Patrick, thank you for your response and the link. I will need to read it
more carefully and try to understand as it since it deals with concepts I
am not familiar with. However, wouldn't the solution you proposed restrict
all values of the rec Dict to the same type in any given call. Am I correct
in understanding that they can be of any type, however, they have to be the
same type?
Thank you.
On Wednesday, November 26, 2014 7:17:48 PM UTC-5, Patrick O'Leary wrote:
>
> On Wednesday, November 26, 2014 6:01:56 PM UTC-6, Test This wrote:
>>
>> I have the following function defined to check whether a record exists in
>> a Mongodb database (thanks a million for PyCall, which make it easy to use
>> pymongo
>> to interact with mongodb in julia).
>>
>> function doesrecexist(collectn::PyObject, rec::Dict{ASCIIString,Any})
>> # checks if record exists.
>> r = collectn[:find_one](rec)
>> return r
>> end
>>
>>
>> If I define
>>
>> rec = ["p" => 0.3]
>>
>> julia recognizes it as Dict{ASCIIString, Float64}. Then if I do,
>>
>> doesrecexist(collectn, rec), I get an error saying *ERROR:
>> `doesrecexist` has no method matching doesrecexist(::PyObject,
>> ::Dict{ASCIIString,Float64})*
>>
>> If I remove the type declaration for rec in doesrecexist, things work
>> fine. I have no other method defined for doesrecexist. Does it make sense
>> to get this error, given
>> that I intend to allow any values in the rec Dict by declaring Any? Is
>> there a workaround where I can declare the type for rec when defining the
>> function, without having
>> to define doesrecexist for Float values, Int values, String values etc.
>>
>> Thank you.
>>
>
> You've hit type invariance. In Julia, parametric types are invariant--that
> is, Dict{ASCIIString,Float64} is not a subtype of Dict{ASCIIString,Any}.
> However, you can use generic types instead:
>
> function doesrecexist{T}(collectn::PyObject, rec::Dict{ASCIIString, T})
> ...
> end
>
> Which will match for all T. You can also restrict T to subtypes of a given
> type in the curly-braced section. For instance, if you wanted to guarantee
> that the type T was a subtype of Real:
>
> function doesrecexist{T<:Real}(...)
> ...
> end
>
> For more information on why we use invariant parametric types, search the
> list for "parametric invariant" or similar; there have been a few
> discussions on the topic.
>
> Patrick
>
