Re: [Yhc] CAF vs 0-arity function question

12 May 2006

      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
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