# SCHEME The Scheme Interpreter. 2 Primitives COMPUTER SCIENCE 61A. October 29th, 2012

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1 SCHEME COMPUTER SCIENCE 6A October 29th, 202 In the next part of the course, we will be working with the Scheme programming language. In addition to learning how to write Scheme programs, we will eventually write a Scheme interpreter in Project 4. Scheme is a dialect of the Lisp programming language, a language dating back to 958. The popularity of Scheme within the programming language community stems from its simplicity in fact, previous versions of CS 6A were taught in the Scheme language. The Scheme Interpreter Like Python, the Scheme language also has an interactive interpreter. accounts, you can access this by typing in stk. On your class nova [46] # stk Warning: DISPLAY variable is not set. Graphics will NOT be initialized. Welcome to the STk interpreter version 4.0.-ucb.3.6 [SunOS-5.0-sun4u] Copyright (c Erick Gallesio - I3S - CNRS / ESSI Modifications by UCB EECS Instructional Support Group Questions, comments, or bug reports to STk> Keep this one thing in mind: Everything is an expression. 2 Primitives Scheme has a certain set of atomic primitive expressions. What atomic means is that these expressions cannot be divided up, or at least, not usually created out of smaller

2 DISCUSSION : SCHEME Page 2 expressions. The ones we ll use the most are ones representing numbers (i.e., integers and floats, the two boolean values and symbols. Scheme also has strings but we ll only use them a little. STk> STk> STk> a ; this is a symbol a STk> #t #t STk> #f #f STk> "asdf" "asdf" You ll notice that numbers work pretty much the same in Scheme as they do in Python. Also, instead of True and False, we have #t and #f. Before explaining symbols though, let s look at how we define variables: STk> (define a 3 a STk> a 3 You ll notice this is a lot different. In Scheme, you need to have parentheses and inside, have define, then the name of the variable and then the expression (which first gets evaluated that you want bound to the name. Notice that the space between each of the parts (except the parens matters. Also, notice that to refer back to a, I just typed in a just as in Python. The thing is- a is a symbol. When you type things into the interpreter, Scheme will evaluate it. The rule for evaluating a symbol is to get the value bound to that symbol. This is one difference between strings and symbols- symbols don t evaluate to themselves. However, as you saw above, when you type in a, you get a. This is because when you use the single quote, you re telling Scheme not to follow the normal rules of evaluation and just have the symbol return as itself. You ll also notice that you can use the single quotes on integers and floats, and the boolean values also. However they re unnecessary because those evaluate to themselves. Finally, let s revisit what you wrote when you did (define a 3. Both define and a are symbols! CS6A Fall 202: John Denero, with

3 DISCUSSION : SCHEME Page 3 2. Questions. What gets printed STk> (define a STk> a STk> (define b a STk> b STk> (define c a STk> c 3 Evaluating Function Calls and Special Forms Now just defining variables and printing out primitives isn t very useful. You want to call functions too: STk> (+ 2 3 STk> ( STk> (* STk> (/ STk> (+ (* Functions Now you might notice that Scheme does function calls differently. In Scheme, to a call a function, you give the symbol for the function name first, then you give the arguments (remember the spaces. But just as in Python, what you do is you evaluate the operator (the first expression to the right of the (, then you evaluate each of the arguments and then apply those evaluated arguments to the thing returned by the first expression. So when you evaluate (+ 2, you evaluate the + symbol which is bound to a built-in CS6A Fall 202: John Denero, with

4 DISCUSSION : SCHEME Page 4 addition function, then you evaluate and 2. Finally, you apply and 2 to the function value bound to +. Some important functions you ll want to use are: +, -, *, / eq, =, >, >=, <, <= 3.2 Questions. What do the following return (+ (* 3 (+ (* 3 3 (* 4 4 (define a (define b Special Forms However, there are certain things that look like function calls that aren t. These are called special forms and have their own rules for evaluation. You ve already seen one- define where, of course, the first argument can t be evaluated (or else it d search for unbound variables!. Another one we ll use for this class is if. An if expression looks like this: (if <CONDITION> <THEN> <ELSE> where <CONDITION>, <THEN> and <ELSE> are expressions. How it gets evaluated however is that first, <CONDITION> is evaluated. If it evaluates to #f, then <ELSE> is evaluated. Otherwise, <THEN> is evaluated. Everything that is not #f is a true expression. STk> (if this-evaluates-to-true 2 STk> (if #f (/ 0 this-is-returned this-is-returned There are also special forms for the boolean operators which exhibit the same short circuiting behavior that you see in Python. The return values are either the value that lets you know the expression evaluates to a true value or #f. STk> (and STk> (or 2 3 STk> (or #t (/ 0 CS6A Fall 202: John Denero, with

5 DISCUSSION : SCHEME Page 5 #t STk> (and #f (/ 0 #f STk> (not 3 #f STk> (not #t #f 3.4 Questions. What does the following do STk> (if (or #t (/ 0 (/ 0 STk> (if (> 4 3 ( (+ 3 4 (* 3 2 STk> ((if (< Lambdas, Environments and Defining Functions Scheme has lambdas too! In fact, lambdas are more powerful in Scheme than in Python. The syntax is (lambda (<PARAMETERS> <EXPR>. Like in Python, lambdas are function values. Likewise, in Scheme, when a lambda expression is called, a new frame is created where the symbols defined in the <PARAMETERS> section are bound to the arguments passed in. Then, <EXPR> is evaluated under this new frame. Note that <EXPR> is not evaluated until the lambda value is called. STk> (define x 3 x STk> (define y 4 y STk> ((lambda (x y (+ x y Like in Python, lambda functions are also values! So you can do this to define functions: STk> (define square (lambda (x (* x x square STk> (square 4 6 CS6A Fall 202: John Denero, with

6 DISCUSSION : SCHEME Page 6 You might notice that this is a little tedious though. Luckily Scheme has a way outdefine: STk> (define (square x (* x x square STk> (square 5 25 When you do (define (<FUNCTIONNAME> <PARAMETERS> <EXPR>, Scheme will automatically transform it to (define <FUNCTIONNAME> (lambda (<PARAMETERS> <EXPR> for you. In this way, lambdas are more foundational to Scheme than they are to Python. There is also another special form based around lambda- let. The structure of let is as follows: (let ( (<SYMBOL> <EXPR>... (<SYMBOLN> <EXPRN> <BODY> This special form really just gets transformed to: ( (lambda (<SYMBOL>... <SYMBOLN> <BODY> <EXPR>... <EXPRN> You ll notice that what let does then is bind symbols to expressions. For example, this is useful if you need to reuse a value multiple times, or if you want to make your code more readable: (define (sin x (if (< x x (let ( (recursive-step (sin (/ x 3 (- (* 3 recursive-step (* 4 (expt recursive-step 3 4. Questions. Write a function that calculates factorial. (Note how you haven t been told any methods for iteration. (define (factorial x CS6A Fall 202: John Denero, with

7 DISCUSSION : SCHEME Page 7 2. Write a function that calculates the Nth fibonacci number (define (fib n (if (< n 2 5 Pairs and Lists So far, we have lambdas and a few atomic primitives. How do we create larger more complicated data structures Well, the most important data-structure from which you ll build most complex data structures out of is the pair. A pair is an abstract data type that has the constructor cons which takes two arguments, and it has two accessors car and cdr which get the first and second argument respectively. car and cdr don t stand for anything really now but if you want the history go to " wiki/car_and_cdr" STk> (define a (cons 2 a STk> a (. 2 STk> (car a STk> (cdr a 2 Note that when a pair is printed, the car and cdr element are separated by a period. A common data structure that you build out of pairs is the list. A list is either the empty list whose literal is (, also known as nil, another primitive, or it s a cons pair where the cdr is a list. (Note the similarity to Rlists! STk> ( ( STk> nil ( STk> (cons (cons 2 nil ( 2 STk> (cons (cons 2 (cons 3 nil ( 2 3 CS6A Fall 202: John Denero, with

8 DISCUSSION : SCHEME Page 8 Note that there are no dots here. That is because when a dot is followed by the left parenthesis, the dot and the left parenthesis are deleted; when a left parenthesis is deleted, its matching right parenthesis is deleted also. You can check if a list is nil by using the null function. A shorthand for writing out a list is: STk> ( 2 3 ( 2 3 STk> (define (square x (* x x (define (square x (* x x You might notice that the return value of the second expression looks a lot like Scheme code. That s because Scheme code is made up of lists. When you use the single quote, you re telling Scheme not to evaluate the list, but instead keep it as just a list. This is why Scheme is cool. It can be defined within itself. 5. Questions. Define map where the first argument is a function and the second a list. This should work like Python s map. (define (map fn lst 2. Define reduce where the first argument is a function that takes two arguments, the second a default value and the third a list. This should work like Python s reduce. (define (reduce fn s lst 3. Fill out the following to complete an abstract type for binary trees: (define (make-btree entry left right (cons entry (cons left right (define (entry tree CS6A Fall 202: John Denero, with

9 DISCUSSION : SCHEME Page 9 (define (left tree (define (right tree 4. Using the above definition, write a function that sums over the binary tree. For our purposes, assume that if there is no left or if there is no right branch, the values for those should return 0. (define (btree-sum tree CS6A Fall 202: John Denero, with

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