Red提供的错误处理机制属于异常。
# Red的异常概念
在Red中引入了error!类型来表示一个异常，如果直接执行一个error!，且没有捕获，会导致程序退出。
# 抛出异常
make error!cause-error这在下面的Red异常的用法章节有介绍
# 捕获异常
可以使用 try或者 attempt， try一个异常时返回的是一个error!类型，而attempt返回的是none。
# Red的异常类型
Red的异常有以下几种：
>> help system/catalog/errors
SYSTEM/CATALOG/ERRORS is an object! with the following words and values:
throw object! [code type break return throw continue while-cond]
note object! [code type no-load]
syntax object! [code type invalid missing no-header no-rs-header bad-header malconstruct bad-...
script object! [code type no-value need-value not-defined not-in-context no-arg expect-arg ex...
math object! [code type zero-divide overflow positive]
access object! [code type cannot-open invalid-utf8 no-connect]
user object! [code type message]
internal object! [code type bad-path not-here no-memory wrong-mem stack-overflow too-deep featu...
比如 1 / 0除零异常：
>> probe try [1 / 0]
make error! [
code: 400
type: 'math
id: 'zero-divide
arg1: none
arg2: none
arg3: none
near: none
where: '/
stack: 41781600
]
*** Math Error: attempt to divide by zero
*** Where: /
*** Stack: probe
它的type为math，id为zero-divide。
再举一个例子：
>> probe try [do]
make error! [
code: 304
type: 'script
id: 'no-arg
arg1: 'do
arg2: 'value
arg3: none
near: none
where: 'do
stack: 41781600
]
*** Script Error: do is missing its value argument
*** Where: do
*** Stack: probe
>>
我们知道系统函数do提供了把数据作为代码执行的功能，它需要一个参数，如果程序没有提供参数，就会抛出这样的异常。它的type为script，id为no-arg。
而且我们从这些名称可以大致看出错误的原因。
# Red异常的用法
type为user，id为message的错误是Red提供的默认用户异常，它可以通过make error!创建。
# 通过make error!创建异常
>> probe try [make error! "hello"]
make error! [
code: 600
type: 'user
id: 'message
arg1: "hello"
arg2: none
arg3: none
near: none
where: none
stack: none
]
*** User Error: "hello"
*** Where: ???
另外 make error!可以创建特定类型的异常：
>> probe try [make error! 400]
make error! [
code: 400
type: 'math
id: 'zero-divide
arg1: none
arg2: none
arg3: none
near: none
where: none
stack: none
]
*** Math Error: attempt to divide by zero
*** Where: ???
或者：
>> probe try [make error! [math zero-divide]]
make error! [
code: 400
type: 'math
id: 'zero-divide
arg1: none
arg2: none
arg3: none
near: none
where: none
stack: none
]
*** Math Error: attempt to divide by zero
*** Where: ???
但是，这两种方式创建的异常都不能带有参数。
# 使用cause-error创建异常
>> ? cause-error
USAGE:
CAUSE-ERROR err-type err-id args
DESCRIPTION:
Causes an immediate error throw, with the provided information.
CAUSE-ERROR is a function! value.
ARGUMENTS:
err-type [word!]
err-id [word!]
args [block!]
例如：
>> probe try [cause-error 'math 'zero-divide []]
make error! [
code: 400
type: 'math
id: 'zero-divide
arg1: none
arg2: none
arg3: none
near: none
where: 'do
stack: 41459176
]
*** Math Error: attempt to divide by zero
*** Where: do
*** Stack: probe
cause-error创建异常的功能更加强大，而且可以带参数。
>> probe try [cause-error 'user 'message ["hello" "world"]]
make error! [
code: 600
type: 'user
id: 'message
arg1: "hello"
arg2: "world"
arg3: none
near: none
where: 'do
stack: 41781640
]
*** User Error: "hello"
*** Where: do
*** Stack: probe
# 自定义用户异常
上面例子中的异常都是Red语言提供的异常，有的时候我们需要自定义异常，以区别其他异常。我们可以这样定义一个异常：
>> probe system/catalog/errors/user: make system/catalog/errors/user [my-error: ["info [" :arg1 ": (" :arg2 " " :arg3 ")]"]]
make object! [
code: 800
type: "User Error"
message: [:arg1]
my-error: ["info [" :arg1 ": (" :arg2 " " :arg3 ")]"]
]
== make object! [
code: 800
type: "User Error"
message: [:arg1]
my-error: ["info [" :ar...
>>
然后可以使用cause-error使用该异常类型创建异常：
>> new-error: func [name [word!] arg2 arg3][
[ cause-error 'user 'my-error [name arg2 arg3]
[ ]
== func [name [word!] arg2 arg3][cause-error 'user 'my-error [name arg2 arg3]]
>> probe try [new-error 'func-name "msg1" "msg2"]
make error! [
code: 601
type: 'user
id: 'my-error
arg1: 'func-name
arg2: "msg1"
arg3: "msg2"
near: none
where: 'do
stack: 41781680
]
*** User Error: info [ func-name : ( "msg1" "msg2" )]
*** Where: do
*** Stack: probe
# Red的catch/throw功能
Red提供了catch/throw语句来完成block块的提前返回。
write %file.txt "i am a happy little file with no real purpose"
print catch [
if exists? %file.txt [throw "Doc found"]
"Doc not found"
]
Doc found
catch/throw语句是成对出现的，如果程序中使用了throw而没有用catch捕获，会导致异常：
>> probe try [throw "error"]
*** Throw Error: no catch for throw: "error"
*** Where: throw
*** Stack:
# Red异常系统的问题
# make error!还不能带参数(arg1/arg2/arg3)
make error! [user message ["msg1" "msg2]]
但是可以使用cause-error替代
# 不能捕获特定的异常
或许我们可以增加如下语法：
probe try/with [cause-error 'user 'message][
['user 'message] [probe self]
['script 'no-arg] [probe self]]
如果两种异常都没有捕获，异常继续向上层throw。
当然我们可以捕获后再往上层throw，但是这样不方便使用。如果系统提供捕获特定异常的方式，将能够简化部分异常处理。
我们可以利用bind，实现该功能：
try-catch: func [blk [block!] cond [block!] todo [block!] /local error][
unless error? error: try blk [return error]
if do bind cond 'error [return do bind todo 'error]
error
]
suc: "your todo"
;-- test case 1
command: [1 / 0]
cond: [error/code = 400]
todo: [print ["catch" lf error] suc]
a: try-catch command cond todo
print a = suc
;-- test case 2
command: [1 / 0]
cond: [error/code = 300]
todo: [print ["catch" lf error] 1]
a: try-catch command cond todo
print error? a
;-- test case 3
command: [1 / 1]
cond: [error/code = 300]
todo: [print ["catch" lf error] 1]
a: try-catch command cond todo
print a = 1
# Red中异常处理的最佳实践
# 每个context中提供一个自定义异常
ctx-a: context [
system/catalog/errors/user: make system/catalog/errors/user [ctx-a: ["ctx-a [" :arg1 ": (" :arg2 " " :arg3 ")]"]]
new-error: func [name [word!] arg2 arg3][
cause-error 'user 'ctx-a [name arg2 arg3]
]
func-a: func [][
new-error 'func-a "msg1" "msg2"
]
]