Non-local references

So far, whenever we needed to compute a variable, there  was  a binding for that  variable in the 'local' environment (the environment in which  we were evaluating the expression). But take this code:

def biz(a):

return a + b

>>> b = 6

We need to elaborate, slightly,  how it is that a variable v is computed in an environment E:

• We take if there is a binding for v in E; if so, we stop and return it.

• If not, we compute v in the parent environment of E. If E has no base, we create an error.

It is important to check that this method  will continue up an arbitrarily long chain of environ­ ments  and their parents until either a binding for v is found or there are no more environments to examine.

So, in our case, we will compute the expression a + b in environment E2. We start by calculating a and searching value 2 in E2. Then, we compute b and cannot search it in E2...but we don't panic! We take the parent pointer to E1 and  try again.  We search a binding for b in E1 and  get the value  6. So, the value  of a + b in E2 is 8.