Lecture 4

Question 1

What is a type?

Answer 1

Set of values together with a set of operations on these values.

Example:
Integers: integer numbers + add, sub, div, mul

Question 2

What is static type checking?

Answer 2

Performed before execution, at compile-time

Question 3

What is dynamic type checking?

Answer 3

Run-time type checking.

Performed during execution, before execution of each individual operation.

Question 4

How is a practical language defined from the kernel language?

Answer 4

Add syntactic sugar and linguistic abstractions.

Syntactic sugar: Syntactic conveniences for commonly used functionality.

Linguistic abstractions: Syntactic conveniences that introduce new programming structures without extending the model of computation.

Question 5

Give 2 examples of syntactic sugar

Answer 5

Nested records:
Z = z(y: y(x: 1))

Multiple variable declaration:
local X Y Z in <s> end</s>

Question 6

How can record features be accessed using pattern matching?

Answer 6

case E of label(feature2: value anotherFeature: value4) then
X = value
Y = value4
end

Question 7

What does _ mean in:
case R of label(f1:_ f2:value) then

Answer 7

(‘_’) denotes an unnamed variable, used to keep values we do not care about.

Question 8

What syntactic sugar makes it easier to access record fields?

Answer 8

A = recordName(feature: value)

X = A.feature

Question 9

What does Record.arity return?

Answer 9

List of features

Question 10

What is a keyword?

Answer 10

A lexeme that has some special meaning, may be related to syntactic and semantic rules

Question 11

What are reserved words?

Answer 11

Lexems that can only be used as keywords.

Question 12

What is an atom?

Answer 12

Atom start with lowercase, and any variables/symbols after that

Atom can be any lexem in quotes matching ‘[^`\]’

A record with no fields:
atom == atom()
Syntactically, atom is not equivalent to atom()

Question 13

What is an identifier?

Answer 13

A syntactic entity, part of programs sequence of tokens

Identifier mapped to variable.
Variable bound to value.

Question 14

What is a variable?

Answer 14

A semantic entity, part of the execution of a program.

Question 15

What are declarative variables?

Answer 15

After initial binding, they cannot be reassigned another value.

Dataflow variables.

Question 16

What are partial values?

Answer 16

Can contain unbound variables, e.g. records.

declare X Y in
Y = label(feature: X)

Question 17

Why does Browse not belong to the declarative sequential model of computation?

Answer 17

It starts a new thread in which the browser displays a value

Question 18

What is an expression?

Answer 18

An operation that evaluates to a value.

Examples:
2+2 (evaluates to 4)
A == B (evaluates to true or false)
proc {$ X} X = 1 end (evaluates to a procedure)

Question 19

What is a statement?

Answer 19

Specifies an operation that does not evaluate to a value.

X=1 (unifies X with 1)
{Browse ‘hello’}
skip

Question 20

Define a program in the declarative sequential kernel language in regards to statements and expressions

Answer 20

A program composed exclusively of statements

Question 21

Give an example of if-else constructs being a statement and an expression

Answer 21

Statement:
local X in
if A > B then
X=A
end
end

Expression:
if A > B then
A
end

Question 22

What is a function?

Answer 22

Linguistic abstractions of procedures.

A function definition can be translated into a procedure.

Function of n arguments is a procedure of n + 1

Question 23

What does # mean?

Answer 23

It is the tupling constructor.

Syntactic sugar for building records.

X#Y#Z equals ‘#’(X Y Z)

Instead of X == 1 andthen X==2 andthen X== 3 then <s> end</s>

We can pattern match:
case X#Y#Z of 1#2#3 then <s> end</s>

Question 24

What is a list?

Answer 24

A sequence of records.

The record that form a list has the label ‘|’ and 2 features (1: Head and 2: Holds tail list)

[one two]
= ‘|’ (one ‘|’ (two nil))
= one|two|nil

Question 25

What is the arity of a(c: 1 b:2) and a(c b)

Answer 25

A = a(c: 1 b:2)
B = a(c b)

A.arity: [b c]
B.arity: [1 2]

Arity returns sorted list

Question 26

What does the pattern H|_ mean?

Answer 26

’|’ (1: H 2:_)

Feature 2 can contain whatever value