Reusing Existing Semantics
In this lesson we will learn that, in some cases, we can reuse existing semantics of language features without having to make any change!
Although the definitional style of the basic LAMBDA language changed quite radically in our previous lesson, compared to its original definition in Part 1 of the tutorial, we fortunately can reuse a large portion of the previous definition. For example, let us just cut-and-paste the rest of the definition from Lesson 7 in Part 1 of the tutorial.
krun all the remaining programs from Part 1 of the
tutorial. Everything should work fine, although the store contains lots of
garbage. Garbage collection is an interesting topic, but we do not do it
here. Nevertheless, much of this garbage is caused by the intricate use of
the fixed-point combinator to define recursion. In a future lesson in this
tutorial we will see that a different, environment-based definition of
fixed-points will allocate much less memory.
One interesting question at this stage is: how do we know when we can reuse an existing semantics of a language feature? Well, I'm afraid the answer is: we don't. In the next lesson we will learn how reuse can fail for quite subtle reasons, which are impossible to detect statically (and some non-experts may fail to even detect them at all).