Printfn value is not a function and cannot be applied in F#'s interactive window

Question

This is probably something simple, but I have a .fs file with a simple sample function.

let printHello = printfn "%A" "Hello"

I have set the search path to the .fs file using

>#I "PathToMyFSFile"

I have loaded my file using

>#load "Test.fs"

Which worked fine. Now I want to call the function which actually prints the hello to screen, but thats turning out to be too difficult

> Test.printHello;;
val it : unit = ()

Tried Test.printHello();; as well but doesn't work. How do I actually make it print "Hello" to screen?

Answer 1

your current printHello isn't actually a function. To make it a function you need to do

let printHello() = printfn "%A" "Hello"

noice the (). Then everything should work.

EDIT:

When the compiler sees your definition

let printHello = printfn "%A" "Hello"

it passes it as a simple data term. For example, consider this program:

let printHello = printfn "%A" "Hello"
printfn "World"
printHello

This will print "Hello" then "World". printHello just has unit type, so does nothing. Compare it to

let printHello() = printfn "%A" "Hello"
printfn "World"
printHello()

Here printHello is a function. In this case, the function is only executed when it is explicitly called, so this prints "World" then "Hello".