Solution
ID: past-exam-of-the-computer-science-course-of-the-university-of-oxford/2015/compilers/2/f/solution
Past exam of the computer science course of the University of Oxford 2015 Compilers 2 f Solution by
Codex 0 2026-10-31
No: a procedural parameter can carry an older lexical environment past a newer recursive activation. Here is typed, Pascal-like pseudocode; a function-valued parameter is represented by a programming closure, comprising code and its defining environment.
procedure dummy(): integer;
begin return 0 end;
procedure P(n: integer; f: procedure(): integer): integer;
var x: integer;
procedure Q(): integer;
begin return x end;
begin
x := n;
if n = 1 then return P(2, Q)
else return f()
end;
begin print P(1, dummy) end.Call the first
P activation . It sets its own x to and passes the programming closure into a recursive activation . That activation sets its own x to and calls its parameter f. The resulting activation of Q has its static link pointing to , because that is where the passed procedure was defined. Its dynamic link points to , because that is where it was called.During this call the dynamic chain contains
Q, P2, P1, main, while Q's static chain contains Q, P1, main. The newest surviving activation of P is , but the static chain deliberately uses . Correct lexical scope therefore prints ; blindly consulting current[P] would produce . Both enclosing activations remain alive, so no issue of a returned procedure or dangling pointer is involved.The failed step in part (d) is precisely the assumption that the callee's lexical environment is a suffix of the direct caller's static chain. A passed programming closure can specify a different, still surviving environment. An implementation supporting such calls must preserve that environment; a table implementation requires additional environment installation and restoration rather than merely consulting newest activations.
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