[Yhc] What constitutes an 'evaluated node'?

[Thomas Shackell]

Stefan O'Rear wrote:
> I'm sorry, I seem to have horribly mangled my question.  in the present
> implementation:
> 
> ptr1 -----> Some Big
> ptr2 -----> Full PAP
> 
> after EVAL: 
>            /-----------\
> ptr1 -----/ An Ind-     \--V 
> ptr2 -----> irection ---> Value
> 
> after POP_1:
>              An Ind-
> ptr1 ----->  irection ---> Value
> 
> INT_SWITCH examines Value.
> 
> In the implementation I'm trying for, INT_SWITCH would examine the
> indirection, BUT in my implentation EVAL *does* chase indirections,
> so:
> 
> g(ptr1,ptr2) = ptr1 `seq` ptr2
> 
> ptr1 -----> Some Big
> ptr2 -----> Full PAP
> 
> after EVAL: 
>            /-----------\
> ptr1 -----/ An Ind-     \--V 
> ptr2 -----> irection ---> Value
> 
> after POP_1:
>              An Ind-
> ptr1 ----->  irection ---> Value
> 
> after EVAL: (chases the ind):
>              An Ind-
> ptr1 ---\    irection  /-> Value
>          \------------/
> which is returned.  (yes I know about RETURN_EVAL)

I'm affraid I'm still slightly unclear on your question. In the current
implementation:

	g(ptr1,ptr2) = ptr1 `seq` ptr2

Is compiled into something approximating

g(ptr1,ptr2):
	PUSH_ZAP_ARG ptr1
	EVAL
	POP_1
	PUSH_ZAP_ARG ptr2
	RETURN_EVAL	

Executing this we would get something like

g(ptr1,ptr2):
		ptr1 ---> Some Big Thunk
		ptr2 ----------^
	PUSH_ZAP_ARG ptr1
		ptr1 ---------> ZAP
		ptr2 ---------> Some Big thunk
	EVAL
	   // eval doesn't remove indirections from the result		
		ptr2 ---> Ind ------> Value
	POP_1
		ptr2 ---> Ind ------> Value
	PUSH_ZAP_ARG ptr2
		ptr2 ---> ZAP
	RETURN_EVAL
	    // return_eval must eventually finish with 'return',
	    // and 'return' doesn't remove indirections so the result
             // of g will have an indirection in it.

It would certainly be possible to make RETURN remove indirections from
the result - and it might constitute a small optimization.

> Known, known, known.

Appologies, your question seemed to be asking "why do we need
indirections?", though it's clear now that's not what you mean :-)

> Am doing so since about a week ago :)

Ah excellent :-)

Tom