- Concepts of Programming Languages

Strict and Nonstrict Evaluation

Instructor:

Learning Objectives

How much of an expression should be evaluated when computing a result?

• Perform expression evaluation
• Perform function calls and function argument evaluation

Strict and Nonstrict Operators

• A strict construct evaluates all of its operands before it runs

Conditional Expression

Conditional expression if e1 then e2 else e3

• Evaluate e1; if true then evaluate e2, else evaluate e3

for i <- 0 to 10 do
  print(i)
  if i % 2 == 0
  then println(": even")
  else println(": odd")
end for

Conditionals in C

• Conditional statement: executes return 1 or return n*..., never both

int fact (int n) {
  if (n <= 1) {
    return 1;
  } else {
    return n * fact (n - 1);
  }
}

int fact (int n) {
  return (n <= 1) ? 1 : n * fact (n - 1);
}

What Happens?

• Function calls are strict

def f(b: Boolean, t: Unit, f: Unit) : Unit =
  if b then t else f
for i <- 0 to 10 do
  print(i)
  f(i % 2 == 0,
    println(": even"),
    println(": odd"))
end for

• Both "even" and "odd" are printed every time f is invoked

What Happens?

• Thunks: pass-by-name for explicit nonstrict evaluation of function arguments

def f(b: Boolean, t: => Unit, f: => Unit) : Unit =
  if b then t else f
for i <- 0 to 10 do
  print(i)
  f(i % 2 == 0,
    println(": even"),
    println(": odd"))
end for

• "even" and "odd" are printed again alternating

Summary