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日期：2019-06-16 11:37

CSCI3136

Assignment 9

In this assignment we will further expand the Splat Evaluator to handle closures. We will add

the several features to our evaluator including, set and lambda expressions, the ability to call

functions, and three built in functions to work with lists.

The set expression is used to modify variables in the current scope. The syntax for set is

set id = expression

where the id denotes a bound identifier that is in scope and expression is the expression to be

evaluated whose result is then bound to the id. The result of the set expression is the previous

value that id was bound to. For example, the expressions below yield 42, 42, and 37.

def x = 42

set x = 37

x

The lambda operator allows us to create closures (functions with a reference environment) that

can then be invoked. I.e., just like you can define and use methods in Java or functions in C, we

can define and use closures in Splat. The syntax for lambda comprises two parts:

lambda ( param1, param2, ... ) { expression1 expression2 ... }

where the first part is a comma-separated list of zero or more parameters, which are identifiers, and

the second part is the body of the function, comprising one or more expressions. The expressions

are evaluated when the closure is invoked, and the result of the evaluation is the result of the last

expression in the body. Typically, lambda expressions are used in conjunction with def or let.

In Example 1 (Figure 1), the first expression defines a symbol area and binds it to a closure that

computes the area given the length (l) and width (w). The second expression defines the symbol

square and binds it to a closure that computes the square area, given a width, by invoking the area

closure. The next two expressions simply invoke the area and square closures. The fifth expression

binds the symbol execute to a closure that takes two parameters: a closure (c) and a value (a), and

evaluates the closure, passing the argument a to it twice. The last expression is an invocation of

the closure bound to execute. The evaluation of these expressions is: 2142 49 2601.

In Example 2, the expression comprises a let, which binds x to 42 and then evaluates a second

let expression, which binds foo to a closure that divides x by the passed argument (a), then

evaluates three expression: a third let expression, which binds x to 21 and then invokes foo with

argument 3; a set expression, which changes the value of x to 14; and another invocation of foo

with argument 7. The evaluation of this expressions is 2.

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CSCI3136 Summer 2019 Assignment 9

Example 1 Example 2

def area = lambda (l, w) {

l * w

}

def square = lambda (w) {

area(w, w)

}

area(51, 42)

square(7)

def execute = lambda (c, a) {

c(a, a)

}

execute(area, 51)

let x = 42 {

let foo = lambda (a) {

x / a

}

{

let x = 21 {

foo(3)

}

set x = 14

foo(7)

}

}

Figure 1: Examples of lambda, set, and function calls.

Note: the output for the evaluation of a lambda expression is just <lambda>, i.e, The output

of the following program is <lambda>.

lambda ( ) {

42

}

1. [10 marks] Starting with either your own solution to Assignment 8 or the provided solution

(splat.3.py), add the following productions to your parser/evaluator:

EXPR → SET

→ LAMBDA

SET → 0

set0 SYMBOL 0 =

0 EXPR

LAMBDA → 0lambda0 0(0 PARAMS 0)0 BODY

PARAMS → → SYMBOL P LIST

P LIST → → 0,0 SYMBOL P LIST

CALL → → 0(0 ARGS 0)0

and replace production

VALUE → SYMBOL

with

VALUE → SYMBOL CALL

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CSCI3136 Summer 2019 Assignment 9

2. [30 marks] Implement the evaluation of the set, lambda, and function call expressions

in your evaluator. Your implementation should perform lexical scoping and deep

binding.

Suggestions:

To implement set, you will need to add functionality to your reference environment to

update bindings. The provided solution already implements this.

Do not evaluate the body of closure until it is actually called.

When a closure is called, link in a new frame into the reference environment to hold the

parameter/argument bindings. Once the call completes, this frame is no longer needed.

3. [10 marks] Add three built-in functions into your Splat evaluator: prepend(), head() and

tail(). The prepend() function takes a value and a list and prepends the value to the list,

and returns the new list. The latter two functions take a single argument, which is a list.

The head() function returns the first item in the list. The tail() function returns removes

the head and returns the remainder of the list.

Input Output

def nums = [1, 2, 3, 4]

head(nums)

tail(nums)

prepend(0, nums)

[1, 2, 3, 4]

1

[2, 3, 4]

[0, 2, 3, 4]

Figure 2: Examples of prepend(), head() and tail().

4. [Bonus 5 marks] For bonus marks we want to add the if expression to our evaluator. The

if expression has the form:

if condition body

elseif condition body

...

else body

which consists of one required and two optional parts. The required part consists of an

if followed by a condition expression and a body. There are zero or more elseif parts

which consists of an elseif followed by a condition expression and a body. Lastly, there

is an optional else part, which consists of an else followed by a body. Add the following

productions to your Splat evaluator.

EXPR → IF

IF → 0

if0 EXPR BODY ELSEIF ELSE

ELSEIF → 

→ 0

elseif0 EXPR BODY ELSEIF

ELSE → 

→ 0

else0 BODY

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CSCI3136 Summer 2019 Assignment 9

5. [Bonus 20 marks] For bonus marks implement the evaluation of if expressions.

The if expression is evaluated by first evaluating the condition after the if. If this condition

evaluates to true, the result of the body that follows the condition is evaluated and yields the

overall evaluation. Otherwise, if an elseif follows, if the condition of the elseif evaluates to

true, the body of the elseif is evaluated and yields the result. Otherwise, the next elseif

is evaluated, and so on. If all conditions evaluate to false and there is an else part, the

body of the else is evaluated and yields the result. If there is no else and all conditions

evaluate to false the result of the whole if expression is false.

Input Output

def a = 37

def b = 42

Figure 3: Examples of if expressions.

A set of test cases is provided for you to test your evaluator. Two test scripts test.sh and

bonus.sh are provided to run the tests. Since the choice of language is up to you, you must provide

a standard script called runme.sh to run your interpreter, just like in the previous assignment.

To submit this part of the assignment please use Brightspace.

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CSCI3136: Assignment 9

Summer 2019

Student Name Login ID Student Number Student Signature

Mark

Question 1 /10

Question 1 /30

Functionality /15

Structure /15

Question 3 /10

Question 4 Bonus /5

Question 5 Bonus /20

Functionality /10

Structure /10

Total /50

Comments:

Assignments are due by 9:00am on the due date. Assignments must be submitted electronically

via Brightspace. Plagiarism in assignment answers will not be tolerated. By submitting

their answers to this assignment, the authors named above declare that its content is their

original work and that they did not use any sources for its preparation other than the class

notes, the textbook, and ones explicitly acknowledged in the answers. Any suspected act

of plagiarism will be reported to the Facultys Academic Integrity Officer and possibly to the

Senate Discipline Committee. The penalty for academic dishonesty may range from failing the

course to expulsion from the university, in accordance with Dalhousie University’s regulations

regarding academic integrity.