In early FORTRANs (i.e., I to 77) there was usually no checking of code for
recursive calls. Instead at execution time the recursive call was made, but
the program later fell into an infinite loop. Based on your knowledge of the
implementation of subroutine calls in FORTRAN, explain why this happens.
Design a mechanism which would allow the program to detect such recursive calls
and abort. Your mechanism should also detect mutually recursive or indirectly
recursive calls. (It is sufficient to have a run-time rather than compile-time
mechanism - i.e., figure out how to detect when this happens at run-time.)
Do problem 23 on page 244 of Louden.
A block is like a procedure body which occurs in-line in a program. That
is, it is composed of a series of declarations followed by some executable
code. However it may not be called like a procedure, it is only executed when
the program counter enters the first statement of the block. Please explain
why the static and dynamic links have the same values for blocks.
In class we discussed the use of a run-time stack of activation records for
procedure calls in block-structured languages. In the interpreter you wrote
last week there was no explicit use of a run-time stack. Your (recursive)
interpreter for PCF provides similar support for activation records (though PCF
is simpler than block-structured languages because there are no local variables
and only one parameter for each function call). Explain how your interpreter
provides a stack-like support for activation records by explaining how the
environment changes during the evaluation of the following expression:
((if is_zero(((fn y => pred y)1)) then succ else pred)
((fn y => (pred(pred y)))8))
