Assignment 7
Consider the grammar in Figure 1 where the terminal int denotes an integer and the terminal id
S → Atoms
Atoms →
Atoms → Atom Atoms
Atom → 0 Atom
Atom → id
Atom → int
Atom → ( Atoms )
Figure 1: A simplified grammar for Scheme.
denotes an identifier. The only other terminals in this grammar are the quote ’, the left parenthesis
(, and the right parenthesis ).
Scheme, a derivative of Lisp, is a list based language where all programs are represented by
lists. For example, a simple Scheme program
( + 1 2 )
adds two numbers together and prints out the result. A scheme interpreter, /local/bin/scheme,
is available on timberlea.cs.dal.ca. Feel free to give it a try. A program, which is zero or more
atoms is executed by evaluating each of the atoms. An atom is either an integer, an identifier, or
a list. The evaluation of an integer is the integer, and the evaluation of an identifier, for now, is
simply the identifier. The evaluation of a quoted atom is simply the atom itself. E.g., the evaluation
of ’ ( 1 2 3 ) is ( 1 2 3 ).
A list is evaluated by evaluating the first atom of the list and treating the result as a function.
Then each of the remaining atoms of the list is evaluated, and passed as parameters to the function.
For example, the evaluation of ( + 1 2 ), treats + as a function, and passes 1 and 2 to the function.
Applying + to 1 and 2 yields the result 3. Whereas, the evaluation of ( + 1 ( * 2 3 ) ), evaluates
+ and 1 as before, but then evaluates ( * 2 3 ), yielding 6, which is then passed to the + function,
which yields the result of 7.
1. [8 marks] Consider the following L-attributed grammar, based on the grammar specified
in Figure 1. Describe and justify what it is computing. Be sure to explain the purpose of
the attributes, and what each of the semantic rules is doing. Your description must include
a succinct summary of the purpose of this attribute grammar. I.e., Be sure to state what it
is supposed to do, not just how it is doing it.
1
CSCI3136 Summer 2020 Assignment 7
Symbol Attribute Type
Atoms calls: int synthesized
callable: bool synthesized
symbol table: Symbol Table inherited
Atom calls: int synthesized
callable: bool synthesized
symbol table: Symbol Table inherited
In the attribute grammar below, the symbol ✄ indicates a semantic rule that operates on
代写CSCI3136作业、代做Python，Java
inherited attributes, and the ✁ symbol indicates a semantic rule that operates on synthesized
attributes.
S → Atoms1 ✄ Atoms1.symbol table = getSymbolTable()
✁ print(Atoms1.calls)
Atoms → ✁ Atoms.calls = 0
✁ Atoms.callable = false
Atoms → Atom1 Atoms1 ✄ Atom1.symbol table = Atoms.symbol table
✄ Atoms1.symbol table = Atoms.symbol table
✁ Atoms.calls = Atom1.calls + Atoms1.calls
✁ Atoms.callable = Atom1.callable
Atom → 0 Atom1 ✄ Atom1.symbol table = Atom.symbol table
✁ Atom.calls = 0
✁ Atom.callable = false
Atom → id ✁ Atom.calls = 0
✁ Atom.callable = Atom.symbol table.isFunction(id)
Atom → int ✁ Atom.calls = 0
✁ Atom.callable = false
Atom → ( Atoms1 ) ✄ Atoms1.symbol table = Atom.symbol table
✁ if Atoms1.callable then Atom.calls = Atoms1.calls + 1
✁ Atom.callable = Atoms1.callable
2. [8 marks] Suppose you wanted to construct an abstract syntax tree from parse tree derived
by using the grammar specified in Figure 1. Give an S-attributed grammar that creates an
abstract syntax tree composed of the following kinds of tree nodes as illustrated in the UML
diagram below.
2
CSCI3136 Summer 2020 Assignment 7
All that is required is to create an abstract syntax tree. Observe that all you will need for
most types of nodes is just the constructor (as shown) but for ListASTNode two methods
are provided to prepend and append nodes to a list. You will need one of these methods,
depending on how you structure your attribute grammar.
Please provide both the semantic rules and the attributes using notation similar to that in
question 1.
Provide a brief explanation of how the grammar works.
3. [9 marks] Suppose that the base ASTNode class had a setNesting(int n) method that sets
the nesting level of the node in the abstract syntax tree. The nesting level is simply the number
of parentheses surrounding the symbol contained in the node. E.g., for ((a) b (c (d e)))
the outer list is at nesting 0; the atoms (a), b, and (c (d e)) are at nesting 1; atoms a, c,
and (d e) are at nesting 2; and atoms d and e are at nesting 3.
Extend the S-attributed grammar from Question 2 into an L-attributed grammar that also
sets the nesting level of each node in the abstract syntax tree as it is created.
Provide a brief explanation of how the grammar works.
3
CSCI3136 Summer 2020 Assignment 7
Marking Scheme
1. Marking scheme for Question 1
4 points 3 points 2 points 1 point 0 points
Attributes
Use of attributes
properly explained
Use of attributes
somewhat explained
No proper explanation
of attributes
Rules All semantic
rules explained
Most semantic
rules explained
Some semantic
rules explained
Few semantic
rules explained
No semantic
rules explained
Description
Purpose of
grammar is
described
Attempt made
to describe
purpose
No attempt
to describe
purpose
2. Marking scheme for Question 2
4 points 3 points 2 points 1 point 0 points
Attributes
Appropriate
attributes
specified
Somewhat
appropriate
attributes
specified
Attributes not
specified
Rules Semantic rules
are correct
Most semantic
rules are correct
Some semantic
rules are correct
Few semantic
rules are correct
No semantic
rules specified
Explanation
Function of
grammar
explained
Function of
grammar poorly
explained
Function of
grammar not
explained
3. Marking scheme for Question 3
4 points 3 points 2 points 1 point 0 points
Attributes
Appropriate
attributes
specified
Somewhat
appropriate
attributes
specified
Attributes not
specified
Rules Semantic rules
are correct
Most semantic
rules are correct
Some semantic
rules are correct
Few semantic
rules are correct
No semantic
rules specified
Explanation
Function of
grammar
explained
Function of
grammar poorly
explained
Function of
grammar not
explained
Notation
Notation differentiates
between
synthesized and
inherited
attributes
Notation does
not differentiate
between
synthesized
and inherited
attributes
4
CSCI3136: Assignment 7
Summer 2020
Student Name Login ID Student Number Student Signature
Mark
Question 1 /8
Question 2 /8
Question 3 /9
Total /25
Comments:
Assignments are due by 9:00am on the due date. Assignments must be submitted electronically
via Brightspace. Please submit a PDF for the written work. You can do your work on paper
and then scan in and submit the assignment.
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 Universitys regulations regarding academic integrity.
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