convert(Array{Float64}, a) 2×3 Array{Float64,2}: 1.0 2.0 3.0 4.0 5.0 6.0 类型转换并不总是可行的，有时 convert 函数并不知道该如何执行所请求的类型转换就会抛出 MethodError 错误。例如下例： julia> convert(AbstractFloat, "foo") ERROR: MethodError: Cannot `convert` x。相反地，如果 T 是一个可变的集合类型，那 么 T(x) 应该总是创建一个新的集合（从 x 复制元素）。 封装器类型 对于某些「封装」其它值的类型，构造函数可能会将其参数封装在一个新对象中，即使它已经是 所请求的类型。例如，用 Some(x) 表示封装了一个 x 值（在上下文中，其结果可能是一个 Some 或 nothing）。但是，x 本身可能是对象 Some(y)，在这种情况下，结果为 Some(Some(y))，封装了两层。 convert(::Type{T}, x::Number) where {T<:Number} = T(x) 这意味着新的 Number 类型只需要定义构造函数，因为此定义将为它们处理 convert。在参数已经是 所请求的类型的情况下，用恒同变换来处理 convert。 158 CHAPTER 15. 类型转换和类型提升 convert(::Type{T}, x::T) where {T<:Number} = x

com", "github.com", "julialang.org") @async begin conn = connect(hostname, 80) write(conn, "GET / HTTP/1.1\r\nHost:$(hostname)\r\n\r\n") readline(conn, keep=true) println("Finished connection to $(hostname)") delays=rand(5:10, 2)) retry(f, delays=Base.ExponentialBackOff(n=3, first_delay=5, max_delay=1000)) retry(http_get, check=(s,e)->e.status == "503")(url) retry(read, check=(s,e)->isa(e, IOError))(io, 128; all=false) These pairs are passed as headers when downloading URLs with protocols that supports them, such as HTTP/S. The timeout keyword argument specifies a timeout for the download to complete in seconds, with

com", "github.com", "julialang.org") @async begin conn = connect(hostname, 80) write(conn, "GET / HTTP/1.1\r\nHost:$(hostname)\r\n\r\n") readline(conn, keep=true) println("Finished connection to $(hostname)") delays=rand(5:10, 2)) retry(f, delays=Base.ExponentialBackOff(n=3, first_delay=5, max_delay=1000)) retry(http_get, check=(s,e)->e.status == "503")(url) retry(read, check=(s,e)->isa(e, IOError))(io, 128; all=false) These pairs are passed as headers when downloading URLs with protocols that supports them, such as HTTP/S. The timeout keyword argument specifies a timeout for the download to complete in seconds, with

com", "github.com", "julialang.org") @async begin conn = connect(hostname, 80) write(conn, "GET / HTTP/1.1\r\nHost:$(hostname)\r\n\r\n") readline(conn, keep=true) println("Finished connection to $(hostname)") delays=rand(5:10, 2)) retry(f, delays=Base.ExponentialBackOff(n=3, first_delay=5, max_delay=1000)) retry(http_get, check=(s,e)->e.status == "503")(url) retry(read, check=(s,e)->isa(e, IOError))(io, 128; all=false) These pairs are passed as headers when downloading URLs with protocols that supports them, such as HTTP/S. The timeout keyword argument specifies a timeout for the download to complete in seconds, with

com", "github.com", "julialang.org") @async begin conn = connect(hostname, 80) write(conn, "GET / HTTP/1.1\r\nHost:$(hostname)\r\n\r\n") readline(conn, keep=true) println("Finished connection to $(hostname)") delays=rand(5:10, 2)) retry(f, delays=Base.ExponentialBackOff(n=3, first_delay=5, max_delay=1000)) retry(http_get, check=(s,e)->e.status == "503")(url) retry(read, check=(s,e)->isa(e, IOError))(io, 128; all=false) These pairs are passed as headers when downloading URLs with protocols that supports them, such as HTTP/S. The timeout keyword argument specifies a timeout for the download in seconds, with a resolution

com", "github.com", "julialang.org") @async begin conn = connect(hostname, 80) write(conn, "GET / HTTP/1.1\r\nHost:$(hostname)\r\n\r\n") readline(conn, keep=true) println("Finished connection to $(hostname)") Main binary filename not available. Serving web UI on http://localhost:62261 "alloc-profile.pb.gz" Then you can view the profile by navigating to http://localhost:62261, and the profile is saved to disk delays=rand(5:10, 2)) retry(f, delays=Base.ExponentialBackOff(n=3, first_delay=5, max_delay=1000)) retry(http_get, check=(s,e)->e.status == "503")(url) retry(read, check=(s,e)->isa(e, IOError))(io, 128; all=false)

com", "github.com", "julialang.org") @async begin conn = connect(hostname, 80) write(conn, "GET / HTTP/1.1\r\nHost:$(hostname)\r\n\r\n") readline(conn, keep=true) println("Finished connection to $(hostname)") Main binary filename not available. Serving web UI on http://localhost:62261 "alloc-profile.pb.gz" Then you can view the profile by navigating to http://localhost:62261, and the profile is saved to disk delays=rand(5:10, 2)) retry(f, delays=Base.ExponentialBackOff(n=3, first_delay=5, max_delay=1000)) retry(http_get, check=(s,e)->e.status == "503")(url) retry(read, check=(s,e)->isa(e, IOError))(io, 128; all=false)

@async begin conn = connect(hostname, 80) CHAPTER 21. NETWORKING AND STREAMS 288 write(conn, "GET / HTTP/1.1\r\nHost:$(hostname)\r\n\r\n") readline(conn, keep=true) println("Finished connection to $(hostname)") delays=rand(5:10, 2)) retry(f, delays=Base.ExponentialBackOff(n=3, first_delay=5, max_delay=1000)) retry(http_get, check=(s,e)->e.status == "503")(url) retry(read, check=(s,e)->isa(e, IOError))(io, 128; all=false) These pairs are passed as headers when downloading URLs with protocols that supports them, such as HTTP/S. The timeout keyword argument specifies a timeout for the download in seconds, with a resolution

@async begin conn = connect(hostname, 80) CHAPTER 21. NETWORKING AND STREAMS 288 write(conn, "GET / HTTP/1.1\r\nHost:$(hostname)\r\n\r\n") readline(conn, keep=true) println("Finished connection to $(hostname)") delays=rand(5:10, 2)) retry(f, delays=Base.ExponentialBackOff(n=3, first_delay=5, max_delay=1000)) retry(http_get, check=(s,e)->e.status == "503")(url) retry(read, check=(s,e)->isa(e, IOError))(io, 128; all=false) These pairs are passed as headers when downloading URLs with protocols that supports them, such as HTTP/S. The timeout keyword argument specifies a timeout for the download in seconds, with a resolution

@async begin conn = connect(hostname, 80) CHAPTER 21. NETWORKING AND STREAMS 288 write(conn, "GET / HTTP/1.1\r\nHost:$(hostname)\r\n\r\n") readline(conn, keep=true) println("Finished connection to $(hostname)") delays=rand(5:10, 2)) retry(f, delays=Base.ExponentialBackOff(n=3, first_delay=5, max_delay=1000)) retry(http_get, check=(s,e)->e.status == "503")(url) retry(read, check=(s,e)->isa(e, IOError))(io, 128; all=false) These pairs are passed as headers when downloading URLs with protocols that supports them, such as HTTP/S. The timeout keyword argument specifies a timeout for the download in seconds, with a resolution