Of course this was a simplified example to show the problem. A bad one, I
admit.
My question relates to the problem you (Simon) provided one answer already.
I certainly have read Metaprogramming chapter and tried everything I could
think of without luck. However, it could well be that I am just too thick...
Returning to the real problem:
julia> macro gentype(N, typename)
fields = [:($(symbol("I_$i"))::T) for i=1:N]
quote
immutable $(typename){T}
$(fields...)
end
end
end
julia> n = 3
julia> @gentype n Uint8
ERROR: `colon` has no method matching colon(::Int64, ::Symbol)
julia> macroexpand(:(@gentype n Uint8))
:($(Expr(:error, MethodError(colon,(1,:n)))))
So, the problem seems to be that variable argument is treated as a symbol
and is not interpolated in parse time, that lead me to try eval(), which is
a no-no.
If I replace N -> $N, i get "ERROR: error compiling anonymous: syntax:
prefix $ in non-quoted expression"
Any suggestions are appreciated, there must be a way....
Thanks,
Kaj
On Wednesday, March 11, 2015 at 3:40:38 PM UTC+2, Simon Danisch wrote:
>
> This is most likely not the right place to use eval!
> You need to define your problem better. What you describe here doesn't
> need a macro whatsoever.
> Macros are for manipulating the syntax tree, which is why the arguments
> are not the values, but expressions.
> What a macro is intended to do is more something like this:
>
> macro testmacro(N)
> quote
> for i = 1:$N
> println("Hello!")
> end
> end
> end
> n = 10
>
> @testmacro n
>
> So all the code inside a macro should be used to transform an expression,
> which than replaces the original expression that you gave the macro via its
> arguments
>
> Am Mittwoch, 11. März 2015 13:37:55 UTC+1 schrieb Kaj Wiik:
>>
>> I have a problem in using variables as argument for macros. Consider a
>> simple macro:
>>
>> macro testmacro(N)
>> for i = 1:N
>> println("Hello!")
>> end
>> end
>>
>> @testmacro 2
>>
>> Hello!
>> Hello!
>>
>>
>> So, all is good. But if I use a variable as an argument,
>>
>> n = 2
>> @testmacro n
>>
>>
>> I get an (understandable) error message "ERROR: `colon` has no method
>> matching colon(::Int64, ::Symbol)".
>>
>> Is this the correct place to use eval() in macros, like
>>
>> macro testmacro(N)
>> for i = 1:eval(N)
>> println("Hello!")
>> end
>> end
>>
>> This seems to work as expected. I tried multitude of combinations of
>> dollar signs, esc, quotes and brackets, none of them worked :-), got
>> "ERROR: error compiling anonymous: syntax: prefix $ in non-quoted
>> expression"...
>>
>> Are there better ways to do this, is it OK to use eval() in this context?
>>
>> Thanks,
>> Kaj
>>
>>
