[Yhc] CAF vs 0-arity function question

[Tom Shackell]

Upon further inspection of the code it seems I really don't remember 
what the constant were for :-)

The difference is probably best illustrated by an example:

    f :: Int
    f = 2 + 2

    g :: Int -> Int
    g x = f

Here g would be compiled to:

function g(x):
    PUSH_CONST 0
    RETURN_EVAL

constants
    0.         CAF(A)        f
  
Here f is a CAF constant because f is a CAF. However in the next example ...

    f :: Int -> Int
    f x = 2 + 2

    g :: Int -> Int
    g = f

In this case g would be compiled to:

function g():
    PUSH_CONST 0
    RETURN

constants
    0.         FUN0(0)         f

Here f is a FUN0 because although f is not a CAF (it takes more than one 
argument) we want the object that represents the currying
of the function f that does take no arguments.

So in some sense Simon is right, FUN0 is sort of like a non-updatable 
CAF, although if my memory serves me correctly the no-argument currying of
a function that does take arguments is not defined as a CAF.

However, the distinction is still academic, the runtime treats A, Z, and 
0 as being exactly the same .

I shall update the wiki to make it clearer ...

thanks :-)


Tom


Tom Shackell wrote:

> Hi Rob,
>
> The constant table item constants are somewhat of a legacy. The 
> original constants were chosen to correspond to nhc's constants, 
> however as far as the Yhc runtime is concerned:
>
> - A and Z are simply references to heap nodes and are treated in 
> exactly the same way.
> - F, 0, C, P, X are all references to Info structures and are also 
> treated in exactly the same way.
>
> However, you are quite right, looking at the C code 0 is mistakenly 
> included with the A&Z code. This has likely not proved a problem 
> because '0' is infact entirely redundant. The only thing you could do 
> with a 0-arity FInfo is make an application to it, but why would you 
> want to when you can just push the CAF instead?
>
> Ultimately we should tidy up the constants to a more simple
>
> - Some constant value (i, l, f, d, s)
> - References to heap nodes (N)
> - References to FInfo or CInfo (I)
>
> For the moment I shall change the C code to make using '0' an error :-)
>
> Thanks
>
> Tom
>
>
>
> Robert Dockins wrote:
>
>> The HBC bytecode format has different constant tags for the folloing:
>> 1) CAF, tag 'A'
>> 2) 0-arity function, tag '0'
>>
>> http://haskell.org/haskellwiki/Yhc/RTS/hbc
>>
>> Why the distinction?  Maybe I don't fully understand, but I thought 
>> that a 0-arity function _was_ a CAF?
>>
>> The runtime seems to treat them very much the same (although I can't 
>> be quite sure -- reading C gives me a headache ;)
>>
>>
>> Rob Dockins
>> _______________________________________________
>> Yhc mailing list
>> Yhc at haskell.org
>> http://www.haskell.org//mailman/listinfo/yhc
>>  
>>
>
> _______________________________________________
> Yhc mailing list
> Yhc at haskell.org
> http://www.haskell.org//mailman/listinfo/yhc