On Fri, May 29, 2015 at 2:12 PM, andrew cooke <[email protected]> wrote:
> then Julia needs a Maybe type as well?
Note that different from `Nullable{T}`, which is a type by itself,
`Maybe{T}` as proposed in the issue and you email, is just a type
alias. In this sense, it is already defined (just write
`Union(Nothing, ...)`) and it can already appear in type inference.
```
julia> f(a) = a > 0 ? nothing : 1
f (generic function with 1 method)
julia> @code_typed f(1)
1-element Array{Any,1}:
:($(Expr(:lambda, Any[:a],
Any[Any[],Any[Any[:a,Int64,0]],Any[],Any[]], :(begin # none, line 1:
unless (top(slt_int))(0,a::Int64)::Bool goto 0
return nothing
0:
return 1
end::Union(Int64,Void)))))
```
It might make sense to add such a type alias if a lot of people need
to write it but I personally don't think that's the case.
First of all, if the function returns a certain concrete type or
nothing, the type inference can figure that out by itself (as shown
above) and you don't need to tell it anything about that.
Second, you probably don't want to write that as the type constraint
of the argument either. If the function doesn't have any other
methods, writing that is meaningless (except as sanity check probably)
since julia will specialize on the concrete type anyway. If the
function has other methods, you will introduce ambiguity.
```julia
julia> f(::Union(Int, Void)) = 1
f (generic function with 1 method)
julia> f(::Union(Float64, Void)) = 2
Warning: New definition
f(Union(Float64, Void)) at none:1
is ambiguous with:
f(Union(Int64, Void)) at none:1.
To fix, define
f(Void)
before the new definition.
f (generic function with 2 methods)
```
For your usecase, according to what you've described, I don't think
you will need to explicitly writh `Union(Nothing, ...)`
1. If you want to dispatch on a single Nothing and if the input has
arbitrary types, just check for nothing explicitly or defining
`f(::Nothing)`
2. If you want to distinguish between different types of Nothing (i.e.
if the input is a missing Int or a missing Float64), you can probably
use `Nullable{T}()` as the returned missing value. Whether you want to
return Nullable or not for non-missing value depends on whether you
want the user function (that generates the input value) to be type
stable.
>
> On Friday, 29 May 2015 10:16:40 UTC-3, andrew cooke wrote:
>>
>>
>> I'm not sure if I'm confused, or if there's a problem here, and I don't
>> know what any fix would be anyway, so apologies for the open-ended post
>> but...
>>
>> I cannot find on "option" type in Julia that I can dispatch on, so that I
>> have a method call different functions, depending on whether a value is
>> present or not.
>>
>> What I think I need is:
>>
>> typealias Maybe{T} Union(T,Nothing}
>>
>>
>>
>> That allows me to do:
>>
>> julia> foo(::Int) = "int"
>> foo (generic function with 1 method
>>
>> julia> foo(::Nothing) = "nothing"
>> foo (generic function with 2 methods)
>>
>> julia> bar(x::Maybe{Int}) = foo(x)
>> bar (generic function with 1 method)
>>
>> julia> foo(1)
>> "int"
>>
>> julia> foo(nothing)
>> "nothing"
>>
>> julia> bar(nothing)
>> "nothing"
>>
>> julia> bar(1)
>> "int"
>>
>>
>> which seems like what I want.
>>
>> HOWEVER, there's also Nullable, described at
>> http://julia.readthedocs.org/en/latest/manual/types/#nullable-types-representing-missing-values
>> and also at
>> http://docs.julialang.org/en/latest/manual/faq/#nothingness-and-missing-values
>> which seems like what I "should" be using. But I can't see how to dispatch
>> on it.
>>
>> So it seems like one of the following is true
>>
>> 1 - There is a way to dispatch on Nothing, and please someone explain it
>> to me
>>
>> 2 - Nullable should be changed so that it can be dispatched on
>>
>> 3 - We need Maybe in Base as well as Nothing.
>>
>> 1 or 2 sounds fine (although I personally don't see how you can do 2). 3
>> seems like the worst option, but still is presumably better than everyone
>> and their dog defining this type in their own code.
>>
>> Any enlightenment appreciated,
>> Andrew
>>
>> PS More references
>>
>> https://github.com/JuliaLang/julia/issues/3332
>>
>> https://github.com/JuliaLang/julia/issues/1134
>>
>> https://github.com/JuliaLang/julia/pull/8152
