Use conditional logic to create multiple execution paths
Let's say you have a program that creates a backup file based on the files in a folder. The program consists of a few lines of code that locate and compress a set of files. What if you decide that you want to add logic that creates a backup file only if a backup hasn't been made that day? The program would need a way to check for that condition (has a backup already been made?), and then do something different based on whether the condition is true (if a backup file for the day exists, don't run backup).
Create a code branch by using an if construct
An if construct evaluates a Boolean expression. If the expression evaluates as true, the program runs a particular branch of code. Here's what the syntax of an if construct looks like:
if <condition> then <expression>
A more tangible code example looks like this example:
let age = 66
if age > 65
then printfn "Senior citizen"
Define an alternate branch by using an else clause
When an if statement evaluates as false, use an else clause with the if construct to run a different branch of code.
Here's how you can use an else clause:
let age = 66
if age > 65
then printfn "Senior citizen"
else printfn "Citizen"
Use an if construct as an expression
So far, you've seen how you can use if and else to create different branches in the code. When the code follows different branches, it produces different results based on whether a Boolean condition evaluates as true or false. What you might not be aware of, is that when an expression is run, a value is produced.
If the code is written like this example, the result becomes clear:
let age = 64
let message = if age > 65 then "Senior citizen" else "Citizen"
printfn "%s" message
In this case, if and else work more like a ternary expression. If a condition is true, it returns one value. If it's false, it returns the other value. In either case, a value is returned. A fact that's true for all if expressions.
Let's revisit an earlier code example and look behind the scenes:
let age = 66
if age > 65 then printfn "Senior citizen" else printfn "Citizen"
It's the same code example you saw earlier, but now, if and else are on the same line. Because the important actions of the printf operation occur in the function, the function doesn't have to return an actual value. Therefore, the return value of the printfn "Senior citizen" expression produces a value of type unit.
Note
unit is a value that acts as a placeholder when no actual value is returned. You can think of it like void or None.
Another thing to keep in mind is that you can't mix types. if and else must return the same data type or you'll get an error. For example, you can't write code that looks like this example:
let someCondition = true
let value = if someCondition then "hello" else 1 // not allowed
In this case, if determines that the return type is string, so else can't change that type to int. For the preceding code to work, you must modify it:
let someCondition = true
let value = if someCondition then "hello" else "1" // allowed
Use an elif construct
So far, you've seen how if and else work. elif is another construct that you can use similar to the way you'd use if. Just like an if construct, the elif construct takes a Boolean value and should be added right after if. The elif construct is executed when the if condition evaluates as false. Here's an example of how to use elif:
let cardValue = 1
let cardDescription = if cardValue = 1 then "Ace" elif cardValue = 14 then "Ace" else "A card"
printfn "%s" cardDescription