CTM: Exercise 4-1

[ Posted by Urban Hafner Wed, 02 Nov 2005 16:10:56 GMT ]

The problem

Thread semantics. Consider the following variation of the statement used in section 4.1.3 to illustrate thread semantics:

1
2
3
4
5

local B in
   thread B=true end
   thread B=false end
   if B then {Browse yes} end
end

For this exercise, do the following:

(a) Enumerate all possible executions of this statement.

(b) Some of these executions cause the program to terminate abnormally. Make a small change to the program to avoid these abnormal terminations.

My solution

(a) Line 4 has to wait until B becomes bound, so either line 2 or 3 has to execute first. The following combinations are possible:

  • 2,3,4
  • 2,4,3
  • 3,2,4
  • 3,4,2

(b) Actually all executions terminate abnormally because all of the eventually try to unify true and false. So what we have to check in the thread in line 2 and 3 is if B is already bound. This leads to the following program:

local B in
   thread if {IsDet B} then skip else B=true end end
   thread if {IsDet B} then skip else B=false end end
   if B then {Browse yes} end
end

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