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.
>>
>
