MLton 20210117

# DefineTypeBeforeUse

Standard ML requires types to be defined before they are used. Because of type inference, the use of a type can be implicit; hence, this requirement is more subtle than it might appear. For example, the following program is not type correct, because the type of r is t option ref, but t is defined after r.

    val r = ref NONE
    datatype t = A | B
    val () = r := SOME A

MLton reports the following error, indicating that the type defined on line 2 is used on line 1.

    Error: z.sml 3.10-3.20.
      Function applied to incorrect argument.
        expects: _ * [???] option
        but got: _ * [t] option
        in: := (r, SOME A)
        note: type would escape its scope: t
        escape from: z.sml 2.10-2.10
        escape to: z.sml 1.1-1.16
    Warning: z.sml 1.5-1.5.
      Type of variable was not inferred and could not be generalized: r.
        type: ??? option ref
        in: val r = ref NONE

While the above example is benign, the following example shows how to cast an integer to a function by (implicitly) using a type before it is defined. In the example, the ref cell r is of type t option ref, where t is defined after r, as a parameter to functor F.

    val r = ref NONE
    functor F (type t
               val x: t) =
       struct
          val () = r := SOME x
          fun get () = valOf (!r)
       end
    structure S1 = F (type t = unit -> unit
                      val x = fn () => ())
    structure S2 = F (type t = int
                      val x = 13)
    val () = S1.get () ()

MLton reports the following error.

    Warning: z.sml 1.5-1.5.
      Type of variable was not inferred and could not be generalized: r.
        type: ??? option ref
        in: val r = ref NONE
    Error: z.sml 5.16-5.26.
      Function applied to incorrect argument.
        expects: _ * [???] option
        but got: _ * [t] option
        in: := (r, SOME x)
        note: type would escape its scope: t
        escape from: z.sml 2.17-2.17
        escape to: z.sml 1.1-1.16
    Warning: z.sml 6.11-6.13.
      Type of variable was not inferred and could not be generalized: get.
        type: unit -> ???
        in: fun get () = (valOf (! r))
    Error: z.sml 12.10-12.18.
      Function not of arrow type.
        function: [unit]
        in: (S1.get ()) ()