On May 12, 2006, at 5:52 AM, Tom Shackell wrote:

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.

Having looked at some bytecode myself, it looks like the FUN0 is just a shorthand for creating a partial application; you can use the PUSH_CONST instruction instead of the MK_PAP instruction (which takes two arguments). That is perhaps a slight advantage. It doesn't mean "0 arity function" it seems to mean "a partial application with no arguments applied". Con0 is similar: the generated bytecode uses a PUSH_CONST instruction rather than a MK_CON. Here, however, it seems to actually only do this for 0 arity constructors. I'm not sure why one would prefer that in this case, however....

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@haskell.org http://www.haskell.org//mailman/listinfo/yhc

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