Conditional Expressions: if...then...else

The if...then...else expression runs different branches of code and also evaluates to a different value depending on the Boolean expression given.


if boolean-expression then expression1 [ else expression2 ]


In the previous syntax, expression1 runs when the Boolean expression evaluates to true; otherwise, expression2 runs.

Unlike in other languages, the if...then...else construct is an expression, not a statement. That means that it produces a value, which is the value of the last expression in the branch that executes. The types of the values produced in each branch must match. If there is no explicit else branch, its type is unit. Therefore, if the type of the then branch is any type other than unit, there must be an else branch with the same return type. When chaining if...then...else expressions together, you can use the keyword elif instead of else if; they are equivalent.


The following example illustrates how to use the if...then...else expression.

let test x y =
  if x = y then "equals"
  elif x < y then "is less than"
  else "is greater than"

printfn "%d %s %d." 10 (test 10 20) 20

printfn "What is your name? "
let nameString = System.Console.ReadLine()

printfn "What is your age? "
let ageString = System.Console.ReadLine()
let age = System.Int32.Parse(ageString)

if age < 10
then printfn "You are only %d years old and already learning F#? Wow!" age
10 is less than 20
What is your name? John
How old are you? 9
You are only 9 years old and already learning F#? Wow!

See also