Please see this section of the manual
http://julia.readthedocs.org/en/latest/manual/metaprogramming/?highlight=quotenode#hygiene
The following is not the best way to solve this problem (you probably
don't want to escape everything) but it's a simple enough way to get
this particular case working
```julia
using Base.Test
import Base.hash
function auto_hash(typ::Expr)
@assert typ.head == :type
quote
function hash(a::$(typ.args[2]))
1
end
end
end
macro auto(typ)
@assert typ.head == :type
hash = auto_hash(typ)
r = quote
$typ
$hash
end
esc(r)
end
println(macroexpand(:(@auto type A end)))
@auto type A end
@test typeof(A()) == A
@test hash(A()) == 1
println("ok")
```
On Fri, Jun 5, 2015 at 7:42 PM, andrew cooke <[email protected]> wrote:
> i'm trying to write a macro that will define == and hash() for a composite
> type.
>
> my initial effort is just to define a hash of 1
>
> using Base.Test
> import Base.hash
>
> function auto_hash(typ::Expr)
> @assert typ.head == :type
> quote
> function hash(a::$(typ.args[2]))
> 1
> end
> end
> end
>
> macro auto(typ)
> @assert typ.head == :type
> hash = auto_hash(typ)
> r = quote
> $typ
> $hash
> end
> show(r)
> r
> end
>
> @auto type A end
> @test typeof(A()) == A
> @test hash(A()) == 1
>
> println("ok")
>
> unfortunately this fails - the hash value is some "random" value.
>
> yet the generated code, displayed by show, works when i cut + paste it into
> julia. here's the result of running the program:
>
> andrew@laptop:~/project/ParserCombinator-0.4> julia-trunk test/auto.jl
> quote # /home/andrew/.julia/v0.4/ParserCombinator/test/auto.jl, line 18:
> type A
> end # line 19:
> begin # /home/andrew/.julia/v0.4/ParserCombinator/test/auto.jl, line 8:
> function hash(a::A) #
> /home/andrew/.julia/v0.4/ParserCombinator/test/auto.jl, line 9:
> 1
> end
> end
> end
> ERROR: LoadError: test failed: 0x0d6a95a0cf175e49 == 1
>
> here's some cut + pasting:
>
> julia> import Base.hash
>
> julia> type A end # line 19:
>
> julia> begin # /home/andrew/.julia/v0.4/ParserCombinator/test/auto.jl, line
> 8:
> function hash(a::A) #
> /home/andrew/.julia/v0.4/ParserCombinator/test/auto.jl, line 9:
> 1
> end
> end
> hash (generic function with 36 methods)
>
> julia> hash(A())
> 1
>
> so why, oh why, doesn't my macro work?
>
> *something* is being evaluated ok because the type A is being created.
>
> thanks,
> andrew
>
>
>
