> On Jun 18, 2021, at 6:00 PM, Nick Rosbrook <rosbro...@gmail.com> wrote:
>
> On Fri, Jun 18, 2021 at 04:18:44PM +0000, George Dunlap wrote:
>>
>>
>>> On Jun 18, 2021, at 4:08 PM, Nick Rosbrook <rosbro...@gmail.com> wrote:
>>>
>>> On Fri, Jun 18, 2021 at 02:44:15PM +0000, George Dunlap wrote:
>>>>
>>>>
>>>>> On May 24, 2021, at 9:36 PM, Nick Rosbrook <rosbro...@gmail.com> wrote:
>>>>>
>>>>> Add a ContextOption type to support functional options in NewContext.
>>>>> Then, add a variadic ContextOption parameter to NewContext, which allows
>>>>> callers to specify 0 or more configuration options.
>>>>>
>>>>> For now, just add the WithLogLevel option so that callers can set the
>>>>> log level of the Context's xentoollog_logger. Future configuration
>>>>> options can be created by adding an appropriate field to the
>>>>> contextOptions struct and creating a With<OptionName> function to return
>>>>> a ContextOption
>>>>>
>>>>> Signed-off-by: Nick Rosbrook <rosbro...@ainfosec.com>
>>>>> ---
>>>>> tools/golang/xenlight/xenlight.go | 44 +++++++++++++++++++++++++++++--
>>>>> 1 file changed, 42 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/tools/golang/xenlight/xenlight.go
>>>>> b/tools/golang/xenlight/xenlight.go
>>>>> index f68d7b6e97..65f93abe32 100644
>>>>> --- a/tools/golang/xenlight/xenlight.go
>>>>> +++ b/tools/golang/xenlight/xenlight.go
>>>>> @@ -136,7 +136,7 @@ func sigchldHandler(ctx *Context) {
>>>>> }
>>>>>
>>>>> // NewContext returns a new Context.
>>>>> -func NewContext() (ctx *Context, err error) {
>>>>> +func NewContext(opts ...ContextOption) (ctx *Context, err error) {
>>>>> ctx = &Context{}
>>>>>
>>>>> defer func() {
>>>>> @@ -146,8 +146,19 @@ func NewContext() (ctx *Context, err error) {
>>>>> }
>>>>> }()
>>>>>
>>>>> + // Set the default context options. These fields may
>>>>> + // be modified by the provided opts.
>>>>> + copts := &contextOptions{
>>>>> + logLevel: LogLevelError,
>>>>> + }
>>>>> +
>>>>> + for _, opt := range opts {
>>>>> + opt.apply(copts)
>>>>> + }
>>>>> +
>>>>> // Create a logger
>>>>> - ctx.logger = C.xtl_createlogger_stdiostream(C.stderr, C.XTL_ERROR, 0)
>>>>> + ctx.logger = C.xtl_createlogger_stdiostream(C.stderr,
>>>>> + C.xentoollog_level(copts.logLevel), 0)
>>>>>
>>>>> // Allocate a context
>>>>> ret := C.libxl_ctx_alloc(&ctx.ctx, C.LIBXL_VERSION, 0,
>>>>> @@ -201,6 +212,35 @@ func (ctx *Context) Close() error {
>>>>> return nil
>>>>> }
>>>>>
>>>>> +type contextOptions struct {
>>>>> + logLevel LogLevel
>>>>> +}
>>>>> +
>>>>> +// ContextOption is used to configure options for a Context.
>>>>> +type ContextOption interface {
>>>>> + apply(*contextOptions)
>>>>> +}
>>>>> +
>>>>> +type funcContextOption struct {
>>>>> + f func(*contextOptions)
>>>>> +}
>>>>> +
>>>>> +func (fco *funcContextOption) apply(c *contextOptions) {
>>>>> + fco.f(c)
>>>>> +}
>>>>
>>>> Why all this convolution with interfaces and such, rather than just
>>>> defining ContextOption as a function pointer? Is it just to keep
>>>> contextOptions out of the documentation page?
>>>
>>> Part of the motivation for using functional options is to abstract the
>>> "options" struct, yes. This allows internal defaults to be applied more
>>> easily -- if you require e.g. a ContextOptions struct to be passed by
>>> the caller, how do you know if they intended to override a default, or
>>> if they just didn't set the field? Additionally, using the ContextOption
>>> as an interface allows variadic arguments, which are just convenient for
>>> API users -- the same NewContext function can be used whether you need
>>> to pass 3 options or 0.
>>>
>>> The reason we use ContextOption as an interface, rather than function
>>> pointer of sorts is for flexibility in the signatures of ContextOption
>>> implementations. E.g., we could have
>>>
>>> func WithLogLevel(lvl LogLevel) ContextOption
>>> func WithLogContext(s string) ContextOption
>>> func WithFooAndBar(s string, n int) ContextOption
>>>
>>> See [1] for more background on this pattern.
>>>
>>> Thanks,
>>> NR
>>>
>>> [1] https://dave.cheney.net/2014/10/17/functional-options-for-friendly-apis
>>
>> Yes, I frequently use a pattern like the one described in that blog post
>> myself. But that blog post doesn’t use interfaces — the final slide actually
>> has the “option function” type as an open-coded function pointer type.
>>
>> So my question was, why not do something like this:
>>
>> type ContextOption func(*contextOptions) error
>>
>> func WithLogLevel(level LogLevel) ContextOption {
>> return func(co *contextOptions) {
>> co.logLevel = level
>> }
>> }
>>
>> ATM the only advantage I can see of defining ContextOption as an interface
>> rather than as a function pointer is that the godoc for ContextOption would
>> look like:
>>
>> type ContextOption interface {
>> // contains filtered or unexported fields
>> }
>>
>> Rather than
>>
>> type ContextOption func(*contextOptions) error
>>
>> Which shows you the name of the unexported field.
>>
>> Is there another reason I missed?
>
> Technically it does allow more flexibility in implementing
> ContextOption, e.g. you could do...
>
> func (lvl LogLevel) apply(co *contextOptions) { co.logLevel = lvl }
>
> ...and then pass a LogLevel directly as a ContextOption. But generally
> everyone implements these things as funcs.
>
> I will admit that when it comes to my choice of using the interface
> version instead of function pointers, I am just more familiar with the
> former and encounter it more often in other Go packages I use.
OK. It seems a bit weird to me, but that’s not really a good reason to block
it. :-) I just wanted to make sure I understood why it was being chosen.
Acked-by: George Dunlap <george.dun...@citrix.com>
