Here are the places where MLton deviates from The Definition of Standard ML (Revised). In general, MLton complies with the Definition quite closely, typically much more closely than other SML compilers (see, e.g., our list of SML/NJ’s deviations). In fact, the four deviations listed here are the only known deviations, and we have no plans to fix them. If you find a deviation not listed here, please report a Bug.

We don’t plan to fix these bugs because the first (parsing nested cases) has historically never been accepted by any SML compiler, the other three clearly indicate problems in the Definition.

  • MLton does not correctly parse case expressions nested within other matches. For example, the following fails.

    fun f 0 y =
          case x of
             1 => 2
           | _ => 3
      | f _ y = 4
    

    To do this in a program, simply parenthesize the case expression.

    Allowing such expressions, although compliant with the Definition, would be a mistake, since using parentheses is clearer and no SML compiler has ever allowed them. Furthermore, implementing this would require serious yacc grammar rewriting followed by postprocessing.

  • MLton does not raise the Bind exception at run time when evaluating val rec (and fun) declarations that redefine identifiers that previously had constructor status. (By default, MLton does warn at compile time about val rec (and fun) declarations that redefine identifiers that previously had constructors status; see the valrecConstr ML Basis annotation.) For example, the Definition requires the following program to type check, but also (bizarelly) requires it to raise the Bind exception

    val rec NONE = fn () => ()
    

    The Definition’s behavior is obviously an error, a mismatch between the static semantics (rule 26) and the dynamic semantics (rule 126). Given the comments on rule 26 in the Definition, it seems clear that the authors meant for val rec to allow an identifier’s constructor status to be overridden both statically and dynamically. Hence, MLton and most SML compilers follow rule 26, but do not follow rule 126.

  • MLton does not hide the equality aspect of types declared in abstype declarations. So, MLton accepts programs like the following, while the Definition rejects them.

    abstype t = T with end
    val _ = fn (t1, t2 : t) => t1 = t2
    
    abstype t = T with val a = T end
    val _ = a = a
    

    One consequence of this choice is that MLton accepts the following program, in accordance with the Definition.

    abstype t = T with val eq = op = end
    val _ = fn (t1, t2 : t) => eq (t1, t2)
    

    Other implementations will typically reject this program, because they make an early choice for the type of eq to be ''a * ''a -> bool instead of t * t -> bool. The choice is understandable, since the Definition accepts the following program.

    abstype t = T with val eq = op = end
    val _ = eq (1, 2)
    
  • MLton (re-)type checks each functor definition at every corresponding functor application (the compilation technique of defunctorization). One consequence of this implementation is that MLton accepts the following program, while the Definition rejects it.

    functor F (X: sig type t end) = struct
        val f = id id
    end
    structure A = F (struct type t = int end)
    structure B = F (struct type t = bool end)
    val _ = A.f 10
    val _ = B.f "dude"
    

    On the other hand, other implementations will typically reject the following program, while MLton and the Definition accept it.

    functor F (X: sig type t end) = struct
        val f = id id
    end
    structure A = F (struct type t = int end)
    structure B = F (struct type t = bool end)
    val _ = A.f 10
    val _ = B.f false
    

    See DreyerBlume07 for more details.