Editing Nested function (section)

=== Access of non-local objects ===

There are several ways to implement nested procedures in a lexically scoped language, but the classic way is as follows:

:Any [[non-local object]], X, is reached via access-links in the [[call stack|activation frames]] on the machine stack. The caller, C, assists the called procedure, P, by pushing a ''direct'' link to the ''latest'' activation of P's immediate lexical encapsulation, (P), prior to the call itself. P may then quickly find the right activation for a certain X by following a ''fixed number'' (P.depth – X.depth) of links (normally a small number).

:The caller creates this direct link by (itself) following C.depth – P.depth + 1 older links, leading up to the latest activation of (P), and then ''temporarily'' bridging over these with a direct link to that activation; the link later disappears together with P, whereby the older links beneath it may come into use again.

:Note that P is visible for, and may therefore be called by, C if (P) = C / (C) / ((C)) / etc.

This original method is faster than it may seem, but it is nevertheless often optimized in practical modern compilers (using [[Call stack#Display|''displays'']] or similar techniques).

Another way to implement nested functions that is used by some compilers is to convert ("lift") nested functions into non-nested functions (where extra, hidden, parameters replace the access links) using a process known as [[lambda lifting]] during an intermediate stage in the compilation.