evaluation order

evaluation order

[Christophe Raffalli]

Hello,

In OCaml-3.11.1 (I did not try other version),
the following code print 0 when compiled in bytecode and 1 in nativecode
for obvious reason of different evaluation order in the pair ...


let ptr = ref 0
let fn b =
  if b then incr ptr else decr ptr
let c = fn true, !ptr
let _ = print_int (snd c); print_newline ()

Is any difference between ocamlc and ocamlopt a bug ?

Cheers,
Christophe

Re: evaluation order

[Rémi Vanicat]

Christophe Raffalli <raffalli@univ-savoie.fr> writes:

> Hello,
>
> In OCaml-3.11.1 (I did not try other version),
> the following code print 0 when compiled in bytecode and 1 in nativecode
> for obvious reason of different evaluation order in the pair ...
>
>
> let ptr = ref 0
> let fn b =
>  if b then incr ptr else decr ptr
> let c = fn true, !ptr
> let _ = print_int (snd c); print_newline ()
>
> Is any difference between ocamlc and ocamlopt a bug ?

Both must follow the specification, but order of evaluation is left
unspecified in ocaml, so this difference is not a bug. The bug is in
your code that depend of an unspecified feature of one of the compiler.
-- 
Rémi Vanicat

Re: [Caml-list] evaluation order

[Jon Harrop]

On Sunday 14 June 2009 17:36:39 Christophe Raffalli wrote:
> Hello,
>
> In OCaml-3.11.1 (I did not try other version),
> the following code print 0 when compiled in bytecode and 1 in nativecode
> for obvious reason of different evaluation order in the pair ...
>
>
> let ptr = ref 0
> let fn b =
>   if b then incr ptr else decr ptr
> let c = fn true, !ptr
> let _ = print_int (snd c); print_newline ()
>
> Is any difference between ocamlc and ocamlopt a bug ?

Not at all, no. They are expected to have different behaviour with respect to 
floating point arithmetic. In this case, they have chosen to evaluate 
subexpressions in a different order, which is fine because the order of 
evaluation is officially undefined.

-- 
Dr Jon Harrop, Flying Frog Consultancy Ltd.
http://www.ffconsultancy.com/?e

Re: [Caml-list] evaluation order

[Christophe Raffalli]

Jon Harrop a écrit :
> On Sunday 14 June 2009 17:36:39 Christophe Raffalli wrote:
>   
>> Hello,
>>
>> In OCaml-3.11.1 (I did not try other version),
>> the following code print 0 when compiled in bytecode and 1 in nativecode
>> for obvious reason of different evaluation order in the pair ...
>>
>>
>> let ptr = ref 0
>> let fn b =
>>   if b then incr ptr else decr ptr
>> let c = fn true, !ptr
>> let _ = print_int (snd c); print_newline ()
>>
>> Is any difference between ocamlc and ocamlopt a bug ?
>>     
>
> Not at all, no. They are expected to have different behaviour with respect to 
> floating point arithmetic. In this case, they have chosen to evaluate 
> subexpressions in a different order, which is fine because the order of 
> evaluation is officially undefined.
>
>   
Despite the fact that it should not be written in OCaml because the 
result rely on the unspecified evaluation order, my example still seems 
to contradict chapter 11 of OCaml's documentation:

"Compatibility with the bytecode compiler is extremely high: the same 
source code should run identically when compiled with ocamlc and ocamlopt."

This kind of bug could also be a nightmare to debug ... because the 
debugger is only available for bytecode ...

Cheers,
Christophe