Control Structures

Question 1

What are Expressions?

Answer 1

Type of program phrase that returns a value but does not change the state or perform a side-effect.

Functional programming only uses expressions!

Question 2

What are Commands?

Answer 2

Type of program phrase that does not return a value but changes the state or performs a side effect

Question 3

Describe the following notations:
function application notation
prefix notation
infix - notation
postfix - notation

Answer 3

FAN: +(a, b)

prefix: + a b
infix: a + b
postfix: a b +

Question 4

What is the difference between eager evaluation and lazy evaluation?

Answer 4

Eager evaluation is evaluating an application of an operator to expressions by first evaluating the arguments and then applying the correct operation to the values

Lazy evaluation is to start by applying the operation and evaluate the arguments only as/if you need them

Question 5

Two models of variables

Answer 5

A variable points to a value

A variable points to a location in the heap containing a value (the variable being a pointer and all…)

Question 6

What are l and r expressions?

Answer 6

l-expressions denote locations, r-expressions are values storable in those locations

Question 7

An assignment is executed by…

Answer 7

evaluating the LHS to an l-value
evaluating the RHS to an r-value
overwriting the content of the l-value with the r-value

Question 8

What are the basic structured forms of control of a high-level imperative language?

Answer 8

Sequencing
Conditionals
Loops

Question 9

Recursion is an alternative to loops for making a programming language…

Answer 9

Turing-complete

Question 10

A function call is tail recursive if…

Answer 10

a call to function g in a function f is tail recursive if f returns the result of g without performing any further computation

A function is tail recursive if all calls in it are tail calls

A call to a tail recursive function only needs a single frame

Question 11

What is a subprogram?

Answer 11

A function : meant to return a value and typically does not change the outside environment

A procedure: not meant to return a value, but changes the outside environment

Question 12

Describe Call By Value

Answer 12

• input only communication
• parameter can be any expression
• On the call, the parameter is evaluated to an r-value that is associated to the formal parameter
• On the return, the formal parameter is destroyed.

Question 13

Describe Call By Reference

Answer 13

• gives bidirectional communication
• The parameter must be an l-expression
• On the call it is evaluated to an l-value and that is associated to the formal parameter, aliasing it
• On the return. the association of the formal parameter to this l-value is destroyed, but the l-value remains

Question 14

Describe Call By Constant

Answer 14

• If the formal parameter is not modified in the body of the procedure, then call by value can be implemented by call by reference
• The l-value of the actual parameter is passed to the procedure, the r-value is not, it is not copied to the frame of the call
• Semantically this is just call by value but implemented differently

Question 15

Describe Call By Result

Answer 15

• Output only communication
• The actual parameter must be an l-expression, on the call it is evaluated to an l-value.
• on the return the current r-value of the formal parameter is stored in this l-value

Question 16

Describe Call By Value-Result

Answer 16

• Combination of by value and result, gives bidirectional communication
• The actual parameter must be an l-expression. On the call it is evaluated to an l-value and further to an r-value. The r-value is associated to the formal parameter
• Upon return, the final r-value of the formal parameter is stored in the l-value of the actual parameter

Question 17

Describe Higher-order functions

Answer 17

Takes functions as parameters or returns a function

Question 18

Higher-order functions deep binding vs shallow binding

Answer 18

Deep binding: Use the environment active when the link between h and f is established

Shallow binding: Use the environment active when the call to f occurs

Question 19

What happens when an exception is raised?

Answer 19

Active blocks are exited until the closest protected block is found trapping this exception

The handler statically linked to this protected block is executed and the protected block is exited

Exceptions are propagated out along the dynamic chain