How can I retrieve the methods of a Julia macro?

Question

In Julia, the methods function may be used to retrieve the methods of a function.

julia> f(::Int) = 0
f (generic function with 1 method)

julia> f(::String) = ""
f (generic function with 2 methods)

julia> methods(f)
# 2 methods for generic function "f":
f(::String) in Main at REPL[1]:1
f(::Int64) in Main at REPL[0]:1

Macros can also have multiple methods.

julia> macro g(::Int)
           0
       end
@g (macro with 1 method)

julia> macro g(::String)
           ""
       end
@g (macro with 2 methods)

julia> @g 123
0

julia> @g "abc"
""

However, the methods function does not seem to work on macros because Julia first calls the macro, due to the fact that they do not need parentheses.

julia> methods(@g)
ERROR: MethodError: no method matching @g()
Closest candidates are:
  @g(::String) at REPL[2]:2
  @g(::Int64) at REPL[1]:2

I tried using an Expression to contain the macro, but this did not work.

julia> methods(:@g)
# 0 methods for generic function "(::Expr)":

How can I retrieve the methods of a macro?

methods(eval(Symbol("@g"))) worked for me, but there must be a cleaner method — Dan Getz
@DanGetz Nice. But yeah, there should be a cleaner method... (By the way, Main.(Symbol("@g")) is deprecated, and getfield(Main, Symbol("@g") should be used instead.) — Harrison Grodin

HarmonicaMuse HarmonicaMuse · Accepted Answer · 2017-05-16T06:59:21

I would put a generic macro (@methods) inside a module (MethodsMacro) in my ~/.juliarc.jl along with the line: using MethodsMacro. This way you'd have it available at every Julia session, something like:

julia> module MethodsMacro                                               

       export @methods                                              

       macro methods(arg::Expr)                                     
           arg.head == :macrocall || error("expected macro name")   
           name = arg.args[] |> Meta.quot                           
           :(methods(eval($name)))                                  
       end                                                          

       macro methods(arg::Symbol)                                   
           :(methods($arg)) |> esc                                  
       end                                                          

       end                                                          
MethodsMacro                                                             

julia> using MethodsMacro                                                

julia> @methods @methods                                            
# 2 methods for macro "@methods":                                   
@methods(arg::Symbol) at REPL[48]:12                                
@methods(arg::Expr) at REPL[48]:6                                   

julia> f() = :foo; f(x) = :bar                                      
f (generic function with 2 methods)                                 

julia> @methods f                                                   
# 2 methods for generic function "f":                               
f() at REPL[51]:1                                                   
f(x) at REPL[51]:1

How can I retrieve the methods of a Julia macro?

1 Answers