[MLton] patch for CM cc variable support?

[Tom 7]

May I request this small change to the way that CM works in mlton? Right 
now, there are no defined symbols whatsoever, which (as far as I can tell) 
makes it impossible to use the conditional compilation system to make a CM 
file that works with both mlton and the newest sml/nj.

file mlton/cm/parse.sml line 37:
          -- old --

  	   val lex =
  	      Lexer.lexer {strdef = no,
  			   sigdef = no,
  			   fctdef = no,
  			   fsigdef = no,
  			   symval = fn _ => NONE}
  	      (cmfile, ins)

          -- new --

            fun symval "MLTON" = SOME 1
              | symval _ = NONE

            val lex =
               Lexer.lexer {strdef = no,
                            sigdef = no,
                            fctdef = no,
                            fsigdef = no,
                            symval = symval}
               (cmfile, ins)

With this, I can write CM files like

group is

#if MLTON < 1
  $/basis.cm
#endif

file.sml
etc.sml

that compile with both compilers. (Also, if anyone can think of a way to
achieve this without any change to mlton, I'm all ears!)

It might also make sense to modify line 408 of lexer.sml to return
SOME 0 for undefined symbols, as SML/NJ apparently does. I do understand
the desire to avoid the slippery and infinite slope of CM compatibility
hacks, however!

Thanks as always,

Tom

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