Evaluation and universal machines The Eval/Apply Cycle What is the role of evaluation in defining a language? Eval and Apply execute a cycle that unwinds our abstractions Reduces to simple applications of built in

Slide 32.1.1 We have spent some time looking at computational objects that rency and asynchronous computing are built around the use of local state variables and methods for Object oriented approaches lose \referential transparency Referential transparency means equal expressions

The 6.001 Lazy Meta-Circular Evaluator The Core Evaluator (define (l-eval exp env) (cond ((self-evaluating? exp) exp) ((variable? exp)(lookup-variable-value exp env)) ((quoted? exp)(text-of-quotation exp)) ((assignment? exp)(eval-assignment exp env)) ((definition? exp)(eval-definition exp env)) ((if? exp)(eval-if exp env))

Slide 17.5.1 Streams-a different way of structuring Now, let's look at one example in which changing the computation evaluation model allows us to explore a very different kind of computational problem. Our goal is to show how a smal

Slide 17.1.1 Normal Order(Lazy) Evaluation Over the past few lectures, we have been looking at evaluation especially how to implement eval and apply in a anguage such as Scheme, in order to define a language. What we have seen is that by creating, or specifying, eval and its associated procedures, we actually define the semantics of the

signment exp en ((definition? exp)(eval-definition exp env)) ((if? exp)(eval-if exp env)) ((lambda? exp) (make-procedure (lambda-parameters exp)(lambda-body exp) env)) ((begin? exp)(eval-sequence (begin-actions exp)env)

Slide 16.1.1 Building up a language Last time, we completed building our evaluator. And as you saw, we slightly misled you. We started off saying we were going to user Scheme's lexical analyzer and parser, but then build our own evaluator, which we did initially for arithmetic

1. Arithmetic calculator 6 (define (eval exp) 8 ((number? exp) exp) ((sum? exp (eval-sum exp))

Slide 15.6.1 The next stage in the evolution of our evaluator is to pull the 5. Environment as explicit parameter environment out as an explicit parameter. Up until now we could rely on just having a single environment in which to store (eva1'(pus*64)) bindings for variables. It made sense to have a global

Slide 15.1.1 Why do we need an interpreter? Our goal over the next few lectures is to build an interpreter which in a very basic sense is the ultimate in programming, since doing so will allow us to define our language