Re: [julia-users] Colon as an argument

[Yichao Yu]

On Thu, Jun 4, 2015 at 8:50 AM, Christoph Ortner
christophortn...@gmail.com wrote:

 When implementing a function (overloading getindex to be precise) to allow :
 as an argument I expected that I would need to do the following:

 funA(_::Colon) = println(funA)

 but the following works as well

 funB(:) = println(funB)

You are just defining a method with `:` being it's (weird) argument name.

`:` is a global constant but it doesn't have any special meaning for
the parser. You can override it with local variables just like any
other globals.

Here's how you can figure out the signature of that method.

```julia
julia f(:) = 1
f (generic function with 1 method)

julia methods(f, Tuple{Any})
1-element Array{Any,1}:
f(:) at none:1

julia methods(f, Tuple{Any})[1].sig
Tuple{Any}
```


 Why? When I type ?: in the REPL I find that : is an instance of Colon. This
 is unexpected for me, it does not seem to be the same as Value Types? Can
 somebody clarify for me what is happening?

 Many thanks,
Christoph

Re: [julia-users] Colon as an argument

[Christoph Ortner]

ah - extremely embarrassing mistake. Thank you.

So was funA the correct way to implement this?

Christoph

Re: [julia-users] Colon as an argument

[Yichao Yu]

On Thu, Jun 4, 2015 at 9:10 AM, Christoph Ortner
christophortn...@gmail.com wrote:
 ah - extremely embarrassing mistake. Thank you.

 So was funA the correct way to implement this?

IIRC, currently `a[:]` gets lowered to `a[1:endof(a)]` (see
`expand(:(a[:]))`) so this won't work for get/set index. (I remember
there's a PR about this lowering, maybe it will change soon?)

But IMHO it is the right way if you want to dispatch on `:` in a
function in general.


 Christoph

Re: [julia-users] Colon as an argument

[Yichao Yu]

On Thu, Jun 4, 2015 at 9:13 AM, Yichao Yu yyc1...@gmail.com wrote:
 On Thu, Jun 4, 2015 at 9:10 AM, Christoph Ortner
 christophortn...@gmail.com wrote:
 ah - extremely embarrassing mistake. Thank you.

 So was funA the correct way to implement this?

 IIRC, currently `a[:]` gets lowered to `a[1:endof(a)]` (see
 `expand(:(a[:]))`) so this won't work for get/set index. (I remember
 there's a PR about this lowering, maybe it will change soon?)

I didn't follow it but this[1] is what I'm thinking about.

[1] https://github.com/JuliaLang/julia/pull/10331#issuecomment-108682886


 But IMHO it is the right way if you want to dispatch on `:` in a
 function in general.

Oh. except that you don't need that dummy `_` if you don't need the
argument. Just `f(::Colon)` should be enough



 Christoph

Re: [julia-users] Colon as an argument

[Christoph Ortner]

great - thank you!

On Thursday, 4 June 2015 14:16:42 UTC+1, Yichao Yu wrote:

 On Thu, Jun 4, 2015 at 9:13 AM, Yichao Yu yyc...@gmail.com javascript: 
 wrote: 
  On Thu, Jun 4, 2015 at 9:10 AM, Christoph Ortner 
  christop...@gmail.com javascript: wrote: 
  ah - extremely embarrassing mistake. Thank you. 
  
  So was funA the correct way to implement this? 
  
  IIRC, currently `a[:]` gets lowered to `a[1:endof(a)]` (see 
  `expand(:(a[:]))`) so this won't work for get/set index. (I remember 
  there's a PR about this lowering, maybe it will change soon?) 

 I didn't follow it but this[1] is what I'm thinking about. 

 [1] https://github.com/JuliaLang/julia/pull/10331#issuecomment-108682886 

  
  But IMHO it is the right way if you want to dispatch on `:` in a 
  function in general. 

 Oh. except that you don't need that dummy `_` if you don't need the 
 argument. Just `f(::Colon)` should be enough 

  
  
  Christoph