(The crusade continues)
Never fear though, this doesn't mean you have to write more code! Julia
supports the use of type variables to express generics. So in your case,
instead of:
function func(a::Params, b::String, c::Dict{String, Array{Int, 1}},
d::Dict{String, Array{Int, 1}})
...
end
which has the aforementioned invariance problem, you can write
function func{T<:String}(a::Params, b::T, c::Dict{T, Array{Int, 1}},
d::Dict{T, Array{Int, 1}})
...
end
which defines a family of methods for any subtype of String. These methods
are called, in Julia terms, "parametric methods", and are discussed in the
manual here:
http://docs.julialang.org/en/release-0.3/manual/methods/#parametric-methods
Hope this helps,
Patrick
On Wednesday, April 22, 2015 at 8:54:12 AM UTC-5, Avik Sengupta wrote:
>
> So a one line answer to this is julia container types are invariant.
>
> Lets take this step by step
>
> julia> f(x::String) = "I am $x"
> f (generic function with 2 methods)
>
> julia> f("abc")
> "I am abc"
>
> julia> g(x::Dict) = "I am a dict of type: $(typeof(x))"
> g (generic function with 1 method)
>
> julia> g(Dict("abc"=>1))
> "I am a dict of type: Dict{ASCIIString,Int64}"
>
> julia> h(x::Dict{String, Int}) = "I am a dict of {String=>Int}"
> h (generic function with 1 method)
>
> julia> h(Dict("abc"=>1))
> ERROR: MethodError: `h` has no method matching h(::Dict{ASCIIString,Int64})
>
>
> Basically, while an "ASCIIString" is a subtype of the abstract type
> "String" , a Dict{ASCIIString, Int} is not a subtype of Dict{String, Int}
>
> See here for more:
> http://docs.julialang.org/en/release-0.3/manual/types/?highlight=contravariant#man-parametric-types
>
> Regards
> -
> Avik
>
> On Wednesday, 22 April 2015 14:14:06 UTC+1, Test This wrote:
>>
>>
>> I defined a function
>>
>> function func(a::Params, b::String, c::Dict{String, Array{Int, 1}},
>> d::Dict{String, Array{Int, 1}})
>> ...
>> end
>>
>> When I run the program, calling this function with func(paramvalue, "H",
>> d1, d2), I get an error saying func has no method matching
>> (::Params, ::ASCIIString, ::Dict{ASCIIString,Array{Int64,1}},
>> ::Dict{ASCIIString,Array{Int64,1}})
>>
>> The code works if I change String to ASCIIString in the function
>> definition. But I thought (and the REPL seems to agree) that
>> any value of type ASCIIString is also of type String. Then, why am I
>> getting this error.
>>
>> Thanks in advance for your help.
>>
>
