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smlmlthunk

Are there multiple ways of writing a thunk in SML?

Basic noob question:

I have data I need to evaluate lazily, the most common way of doing this is by making a thunks; the way I'm used to do it is like this:

fun someFunc () = 
  let
    fun myThunk () = 2 + 2
  in
    (* body *)
  end

But every guide on Standard ML tells me that it's done like this:

fun someFunc () = 
  let
    val myThunk = fn () => 2 + 2
  in
    (* body *)
  end

So my questions are: Does it really matter? Isn't the output the same anyway? And what is the difference between making a function, and binding a lambda to a value anyway?

Solution

  • There are no noticeable difference in your case. The fun keyword is syntactic sugar for val rec, which makes sure that you can reference the name that you are binding your function to in a recursive manner.

    Thus 

    fun myThunk () = 2 + 2

val rec myThunk = fn () => 2 + 2

val myThunk = fn () => 2 + 2

    will all yield the same result.