In fact, if environments could be reifyed, I'd go so far as to say that
Julia should have a separate syntax for a) evaluating an expression in its
defining environment and b) evaluating an expression in its evaluation
environment.
On Wednesday, July 8, 2015 at 3:53:29 PM UTC-4, Brandon Taylor wrote:
>
> For comparison, here is the analogous code from DataFramesMeta.
>
> replace_syms(x, membernames) = x
> function replace_syms(e::Expr, membernames)
> if e.head == :call && length(e.args) == 2 && e.args[1] == :^
> return e.args[2]
> elseif e.head == :. # special case for :a.b
> return Expr(e.head, replace_syms(e.args[1], membernames),
> typeof(e.args[2]) == Expr && e.args[2].head ==
> :quote ? e.args[2] : replace_syms(e.args[2], membernames))
> elseif e.head != :quote
> return Expr(e.head, (isempty(e.args) ? e.args : map(x ->
> replace_syms(x, membernames), e.args))...)
> else
> if haskey(membernames, e.args[1])
> return membernames[e.args[1]]
> else
> a = gensym()
> membernames[e.args[1]] = a
> return a
> end
> end
> end
>
> function with_helper(d, body)
> membernames = Dict{Symbol, Symbol}()
> body = replace_syms(body, membernames)
> funargs = map(x -> :( getindex($d, $(Meta.quot(x))) ),
> collect(keys(membernames)))
> funname = gensym()
> return(:( function $funname($(collect(values(membernames))...)) $body
> end; $funname($(funargs...)) ))
> end
>
> macro with(d, body)
> esc(with_helper(d, body))
> end
>
> On Wednesday, July 8, 2015 at 3:50:12 PM UTC-4, Brandon Taylor wrote:
>>
>> It should be noted that in R, environments are simply pointers. They take
>> up no memory and shouldn't cause drastic slowdowns (I don't think).
>>
>> On Wednesday, July 8, 2015 at 3:48:08 PM UTC-4, Brandon Taylor wrote:
>>>
>>> For example, here's a code snippet from R:
>>>
>>> function (x, data) eval(x$expr, data, x$env)
>>>
>>> x is an expression which contains an environment attribute. It is
>>> evaluated first checking the bindings in data (data being a dataframe) and
>>> then checing the bindings in x's environment.
>>>
>>> On Wednesday, July 8, 2015 at 3:37:04 PM UTC-4, Brandon Taylor wrote:
>>>>
>>>> *reifying. Deifying environments might not be the best idea.
>>>>
>>>> On Wednesday, July 8, 2015 at 3:34:53 PM UTC-4, Brandon Taylor wrote:
>>>>>
>>>>> I was aware of those packages (though I hadn't read the discussions
>>>>> referenced). Macros are great but they are incredibly difficult to reason
>>>>> with concerning issues of scope (at least for me). Deifying environments
>>>>> could solve all of these issues (and so much more) in one fell swoop.
>>>>>
>>>>> On Wednesday, July 8, 2015 at 3:20:00 PM UTC-4, David Gold wrote:
>>>>>>
>>>>>> Some of these issues have been thought about fairly extensively by
>>>>>> the stats community in particular, precisely on account of the use cases
>>>>>> you cite:
>>>>>>
>>>>>> https://github.com/JuliaStats/DataFrames.jl/pull/472
>>>>>> https://github.com/JuliaStats/DataFrames.jl/issues/504
>>>>>> <https://www.google.com/url?q=https%3A%2F%2Fgithub.com%2FJuliaStats%2FDataFrames.jl%2Fissues%2F504&sa=D&sntz=1&usg=AFQjCNHgUEZP8TyJ_BuUyyFA5SIxneOJTA>
>>>>>>
>>>>>> I think that the matter is still very much an open question. I have
>>>>>> no sense that anything is going to be added to Base Julia itself.
>>>>>> Currently, the best way (that I know of, anyway) to achieve the delayed
>>>>>> evaluation effect is via the use of macros. See for instance:
>>>>>>
>>>>>> https://github.com/JuliaStats/DataFramesMeta.jl
>>>>>> https://github.com/one-more-minute/Lazy.jl
>>>>>>
>>>>>> I'm hope somebody else will be able to pop in an give a more thorough
>>>>>> answer, but the above may at least be a place to start.
>>>>>>
>>>>>> On Wednesday, July 8, 2015 at 2:03:45 PM UTC-4, Brandon Taylor wrote:
>>>>>>>
>>>>>>> Hadley Wickham's lazyeval package in R is pretty cool in that you
>>>>>>> can attach an environment to an expression, pass it in and out of
>>>>>>> functions
>>>>>>> with various modifications, and then evaluate the expression within the
>>>>>>> original environment (or any other environment that you choose). R in
>>>>>>> general has the functions like list2env and list(environment()) that
>>>>>>> allow
>>>>>>> one to convert an environment into a list and back again (list being
>>>>>>> the R
>>>>>>> equivalent of a Dict). Are there any plans to add these kind of
>>>>>>> features to
>>>>>>> Julia?
>>>>>>>
>>>>>>
