## MLton 20241230+git20251029+dfsg-5

# SMLNJDeviations

Here are some deviations of SML/NJ from The Definition of Standard ML (Revised). Some of these are documented in the SML '97 Conversion Guide. Since MLton does not deviate from the Definition, you should look here if you are having trouble porting a program from MLton to SML/NJ or vice versa. If you discover other deviations of SML/NJ that aren’t listed here, please send mail to `MLton-devel@mlton.org`.

- SML/NJ allows spaces in long identifiers, as in `S . x`. Section 2.5 of the Definition implies that `S . x` should be treated as three separate lexical items.

- SML/NJ allows `op` to appear in `val` specifications:

      signature FOO = sig
         val op + : int * int -> int
      end

  The grammar on page 14 of the Definition does not allow it. Recent versions of SML/NJ do give a warning.

- SML/NJ rejects

      (op *)

  as an unmatched close comment.

- SML/NJ allows `=` to be rebound by the declaration:

      val op = = 13

  This is explicitly forbidden on page 5 of the Definition. Recent versions of SML/NJ do give a warning.

- SML/NJ allows rebinding `true`, `false`, `nil`, `::`, and `ref` by the declarations:

      fun true () = ()
      fun false () = ()
      fun nil () = ()
      fun op :: () = ()
      fun ref () = ()

  This is explicitly forbidden on page 9 of the Definition.

- SML/NJ extends the syntax of the language to allow vector expressions and patterns like the following:

      val v = #[1,2,3]
      val #[x,y,z] = v

  MLton supports vector expressions and patterns with the `allowVectorExpsAndPats` ML Basis annotation.

- SML/NJ extends the syntax of the language to allow or patterns like the following:

      datatype foo = Foo of int | Bar of int
      val (Foo x | Bar x) = Foo 13

  MLton supports or patterns with the `allowOrPats` ML Basis annotation.

- SML/NJ allows higher-order functors, that is, functors can be components of structures and can be passed as functor arguments and returned as functor results. As a consequence, SML/NJ allows abbreviated functor definitions, as in the following:

      signature S =
        sig
          type t
          val x: t
        end
      functor F (structure A: S): S =
        struct
          type t = A.t * A.t
          val x = (A.x, A.x)
        end
      functor G = F

- SML/NJ extends the syntax of the language to allow `functor` and `signature` declarations to occur within the scope of `local` and `structure` declarations.

- SML/NJ allows duplicate type specifications in signatures when the duplicates are introduced by `include`, as in the following:

      signature SIG1 =
         sig
            type t
            type u
         end
      signature SIG2 =
         sig
            type t
            type v
         end
      signature SIG =
         sig
            include SIG1
            include SIG2
         end

  This is disallowed by rule 77 of the Definition.

- SML/NJ allows sharing constraints between type abbreviations in signatures, as in the following:

      signature SIG =
         sig
            type t = int * int
            type u = int * int
            sharing type t = u
         end

  These are disallowed by rule 78 of the Definition. Recent versions of SML/NJ correctly disallow sharing constraints between type abbreviations in signatures.

- SML/NJ disallows multiple `where type` specifications of the same type name, as in the following

      signature S =
        sig
           type t
           type u = t
        end
        where type u = int

  This is allowed by rule 64 of the Definition.

- SML/NJ allows `and` in `sharing` specs in signatures, as in

      signature S =
         sig
            type t
            type u
            type v
            sharing type t = u
            and type u = v
         end

- SML/NJ does not expand the `withtype` derived form as described by the Definition. According to page 55 of the Definition, the type bindings of a `withtype` declaration are substituted simultaneously in the connected datatype. Consider the following program.

      type u = real ;
      datatype a =
          A of t
        | B of u
      withtype u = int
      and t = u

  According to the Definition, it should be expanded to the following.

      type u = real ;
      datatype a =
          A of u
        | B of int ;
      type u = int
      and t = u

  However, SML/NJ expands `withtype` bindings sequentially, meaning that earlier bindings are expanded within later ones. Hence, the above program is expanded to the following.

      type u = real ;
      datatype a =
          A of int
        | B of int ;
      type u = int
      type t = int

- SML/NJ allows `withtype` specifications in signatures.

  MLton supports `withtype` specifications in signatures with the `allowSigWithtype` ML Basis annotation.

- SML/NJ allows a `where` structure specification that is similar to a `where type` specification. For example:

      structure S = struct type t = int end
      signature SIG =
        sig
           structure T : sig type t end
        end where T = S

  This is equivalent to:

      structure S = struct type t = int end
      signature SIG =
        sig
           structure T : sig type t end
        end where type T.t = S.t

  SML/NJ also allows a definitional structure specification that is similar to a definitional type specification. For example:

      structure S = struct type t = int end
      signature SIG =
        sig
           structure T : sig type t end = S
        end

  This is equivalent to the previous examples and to:

      structure S = struct type t = int end
      signature SIG =
        sig
           structure T : sig type t end where type t = S.t
        end

- SML/NJ disallows binding non-datatypes with datatype replication. For example, it rejects the following program that should be allowed according to the Definition.

      type ('a, 'b) t = 'a * 'b
      datatype u = datatype t

  This idiom can be useful when one wants to rename a type without rewriting all the type arguments. For example, the above would have to be written in SML/NJ as follows.

      type ('a, 'b) t = 'a * 'b
      type ('a, 'b) u = ('a, 'b) t

- SML/NJ disallows sharing a structure with one of its substructures. For example, SML/NJ disallows the following.

      signature SIG =
         sig
            structure S:
               sig
                  type t
                  structure T: sig type t end
               end
            sharing S = S.T
         end

  This signature is allowed by the Definition.

- SML/NJ disallows polymorphic generalization of refutable patterns. For example, SML/NJ disallows the following.

      val [x] = [[]]
      val _ = (1 :: x, "one" :: x)

  Recent versions of SML/NJ correctly allow polymorphic generalization of refutable patterns.

- SML/NJ uses an overly restrictive context for type inference. For example, SML/NJ rejects both of the following.

      structure S =
      struct
        val z = (fn x => x) []
        val y = z :: [true] :: nil
      end

      structure S : sig val z : bool list end =
      struct
        val z = (fn x => x) []
      end

  These structures are allowed by the Definition.

## Deviations from the Basis Library Specification

Here are some deviations of SML/NJ from the Basis Library specification.

- SML/NJ exposes the equality of the `vector` type in structures such as `Word8Vector` that abstractly match `MONO_VECTOR`, which says `type vector`, not `eqtype vector`. So, for example, SML/NJ accepts the following program:

      fun f (v: Word8Vector.vector) = v = v

- SML/NJ exposes the equality property of the type `status` in `OS.Process`. This means that programs which directly compare two values of type `status` will work with SML/NJ but not MLton.

- Under SML/NJ on Windows, `OS.Path.validVolume` incorrectly considers absolute empty volumes to be valid. In other words, when the expression

      OS.Path.validVolume { isAbs = true, vol = "" }

  is evaluated by SML/NJ on Windows, the result is `true`. MLton, on the other hand, correctly follows the Basis Library Specification, which states that on Windows, `OS.Path.validVolume` should return `false` whenever `isAbs = true` and `vol = ""`.

  This incorrect behavior causes other `OS.Path` functions to behave differently. For example, when the expression

      OS.Path.toString (OS.Path.fromString "\\usr\\local")

  is evaluated by SML/NJ on Windows, the result is `"\\usr\\local"`, whereas under MLton on Windows, evaluating this expression (correctly) causes an `OS.Path.Path` exception to be raised.