(drop)CommonPrefix: ccs call implementation

[Maarten Faddegon]

Hi Simon,

Thank you for the explanation, I begin to understand the reasoning 
behind your definition now. I have another question though:

If inlining sets the call site's stack as prefix, I would say that the 
following definition:

	call sApp sLam = sApp ++ sLam'
	  | sApp `isPrefixOf` sLam = sLam
	  | otherwise              = sApp ++ sLam

Gives the same result as your definition:

	call sApp sLam = sApp ++ sLam'
	  where (sPre,sApp',sLam') = commonPrefix sApp sLam

Is that correct or are there situation where sApp' is non-empty?

Cheers,

Maarten


On 06/06/14 10:39, Simon Marlow wrote:
> The motivation for this design is to make it so that
>
>    let
>      f = \x . E
>    in
>      ... f y ...
>
> Gives the same call stack as
>
>      ... (\x . E) y ...
>
> That is, inlining a function does not change the call stack.  GHC
> assumes that this is valid, so the call stack semantics should reflect
> that.
>
> Intuitively it works like this: in the first version, the CCS attached
> to the closure for f is some prefix of the CCS that will be in force at
> the call site, and in that case we don't want to push anything on the
> stack for the call.  However, we might have:
>
>    let
>      f = scc "f" (\x . E)
>    in
>      ... f y ...
>
> and in this case we want to push "f" on the stack.
>
> I don't claim that this always works (recursion in particular causes
> problems) but it behaves in a sensible way in most cases, and it was the
> best formulation I was able to find.
>
> It remains to be seen whether there's a semantics that allows the
> inlining equivalence to be formally proven; if you can do that I think
> it would be a significant step forwards.
>
> Cheers,
> Simon
>
>
> On 05/06/2014 09:57, Ömer Sinan Ağacan wrote:
>> Hi Maarten,
>>
>> Where did you find that slides? I have slides for same talk(also
>> attached) but mine has a different definition for call: (on page 39)
>>
>>    call sapp slam = sapp ++ slam'
>>      where (spre, sapp', slam') = commonPrefix sapp slam
>>
>> This one works same with the implementation.
>>
>> ---
>> Ömer Sinan Ağacan
>> http://osa1.net
>>
>>
>>
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