F# Order of Execution using CPS/Partial Application

Question

Why does this print "DIV/0" first and "2" second?

let printZero = printfn "DIV/0"
let printSuccess x = printfn "%d" x

let div ifZero success x y =
    if y = 0
    then ifZero
    else x / y |> success

let printDiv = div printZero printSuccess
printDiv 8 4
printDiv 10 0

Answer 1

printfn "DIV/0" will write to the console immediately, returning unit. So this line:

let printZero = printfn "DIV/0"

...will immediately write DIV/0 and binds unit to the value printZero. Later when you call your div function with y = 0, you just return that value.

What you want is for printZero to be a function. As that function doesn't need any value as input, you can use unit here too (represented as ()) - so you have a function of type unit -> unit:

let printZero() = printfn "DIV/0"
let printSuccess x = printfn "%d" x

let div ifZero success x y =
    if y = 0
    then ifZero()
    else x / y |> success