First Class Environment Variable Binding

[integrii]

Flaggy currently works alongside the default environment binding that comes in the standard library. For example:

var := os.Getenv("MY_VAR") // set the var from the environment values MY_VAR
flaggy.String(&var, "v", "variable", "Just some variable") // Tell flaggy to overwrite the `var` if the `-v` or `--variable` flags are passed.
flaggy.Parse()

However, this gets a lot worse if your environment variable string needs to ultimately get parsed into another type:

durationStr := os.Getenv("DELAY_SECONDS") // set a time.Duration ultimately
duration, err := time.ParseDuration(durationStr)
if err != nil {
    panic("bad duration")
}
flaggy.Duration(&duration, "d", "duration" "Just some duration") // Tell flaggy to overwrite the duration if this flag is used and a duration is passed
flaggy.Parse()

That's a lot more code. Now, if you mixed in loading from configs as a baseline before parsing env vars, things start to get pretty messy.

So, it might be nice if flaggy could extend it's existing ability to parse a string into many different types to incoming environment variables. A simple set of .BindEnv() functions could do the trick:

var duration time.Duration
flaggy.DurationEnv(&duration, "DELAY_SECONDS")
flaggy.Duration(&duration, "d", "duration" "Just some duration") // Tell flaggy to overwrite the duration if this flag is used and a duration is passed
flaggy.Parse()

Alternatively, it could be possible to determine the type by the pointer to the target variable.

var duration time.Duration
flaggy.Env(&duration, "DELAY_SECONDS")
flaggy.Duration(&duration, "d", "duration" "Just some duration") // Tell flaggy to overwrite the duration if this flag is used and a duration is passed
flaggy.Parse()

[integrii]

It also makes me think that we could add a new generic binding for flags that would determine the type to convert into by simply looking at the assignment var, which would eliminate the need to have type-specific funcs.

var duration time.Duration
flaggy.Duration(&duration, "d", "duration" "Just some duration")

becomes

var duration time.Duration
flaggy.Flag(&duration, "d", "duration" "Just some duration")