`@StefanKarpinski`?
On Tuesday, March 22, 2016 at 5:10:03 PM UTC-4, Jeffrey Sarnoff wrote:
>
> afaik, Julia does elegant -- but I have no insight into the situation at
> hand; tossing this to `@stefankarpinski`
>
> On Tuesday, March 22, 2016 at 3:52:23 PM UTC-4, Bill Hart wrote:
>>
>> Thanks, that looks like an elegant solution if we can make it work.
>> Unfortunately it doesn't.
>>
>> I added the following function to Nemo:
>>
>> function promote_rule_exists{T1, T2}(::Type{T1}, ::Type{T2})
>> return Union{} != Base.promote_rule(T1, T2)
>> end
>>
>> But when I do
>>
>> Nemo.promote_rule_exists(Nemo.Poly{Nemo.fmpz_poly}, Nemo.fmpz)
>>
>> after the relevant promote rule has been created, e.g. by
>>
>> using Nemo
>> R, x = PolynomialRing(ZZ, "x")
>> S, y = PolynomialRing(R, "y")
>>
>> one of the side-effects of which is to create the relevant promote_rule,
>> the function returns false.
>>
>> However, if I create the promote_rule_exists function in the REPL and
>> call it instead, it returns true.
>>
>> I have no idea why it would return a different value depending whether it
>> is inside a module or not.
>>
>> Any ideas?
>>
>> Bill.
>>
>>
>>
>> On Tuesday, 22 March 2016 20:03:49 UTC+1, Jeffrey Sarnoff wrote:
>>>
>>> This checks whether a specific promote rule exists:
>>>
>>> promotionExists{T1, T2}( ::Type{T1}, ::Type{T2} ) = (Union{} !=
>>> promote_rule(T1,T2))
>>>
>>>
>>>
>>> On Tuesday, March 22, 2016 at 11:37:56 AM UTC-4, Bill Hart wrote:
>>>>
>>>> I'm having trouble understanding the following behaviour
>>>> in 0.5.0-dev+3171. I wonder if someone can tell me what I'm doing wrong.
>>>>
>>>> module Mymod
>>>> type mytype
>>>> end
>>>> end
>>>>
>>>> sig_table = [x.sig for x in methods(Base.promote_rule)]
>>>>
>>>> V = Tuple{typeof(Base.promote_rule),Type{Mymod.mytype},Type{Int64}}
>>>>
>>>> V in sig_table # returns true!!
>>>>
>>>> for s in sig_table # prints yes
>>>> if V == s
>>>> println("yes")
>>>> end
>>>> end
>>>>
>>>> for s in sig_table # prints nothing
>>>> if s == V
>>>> println("yes")
>>>> end
>>>> end
>>>>
>>>> Can someone explain what the difference between == and "in" is. For
>>>> example, why shouldn't == be symmetric? And why should "in" tell me
>>>> something is in an array that is clearly not in there?
>>>>
>>>> Metaquestion: what is the easiest way of checking if a promote_rule
>>>> already exists? We have to create promote_rules at run time in response to
>>>> user input (so it can't be done statically) and now the Julia compiler
>>>> complains with pages of warnings because we are overwriting existing
>>>> promote rules (actually, we are, harmlessly). We want to get rid of the
>>>> warnings and the easiest way is to check if that promote rule already
>>>> exists before defining it again.
>>>>
>>>> We can't just do method_exists because it always returns true for
>>>> promote_rule, with any signature. So we need to check whether the promote
>>>> rule with the precise signature we want to define already exists. For
>>>> example
>>>>
>>>> method_exists(Base.promote_rule, Tuple{Type{Mymod.mytype}, Type{Int}})
>>>>
>>>> returns true.
>>>>
>>>> Bill.
>>>>
>>>
