Printfn

In F# we often write values to the console. We use printfn with multiple arguments to print values—a format string is required. String interpolation syntax is also available.

The first argument to printfn or printf must be a format string. The second argument (a value) is inserted in it and written. Printfn has a trailing newline. Printf does not.

First example

This program uses printfn with various format codes. We introduce a mutable id variable. We print it as an integer (numeric) value with the code "%d."

let mutable id = 10

// Print the variable as an int.
printfn "%d" id

// Print it as a value.
printfn "%A" id

// Use some surrounding text.
printfn "The ID is %A." id

// Modify the id variable and print it again.
id <- 20
printfn "%A" id
10
10
The ID is 10.
20

String interpolation

In modern F# programs the string interpolation syntax may be the best choice for printing values. We use it with printfn and the "$" character.

let value = "bird"

// Use string interpolation to display the value.
printfn $"The value: {value}"
printfn $"The value uppercase: {value.ToUpper()}"
The value: bird
The value uppercase: BIRD

List, dict

With the "%A" code we can print entire lists or dicts in a single statement. No for-in loop is needed to write all the elements. This makes programs easier to develop.

let codes = [10; 20; 30]
let data = ["cat", 10; "frog", 40; "rat", 100]

// Use printfn on list and dict types.
// ... All included values are printed to the console.
printfn "%A" codes
printfn "%A" data
[10; 20; 30]
[("cat", 10); ("frog", 40); ("rat", 100)]

Printf, no newlines

This method does not add a trailing newline. So we can use it to write multiple values to a single line in a for-in loop. We print values as strings with the "%s" code.

let values = ["parrot"; "finch"; "bluebird"]

// Use printf on values in a list.
// ... No newline is inserted after the values.
for v in values do
    printf "%s... " v
parrot... finch... bluebird...

Sprintf

Sometimes we do not want to write to the console. With sprintf we can insert values into a string, and use that elsewhere in the program.

// Call sprintf to write values to a string.
let result = sprintf "Result: %A, %A" 10 20

// Now print our string.
printfn "%A" result
"Result: 10, 20"

Parenthesis

When calling a method in a printfn statement, we need to surround it with parentheses. This treats the method as something to be evaluated (a value).

With printfn and printf, we write values to the screen (the console). This helps us develop small and readable F# programs. Format codes help simplify the output.