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 MLtondevel@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 higherorder 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 nondatatypes 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.