What you're trying isn't easy because the results of @ntuple, unless contained
inside an Expr, will be eval'ed so you'll still create the tuple. And
expression-tuples are not iterable, so you can't splat them.
Manual construction:
function foo(N)
args = [symbol("xs_",k) for k = 1:N]
:(gradient(A, $(args...)))
end
The Cartesian package had an `@ncall`, but I found that I so often wanted new
combinations of arguments that it wasn't worth it so didn't port it over to
Base.
--Tim
On Tuesday, September 22, 2015 12:11:05 AM Tomas Lycken wrote:
> I’m having real difficulties grok-ing macro hygiene. I’ve read the manual
> entry, but didn’t get much wiser, so I’m hoping the solution to my current
> problem will be another data point I can use to understand (and I need this
> to work :P).
>
> I have a utility function that builds an expression (which will later be
> interpolated into the method body of a generated function, but that’s out
> of scope for my current problem), something like this:
>
> julia> using Base.Cartesian
>
> julia> foo(N) = :(gradient(A, (@ntuple $N k->xs_k)...))
> foo (generic function with 1 method)
>
> julia> macroexpand(foo(2))
>
> :(gradient(A,(xs_1,xs_2)...))
>
> Now, as you see, the generated tuple is splatted at run time. I’d like to
> try to move that splatting to compile time, so that the generated
> expression is rather :(gradient(A, xs_1, xs_2)). I’ve tried variants on the
> following, but as you can see I haven’t had any success.
>
> I assume that I need to escape N somehow - but how?
>
> julia> foo(N) = :(gradient(A, $((@ntuple N k->xs_k)...)))
> ERROR: MethodError: `_ntuple` has no method matching _ntuple(::Symbol,
> ::Expr)
>
> julia> foo(N) = :(gradient(A, $((@ntuple $N k->xs_k)...)))
> ERROR: MethodError: `_ntuple` has no method matching _ntuple(::Expr, ::Expr)
>
> julia> foo(N) = :(gradient(A, $(esc((@ntuple N k->xs_k)...))))
> ERROR: MethodError: `_ntuple` has no method matching _ntuple(::Symbol,
> ::Expr)
>
> julia> foo(N) = :(gradient(A, $(esc((@ntuple :N k->xs_k)...))))
> ERROR: MethodError: `_ntuple` has no method matching _ntuple(::Expr, ::Expr)
>
> julia> foo(N) = :(gradient(A, $(@ntuple esc(N) k->xs_k)...))
> ERROR: MethodError: `_ntuple` has no method matching _ntuple(::Expr, ::Expr)
>
> julia> foo(N) = :(gradient(A, $(@ntuple esc(:N) k->xs_k)...))
> ERROR: MethodError: `_ntuple` has no method matching _ntuple(::Expr, ::Expr)
>
>
