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.
Format codes. 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."
Next We use the "%A" code, which inserts any value. It does not require a specific type of argument.
Also We can use some surrounding text in our printfn statement. The variable is inserted into this string.
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" id10
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... " vparrot... 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.
Dot Net Perls is a collection of tested code examples. Pages are continually updated to stay current, with code correctness a top priority.
Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.
This page was last updated on Nov 23, 2023 (new example).
