Determine which method is dispatched for a given function call

Question

Consider the findfirst function. I can see which methods are defined for findfirst by using methods:

julia> methods(findfirst)
# 9 methods for generic function "findfirst":
[1] findfirst(A::Union{AbstractString, AbstractArray}) in Base at array.jl:1672
[2] findfirst(p::Union{Base.Fix2{typeof(==),T}, Base.Fix2{typeof(isequal),T}}, r::StepRange{T,S}) where {T, S} in Base at array.jl:1758
[3] findfirst(pred::Base.Fix2{#s66,#s65} where #s65<:Union{Int8, UInt8} where #s66<:Union{typeof(==), typeof(isequal)}, a::Union{Array{Int8,1}, Array{UInt8,1}}) in Base at strings/search.jl:22
[4] findfirst(testf::Function, A::Union{AbstractString, AbstractArray}) in Base at array.jl:1754
[5] findfirst(testf::Function, A) in Base at array.jl:1747
[6] findfirst(pattern::AbstractString, string::AbstractString) in Base at strings/search.jl:104
[7] findfirst(ch::AbstractChar, string::AbstractString) in Base at strings/search.jl:124
[8] findfirst(r::Regex, s::AbstractString) in Base at regex.jl:327
[9] findfirst(A) in Base at array.jl:1663

Now suppose I'd like to figure out which of those methods gets dispatched when I call findfirst(iseven, 1:4). How do I do that?

Answer 1

You can use the @which macro:

julia> @which findfirst(iseven, 1:4)
findfirst(testf::Function, A::Union{AbstractString, AbstractArray}) in Base at array.jl:1754

Another example:

julia> @which length(1:7)
length(r::AbstractUnitRange{T}) where T<:Union{Int128, Int64} in Base at range.jl:542

julia> @which length("hello world")
length(s::String) in Base at strings/string.jl:259

Edit:

As @giordano points out in the comments, you can use

@less findfirst(iseven, 1:4)

or

@edit findfirst(iseven, 1:4)

if you want to see the source code for the method that would get dispatched. @less displays the source code using the system pager. @edit will open the file containing the method source code in a text editor. You can control which editor gets opened by setting the JULIA_EDITOR environment variable. For example, you can set your editor to vim by calling

ENV["JULIA_EDITOR"] = "vim"

either at the REPL or in your startup file, ~/.julia/config/startup.jl. Alternatively, you can add export JULIA_EDITOR=vim to your .bashrc or .bash_profile file on linux/macos.