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
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
John 910 is less than 20 You are only 9 years old and already learning F#? Wow!