Now consider the evaluation of . Intuitively we get
This corresponds to the lazy evaluation of
We can imagine using environments even in the final step. For
example, as we compute we get to
. We could now say
let ~in ~
.
Then we look up and evaluate
.
Now in evaluating this application there are many choices. We need a
value for and we could just look it up in the environment to get
.
This is clearly wrong. What happened?
Next time we will examine a way to use environments at each step and
yet get the right answer, .