Sorry to revive this old thread, but I am trying to understand macros, so
please bear with me:
When I try to use this example , replacing expr by Expr so the macro
becomes:
macro dict(syms...)
Expr(:dict, { :($(Expr(:quote,s))=>$(esc(s))) for s in syms })
end
When I now ask for julia> r = @dict(a,b)
I get ERROR: syntax: malformed expression
Also this version:
macro dict(syms...)
Expr(:dict, { :($(string(s))=>$(esc(s))) for s in syms })
end
doesn't work for me either; it gives the same error.
Could someone please explain to me why the expression returned by the macro
is malformed?
This is on Julia 0.3 Windows 8 64bit.
Thanks very much!
Ivo
Op donderdag 7 februari 2013 21:39:27 UTC+1 schreef René Donner:
>
> Thanks a lot, the trick with writing out the expressions first is great to
> know!
>
> May I propose adding this somewhere at the end of the manual section on
> macros? I think it nicely gives a very valuable bit of practical advice,
> before one ventures out into the cold world of trial and error ;-)
>
>
> Just for reference, I modified the macro slightly to use the actual names
> instead of the symbols as the keys:
>
> macro dict(syms...)
> expr(:dict, { :($(string(s))=>$(esc(s))) for s in syms })
> end
>
>
> Thanks again,
>
> Rene
>
>
>
>
> Am 07.02.2013 um 21:25 schrieb Stefan Karpinski <ste...@karpinski.org
> <javascript:>>:
>
> > This does it:
> >
> > macro dict(syms...)
> > expr(:dict, { :($(expr(:quote,s))=>$(esc(s))) for s in syms })
> > end
> >
> > The approach I used, which is generally effective for writing macros is
> to create an example of the expression I want the macro to construct and
> then write code to build that:
> >
> > julia> ex = :([:a => "hello", :b => "world"])
> > :($(expr(:dict, :(:a=>"hello"), :(:b=>"world"))))
> >
> > julia> ex.head
> > :dict
> >
> > julia> ex.args
> > 2-element Any Array:
> > :(:a=>"hello")
> > :(:b=>"world")
> >
> > So I want an Expr object whose head is :dict and whose args is an array
> of expression of the form :(:sym => value). The tricky bits are:
> > • Wrapping s in expr(:quote,s) so that it ends up in the final
> AST as an expression for a symbol rather than splicing the symbol directly
> in since that would be as if you'd written the variable rather than the
> symbol for it.
> > • Wrapping the s in esc(s) so that it is evaluated in the macro
> caller's context, not the macro definition context. Note that this isn't a
> concern for expr(:quote,s) because symbols are the same in any context.
> > Hope that helps.
> >
> >
> > On Thu, Feb 7, 2013 at 3:02 PM, René Donner <li...@donner.at
> <javascript:>> wrote:
> > Hi,
> >
> > I would like to write a macro which easily allows to contruct Dict's
> using the actual variables' names, like so:
> >
> > a = "hello"
> > b = "world"
> > r = @dict a b # r == {"a"=>"hello","b"=>"world"}
> >
> > The following macro does the job, but how could I get rid of the "eval"
> (which I read one is not supposed to use inside a macro, rather just to
> yield an AST).
> >
> > macro dict(a...)
> > quote
> > local r = Dict{Any,Any}()
> > for i in 1:length($a)
> > r[string(($a)[i])]=eval(($a)[i])
> > end
> > r
> > end
> > end
> >
> >
> > Thanks a lot for any advice!
> >
> > Rene
> >
> >
> >
>
>
