[SOLVED] Add method to a macro defined in a different package

Add method to a macro defined in a different package

For example, how could I add, in my own code/package, a method for the @test macro defined in the standard package Test.jl.

Doing the following does not seem to work:

import Test: @test

macro test(ex::SomeArbitraryType,  args...)
    ...
end

More concretely, it does not register an extra method on the actual Test.@test macro, but just defines a new local macro called @test.

Solution

You can add new methods, but the types you can dispatch on are limited: Expr, Symbol, literal numbers:

julia> using Test

julia> @test 1+1==2  # macro called with Expr type
Test Passed

julia> @which @test 1+1==2
var"@test"(__source__::LineNumberNode, __module__::Module, ex, kws...)
     @ Test ~/.julia/dev/julia/usr/share/julia/stdlib/v1.12/Test/src/Test.jl:493

julia> function Test.var"@test"(__source__::LineNumberNode, __module__::Module, ex::Int, kws...)
         println("new method for Int")  # more specific signature
         return __module__
       end

julia> @test 3  # macro called with Int type
new method for Int

If x isa SomeArbitraryType at runtume, this is not visible to the macro. For instance, if we define f(x) = @test x, then the macro method for ::Symbol is called immediately. The macro sees the code involved in defining f, and returns other code which runs once f is called.

julia> macro Test.test(ex::Symbol,  args...)
         println("method for symbol")
         return string(__source__)
       end

julia> f(x) = @test x  # macro expansion happens immediately
method for symbol
f (generic function with 1 method)

julia> f("something")  # now the expanded code is run
"#= REPL[10]:1 =#"

julia> methods(Test.var"@test")
# 3 methods for macro "@test" from Test:
 [1] var"@test"(__source__::LineNumberNode, __module__::Module, ex::Symbol, args...)
     @ Main REPL[6]:1
 [2] var"@test"(__source__::LineNumberNode, __module__::Module, ex::Int64, kws...)
     @ Main REPL[4]:1
 [3] var"@test"(__source__::LineNumberNode, __module__::Module, ex, kws...)
     @ ~/.julia/dev/julia/usr/share/julia/stdlib/v1.12/Test/src/Test.jl:493