[Aldor-l] Generators

[Laurentiu Dragan]

Martin Rubey wrote:
> Peter Broadbery <p_broadbery at hotmail.com> writes:
> 
>>> I don't know whether we said it in tha AUG, but it is intended that you
>>> cannot assign to loop variables inside the loop.
>>>
>>> Consequently they can be used in type expressions and having dependent
>>> crosses as "for" indices should be allowed.
> 
> Oh, that's interesting.  So 
> 
>         for i in g repeat {
>                 (a: MachineInteger, b: SmallPrimeField(a)) == i;
>                 stdout << a << newline;
>                 stdout << b << newline;
>         }
> 
> should be OK?  But it doesn't work with -DDEBUG...
> 
>> Unfortunately, the compiler isn't keen on them being used in types.  
>>
>> "bug2.as", line 10:        (x, 0$SPF(x)) for x in 1..10;
>>                     .........................^
>> [L10 C26] #1 (Error) `x' is used in a type, so must be constant, and so cannot be assigned to.
>>
>> This seems to be built into scobind.c. I'll hit it with a stick & see what
>> happens..
> 
> That would be great!
> 
> Another mistery, at least to me:  Bill sent this snippet:
> 
>        stdout << "test dependent:" << newline;
>        for (a:MachineInteger, b:SmallPrimeField a) in g repeat {
>                stdout << a << " " << b << newline;
>        }
> 
> root at sage:~/aldor-src/aldor/install/aldor# aldor -fx  -lalgebra
> -laldor crossiterator.as
> "crossiterator.as", line 63:
>        for (a:MachineInteger, b:SmallPrimeField a) in g repeat {
> ............^
> [L63 C13] #1 (Error) `a' is used in a type, so must be constant, and
> so cannot be assigned to.
> 
> Any idea why it's C13 here and C26 above?

For type safety, types must constant. By allowing types to change at 
runtime, a variable could have different types at runtime which makes 
type checking at compile time impossible.

--
Laurentiu