CSC 347 - Concepts of Programming Languages

Strict and Nonstrict Evaluation

Instructor: Stefan Mitsch

Learning Objectives

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

• Understand expression evaluation
• Understand function argument evaluation

Strict and Nonstrict Operators

• A strict construct evaluates all of its operands before it runs
• Name some strict constructs
  • Arithmetic operators: +, -, *, /, ...
  • Comparison operators: <, <=, ==, !=, ...
  • Bitwise operators: |, &, ~, ^
  • Function calls: f(x,y)

Strict and Nonstrict Operators

• A strict construct evaluates all of its operands before it runs
• Name some non-strict constructs
  • e1 && e2 is strict in e1, but not e2, useful e.g., p!=null && p.m()
  • e1 || e2 is strict in e1, but not e2, useful e.g., p==null || p.m()
  • if e1 then e2 else e3 is strict in e1, but not e2 or e3

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
  var x = 0
  var y = 0
  val z = (if scala.util.Random.nextInt % 2 == 0 
           then {x=1; 111} 
           else {y=2; 222})
  println(s"x=$x, y=$y, z=$z")

Conditionals in C

• Conditional statement vs conditional expression

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 fcond(b: Boolean, t: Int, f: Int) : Int = 
  if b then t else f
for i <- 1 to 10 do
  var x = 0
  var y = 0
  val z = fcond(scala.util.Random.nextInt % 2 == 0,
                {x=1; 111},
                {y=2; 222})
  println(s"x=$x, y=$y, z=$z")

What Happens?

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

def fcond(b: Boolean, t: => Int, f: => Int) : Int = 
  if b then t else f
for i <- 1 to 10 do
  var x = 0
  var y = 0
  val z = fcond(scala.util.Random.nextInt % 2 == 0,
                {x=1; 111},
                {y=2; 222})
  println(s"x=$x, y=$y, z=$z")

Summary

• Strict evaluation: evaluates all operands when determining the value of an expression
  • Arithmetic operators etc.
  • Function arguments
• Nonstrict evaluation: evaluates only necessary operands when determining the value of an expression
  • Conditional expression
  • Shortcut evaluation of boolean expressions
• Thunks: request explicit nonstrict evaluation of function arguments