Formatting strings with F#

Noor30 Sept

I'm building an F# library for the first time

Using string.Format gives me a complaint saying that the function doesn't exist.

Have I made a mistake, or is this unavailable?

The conversation continues 2

Follow the reasoning

fen_ko30 Sept Permalink
its fully qualified name is:
```
System.String.Format
```
Oskar30 Sept Permalink
To skip spelling out the fully qualified name, F# lets you use open:
```
open System
let greeting = String.Format("Hello {0}", "world")
```
That works in an ordinary compiled application as well as F# interactive (submit the open declaration beforehand). What's important here is spelling String with a capital S. The reson is that C#'s string isn't an ordinary name for a type - it's a keyword referring to System.String.

Another option worth examining is sprintf. This F# alternative to String.Format offers useful advantages - one being that it is checked for correct types:
```
let greeting = sprintf "Hello %s! Number is %d" "world" 56
```
The compiler verifies that the arguments (string and int) agree with their format markers (%s means string, while %d means integers). Partial function application is another situation in which this function fits more naturally:
```
let values = [ 3 .. 12 ]
let descriptions = values |> List.map (sprintf "number %d")
```
The result is a list of strings holding "number 3", "number 4" and so on... Achieving the equivalent with String.Format would require writing out a lambda function.