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
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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.
1
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
2
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
3
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.
4
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.
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