manual/manual.of
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@@ -1538,9 +1538,6 @@ except that its value is @emph{closed} whenever the variable
goes out of scope, including normal block termination,
exiting its block by @Rw{break}/@Rw{goto}/@Rw{return},
or exiting by an error.
If a block ends in a tail call @see{functioncall},
all variables of the caller function go out of scope
before the start of the callee function.
To \emph{close} a value has the following meaning here:
If the value of the variable when it goes out of scope is a function,
@@ -2038,8 +2035,8 @@ A call of the form @T{f'@rep{string}'}
is syntactic sugar for @T{f('@rep{string}')};
that is, the argument list is a single literal string.
A call of the form @T{return @rep{functioncall}} is called
a @def{tail call}.
A call of the form @T{return @rep{functioncall}} not in the
scope of a to-be-closed variable is called a @def{tail call}.
Lua implements @def{proper tail calls}
(or @emph{proper tail recursion}):
in a tail call,
@@ -2049,13 +2046,15 @@ a program can execute.
However, a tail call erases any debug information about the
calling function.
Note that a tail call only happens with a particular syntax,
where the @Rw{return} has one single function call as argument;
this syntax makes the calling function return exactly
the returns of the called function.
where the @Rw{return} has one single function call as argument,
and it is outside the scope of any to-be-closed variable.
This syntax makes the calling function return exactly
the returns of the called function,
without any intervening action.
So, none of the following examples are tail calls:
@verbatim{
return (f(x)) -- results adjusted to 1
return 2 * f(x)
return 2 * f(x) -- result multiplied by 2
return x, f(x) -- additional results
f(x); return -- results discarded
return x or f(x) -- results adjusted to 1