FnMacro.jl

module FnMacro
export @fn

arg_name(n::Int) = Symbol("hole_", n)

rewrite(x,         n::Ref{Int}) = x
rewrite(s::Symbol, n::Ref{Int}) = s == :_ ? arg_name(n[] += 1) : s
rewrite(e::Expr,   n::Ref{Int}) = Expr(e.head, map(a -> rewrite(a, n), e.args)...)

macro fn(e)
    num_args = Ref(0)
    body = rewrite(e, num_args)
    arg_names = map(arg_name, 1:num_args[])
    :(($(arg_names...),) -> $body)
end

end