doc/guide/src/DefineTypeBeforeUse.adoc

   1 DefineTypeBeforeUse
   2 ===================
   3
   4 <:StandardML:Standard ML> requires types to be defined before they are
   5 used.  Because of type inference, the use of a type can be implicit;
   6 hence, this requirement is more subtle than it might appear.  For
   7 example, the following program is not type correct, because the type
   8 of `r` is `t option ref`, but `t` is defined after `r`.
   9
  10 [source,sml]
  11 ----
  12 val r = ref NONE
  13 datatype t = A | B
  14 val () = r := SOME A
  15 ----
  16
  17 MLton reports the following error, indicating that the type defined on
  18 line 2 is used on line 1.
  19
  20 ----
  21 Error: z.sml 3.10-3.20.
  22   Function applied to incorrect argument.
  23     expects: _ * [???] option
  24     but got: _ * [t] option
  25     in: := (r, SOME A)
  26     note: type would escape its scope: t
  27     escape from: z.sml 2.10-2.10
  28     escape to: z.sml 1.1-1.16
  29 Warning: z.sml 1.5-1.5.
  30   Type of variable was not inferred and could not be generalized: r.
  31     type: ??? option ref
  32     in: val r = ref NONE
  33 ----
  34
  35 While the above example is benign, the following example shows how to
  36 cast an integer to a function by (implicitly) using a type before it
  37 is defined.  In the example, the ref cell `r` is of type
  38 `t option ref`, where `t` is defined _after_ `r`, as a parameter to
  39 functor `F`.
  40
  41 [source,sml]
  42 ----
  43 val r = ref NONE
  44 functor F (type t
  45            val x: t) =
  46    struct
  47       val () = r := SOME x
  48       fun get () = valOf (!r)
  49    end
  50 structure S1 = F (type t = unit -> unit
  51                   val x = fn () => ())
  52 structure S2 = F (type t = int
  53                   val x = 13)
  54 val () = S1.get () ()
  55 ----
  56
  57 MLton reports the following error.
  58
  59 ----
  60 Warning: z.sml 1.5-1.5.
  61   Type of variable was not inferred and could not be generalized: r.
  62     type: ??? option ref
  63     in: val r = ref NONE
  64 Error: z.sml 5.16-5.26.
  65   Function applied to incorrect argument.
  66     expects: _ * [???] option
  67     but got: _ * [t] option
  68     in: := (r, SOME x)
  69     note: type would escape its scope: t
  70     escape from: z.sml 2.17-2.17
  71     escape to: z.sml 1.1-1.16
  72 Warning: z.sml 6.11-6.13.
  73   Type of variable was not inferred and could not be generalized: get.
  74     type: unit -> ???
  75     in: fun get () = (valOf (! r))
  76 Error: z.sml 12.10-12.18.
  77   Function not of arrow type.
  78     function: [unit]
  79     in: (S1.get ()) ()
  80 ----