宏 #

macro sayhi(name)
    return :(println("Hello, ", $name, "!"))
end

@sayhi "Julia"

macro twice(ex)
    return quote
        $ex
        $ex
    end
end

@twice println("Hello")

macroexpand(@__MODULE__, :(@sayhi "Julia"))

macro set_x(value)
    :(x = $value)
end

x = 10
@set_x 20
x

macro set_var(var, value)
    :($(esc(var)) = $value)
end

y = 10
@set_var y 20
y

macro safe_swap(a, b)
    temp = gensym(:temp)
    quote
        $temp = $(esc(a))
        $(esc(a)) = $(esc(b))
        $(esc(b)) = $temp
    end
end

x, y = 1, 2
@safe_swap x y
(x, y)

x = 10
@show x + 1

@time sleep(1)

@assert 1 + 1 == 2

@enum Color Red Green Blue
Red
Green

function sum_simd(arr)
    total = 0.0
    @simd for x in arr
        total += x
    end
    total
end

macro timed(ex)
    quote
        local start = time()
        local result = $(esc(ex))
        local elapsed = time() - start
        println("Elapsed: ", elapsed, " seconds")
        result
    end
end

@timed sleep(1)

macro retry(n, ex)
    quote
        local attempts = 0
        local result
        while attempts < $n
            attempts += 1
            try
                result = $(esc(ex))
                break
            catch e
                if attempts == $n
                    rethrow()
                end
            end
        end
        result
    end
end

@retry 3 rand() > 0.9 ? error("fail") : "success"

macro log(level, msg)
    quote
        println("[", $(string(level)), "] ", $msg)
    end
end

@log INFO "Application started"
@log DEBUG "Processing data"

macro cached(ex)
    cache_name = Symbol(:_cache_, ex)
    quote
        if !isdefined(@__MODULE__, $(QuoteNode(cache_name)))
            $(esc(cache_name)) = $(esc(ex))
        end
        $(esc(cache_name))
    end
end

@cached expensive_computation() = begin
    println("Computing...")
    sleep(1)
    42
end

macro check(condition, message="")
    quote
        if !$(esc(condition))
            error("Check failed: ", $(string(condition)), " ", $message)
        end
    end
end

@check 1 + 1 == 2
@check 1 + 1 == 3 "Math is broken"

macro singleton(name, ex)
    quote
        $(esc(name))() = begin
            if !isdefined(@__MODULE__, :_singleton_instance)
                global _singleton_instance = $(esc(ex))
            end
            _singleton_instance
        end
    end
end

@singleton config Dict("host" => "localhost")
config()

macro delegate(source, methods)
    code = Expr(:block)
    for method in methods.args
        push!(code.args, quote
            $method(args...) = $method($(esc(source)), args...)
        end)
    end
    return code
end

struct Wrapper
    data::Vector{Int}
end

@delegate wrapper.data length push!