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
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Rob Dockins
Speak softly and drive a Sherman tank.
Laugh hard; it's a long way to the bank.
-- TMBG
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