> 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?
-George
