That was it!
This is a great community, many thanks for your time all!
Cheers.
Kaj
On Wednesday, March 11, 2015 at 8:56:26 PM UTC+2, Stefan Karpinski wrote:
>
> The way to do this is by wrapping an @eval around the macro invocation and
> splicing $n:
>
> @eval @gentype $n UInt8
>
>
> This might be necessary, e.g. if you're looping over various values of n:
>
> for n = 1:10
> @eval @gentype $n UInt8
> end
>
>
>
>
> On Wed, Mar 11, 2015 at 2:52 PM, Kaj Wiik <kaj....@gmail.com <javascript:>
> > wrote:
>
>>
>> Of course this was a simplified example to show the problem. A bad one, I
>> admit.
>>
>> My question relates to the problem you (Simon) provided one answer
>> already. I certainly have read Metaprogramming chapter and tried everything
>> I could think of without luck. However, it could well be that I am just too
>> thick...
>>
>> Returning to the real problem:
>>
>> julia> macro gentype(N, typename)
>> fields = [:($(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
>>>>
>>>>
>
