Partially apply a function and dynamically call that function at runtime

Question

Using F#, if I partially apply a function like this:

let sleep x =  Async.Sleep x |> Async.RunSynchronously
let log date message= printfn "%s %s" date message
let getDate = DateTime.Now.ToString()
let logg = log getDate

logg "First"
sleep 1000
logg "Second"
sleep 1000

Prints:

30/11/2018 19:54:25 First
30/11/2018 19:54:25 Second

The call to getDate appears to be translated to an actual date in the new partial function it produces. Why is this, and is it possible to make it call getDate on each call to this log function?

Of interest: [What does this '()' notation mean?](https://stackoverflow.com/a/17872004/1243762) — Guy Coder, Nov 30 '18 at 19:46
cheers @GuyCoder so I have to declare all methods in the chain that use getDate with () after them, and call them accordingly to make them reevaluate when called, otherwise they become fixed variables. Is that right? — gmn, Nov 30 '18 at 19:53

Alexey Romanov · Accepted Answer · 2018-11-30T20:30:25.323

4

so I have to declare all methods in the chain that use getDate with () after them, and call them accordingly to make them reevaluate when called, otherwise they become fixed variables. Is that right?

I'd put it the other way around: let always binds a value, but in let getDate () or let sleep x case those values happen to be functions; they are basically equivalent to

let sleep = fun x -> Async.Sleep x |> Async.RunSynchronously

let getDate = fun () -> DateTime.Now.ToString()

Of course, this isn't really what happens if you examine bytecode, but "morally" it does. With these definitions

let sleep = fun x -> Async.Sleep x |> Async.RunSynchronously
let log = fun date message -> printfn "%s %s" date message
let getDate = fun () -> DateTime.Now.ToString()
let logg = fun x -> log (getDate ()) x

logg "First"
sleep 1000
logg "Second"
sleep 1000

you'll get the same output:

30/11/2018 19:59:40 First
30/11/2018 19:59:41 Second

edited Nov 30 '18 at 20:30

answered Nov 30 '18 at 20:03

Alexey Romanov

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I take it you did a copy and paste of his answer with the time. Time examples never come out the same twice and cause you think about them when doing unit testing. – Guy Coder Nov 30 '18 at 20:35
Yes, of course. I considered mentioning "up to a different timestamp" but decided this was obvious enough (as was dependence on the locale). – Alexey Romanov Nov 30 '18 at 20:42
1

You never know if someone is hiding a time machine in the garage. – Guy Coder Nov 30 '18 at 21:20

Partially apply a function and dynamically call that function at runtime

1 Answers1