Make two functions mutually in scope

Question

How do we make two functions mutually aware of each other? For instance, in the following code, how can we make ping and pong call each other?

module PingPong = 

    let pong = 
        printfn "pong"
        ping 
        // error: The value or constructor 'ping' is not defined.

    let ping = 
        printfn "ping"
        pong

    [<EntryPoint>]
    let main argv = 
        ping
        0

Edit:

Since writing the question, we've found that as of F# 4.1 there is now the module rec keyword. Simply using module rec PingPong... though means that on line 3 we receive this new error:

error FS0031: This value will be eventually evaluated as part of its own definition. You may need to make the value lazy or a function. Value 'pong' will evaluate 'ping' will evaluate 'pong'.

Edit:

After doing more research, we learned the difference between values and functions. We were able to make the following work like this:

module rec PingPong = 

    let pong() = 
        printfn "pong"
        ping() 

    let ping () = 
        printfn "ping"
        pong()

    [<EntryPoint>]
    let main _ = 
        ping()
        0

So, as a follow up question for those who might now the answer, how would we have made two functions mutually in scope before F# 4.1 introduced recursive modules?

Taylor Wood · Accepted Answer · 2018-02-23T04:56:33.000

You can make mutually recursive definitions like this:

let pong = 
    printfn "pong"
    ping
and ping = 
    printfn "ping"
    pong

But your lets aren't functions. And you'd need to mark pong as recursive with rec:

let rec pong x = 
    printfn "pong"
    ping (x + 2)
and ping x = 
    printfn "ping"
    pong (x + 1)

This program doesn't really "work", but it shows the concept. Here's something that will eventually finish:

let rec pong x = 
    printfn "pong"
    ping (x - 2)
and ping x = 
    printfn "ping"
    match x with
    | 1 -> 1
    | 2 -> 1
    | x -> pong (x + 1)

> pong 5;;
pong
ping
pong
ping
val it : int = 1

Make two functions mutually in scope

1 Answers1