[MLton] debugging

[Matthew Fluet]

>> Oh, there was never an issue with emitting .stabs data, its rather that we 
>> don't have any reasonable data to emit.  We don't carry source variable 
>> location beyond type-checking, and it isn't clear how to carry such info 
>> though all the optimization passes.
>
> Can't you just add a new intrinsic?

Sure, but it is orders of magnitude more work to decide _how_ to handle 
this new primitive in each and every pass of the compiler.


> fun f(x) = let
> val x = 1
> val y = 2
> in
> Debug.info(<file name>,line,[I x,I y]);
> x + y
> end
>
> just treat Debug.info pesmisticlly as a effectful function (yeah, I know that 
> disables optimizations). So long as your compler is semantics preserving 
> (i.e. it's not buggy) you can just reverse engineer the stack by looking at 
> what is passed to the intrinsic functions.

Fine for integers, but what info are you going to generate for:

datatype 'a t = T of 'a state ref
and 'a state =
    Unevaluated of unit -> 'a
   | Evaluating
   | Evaluated of 'a

fun delay th =
   let
     val state = Unevaluated th
     val promise = T (ref state)
   in
     Debug.info(<file name>,line,[??? state,??? promise]);
     promise
   end