F# printf string

Question

Im puzzled

let test = "aString"

let callMe =
    printfn test

Why isn't this working? Throws below error at compile time:

The type 'string' is not compatible with the type 'Printf.TextWriterFormat<'a>'

This works fine:

printfn "aString"

Solution 1

That's because the format parameter is not actually a string. It's TextWriterFormat<'T> and the F# compiler converts the string format into that type. But it doesn't work on string variables, because the compiler can't convert the string to TextWriterFormat<'T> at runtime.

If you want to print the content of the variable, you shouldn't even try to use printfn this way, because the variable could contain format specifications.

You can either use the %s format:

printfn "%s" test

Or use the .Net Console.WriteLine():

Console.WriteLine test

Don't forget to add open System at the top of the file if you want to use the Console class.

Solution 2

In line with what svick said, you might also try this:

let test = "aString"
let callMe = printfn (Printf.TextWriterFormat<_> test)
callMe

Solution 3

In addition to answers below. You may also write like this:

let test = "aString"
let print =
   printfn $"{test}"

Comments

[arash maleki]

I would add that using Console.WriteLine in F# code is not idiomatic, and the printf version is much more common.

[CodeMonkey]

Ah ok makes sense if the compiler converts it i guess. Ended up with printfn "%s" test

[Stephen Swensen]

you can also do stdout.WriteLine test where I guess the only advantage is that it is one char shorter and you don't need to open System

[jpierson]

Makes me wonder why there isn't just a print function that does no formatting. I guess it would be easy to define. let print value = printfn "%s" value

[Jim Balter]

Nope; value restriction.