[julia-users] Re: Help understanding ASCIIString vs String

2015-04-23 Thread Test This 
Thank you, Avik. I appreciate your help. 

On Wednesday, April 22, 2015 at 1:34:51 PM UTC-4, Avik Sengupta wrote:


 The answer to your first question is easy. subtypes() only display the 
 direct subtypes. ASCIIString is a subtype of DirectIndexString, which in 
 turn is a subtype of String

 julia subtypes(String)
 8-element Array{Any,1}:
  Base.GenericString  
  DirectIndexString   
  RepString   
  RevString{T:AbstractString}
  RopeString  
  SubString{T:AbstractString}
  UTF16String 
  UTF8String  

 julia subtypes(DirectIndexString)
 2-element Array{Any,1}:
  ASCIIString
  UTF32String

 The answer to your second question is more contentious, though a valid 
 answer might be that this is simply a design choice. A generic discussion 
 is here: 
 http://en.wikipedia.org/wiki/Covariance_and_contravariance_%28computer_science%29
  
 and there are various discussions in the mailing list about why this choice 
 has been made in Julia. 

 Regards
 -
 Avik

 On Wednesday, 22 April 2015 18:20:46 UTC+1, Test This wrote:

 Avik and Patrick,

 Thanks to both of you for clarifying this and for the alternative. 

 I have a couple of related questions:

 - Why is ASCIIString not listed when I do subtypes(String)?
 - Why is Dict{ASCIIString, Int} not a subtype of Dict{String, Int}, if 
 ASCIIString is a subtype of String?

 Anyhow, for now, your answers and links are very helpful.




 On Wednesday, April 22, 2015 at 10:38:27 AM UTC-4, Patrick O'Leary wrote:

 (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.

[julia-users] Re: Help understanding ASCIIString vs String

2015-04-22 Thread Patrick O'Leary 
(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.

[julia-users] Re: Help understanding ASCIIString vs String

2015-04-22 Thread Test This 
Avik and Patrick,

Thanks to both of you for clarifying this and for the alternative. 

I have a couple of related questions:

- Why is ASCIIString not listed when I do subtypes(String)?
- Why is Dict{ASCIIString, Int} not a subtype of Dict{String, Int}, if 
ASCIIString is a subtype of String?

Anyhow, for now, your answers and links are very helpful.




On Wednesday, April 22, 2015 at 10:38:27 AM UTC-4, Patrick O'Leary wrote:

 (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.