F# string.Format

If you want to avoid using the full name, you can use open in F#:

open System
let s = String.Format("Hello {0}", "world")

This should work in both F# interactive (enter the open clause first) and in normal compiled applications. The key thing is that you must write String with upper-case S. This is because string in C# isn't a usual type name - it is a keyword refering to the System.String type.

Alternatively, you could also take a look at the sprintf function. It is an F#-specific alternative to String.Format which has some nice benefits - for example it is type checked:

let s = sprintf "Hello %s! Number is %d" "world" 42

The compiler will check that the parameters (string and int) match the format specifiers (%s for string and %d for integers). The function also works better in scenarios where you want to use partial function application:

let nums = [ 1 .. 10 ]
let formatted = nums |> List.map (sprintf "number %d")

This will produce a list of strings containing "number 1", "number 2" etc... If you wanted to do this using String.Format, you'd have to explicitly write a lambda function.

the full name of it is:

System.String.Format