Allow variable expansion

[syntaqx]

• Allow for variable expansion using expand
• expand struct lookup for values that have defaults that are not set.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (3d2b11f) to head (93d7fb2).

Additional details and impacted files

@@            Coverage Diff            @@
##              main        #9   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            4         4           
  Lines          289       325   +36     
=========================================
+ Hits           289       325   +36     

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[ccoVeille]

You should use different name here than host and port, because the example might let people think HOST and PORT already need to be provided as fields

So maybe, something like

Suggested change

	Address string `env:"ADDRESS,default=${HOST}:$PORT,expand"`
	Address string `env:"ADDRESS,default=${ADDR}:$BIND,expand"`

[syntaqx]

This is pretty standard variable expansion behavior, take Bash for example:

HOST="localhost"
PORT="8080"
ADDR="${HOST}:${PORT}"
echo $ADDR

In bash, you do define them in order. Since variables are expected to be defined before you start your code, IE:

HOST=localhost PORT=8080 go run main.go

The behavior of expand would work with that.

The reverse lookup of default values also allows for the code to be able to leverage those, so if values aren't set in the environment, you don't have to run into errors.

[ccoVeille]

good point about the default value for variable used in another variable using expand.

It should be covered by a dedicated test maybe

Setting just

PORT="8081"

and expect to have

type Config struct {
	Debug    bool           `env:"DEBUG"`
	Host     string         `env:"HOST,default=localhost"`
	Port     string         `env:"PORT,default=8080"`
	Address  string         `env:"ADDRESS,default=${HOST}:$PORT,expand"`
}

cfg := Config{
	Host: "localhost"
	Port: "8081"
	Address: "localhost:8081"
}

[ccoVeille]

It's getting too complicated, it was OK when you had 2 fields

you should define a struct with all the fields you need, then create a variable from this struct, then pass a reference to the variable and, that's it

No more code rewrites when adding an new feature, and also less pointers

[syntaqx]

There wasn't really a way to make the default value lookup work without this. I'm open to suggestions, but variable expansion is a strong feature of environment variables.

[ccoVeille]

I could make a suggestion, but I have to get back from holidays, as I only have a phone with me right now.

So, I'm opening a refactoring issue

[ccoVeille]

I created

• refactoring needed: unmarshalField is getting too complicated #10

[ccoVeille]

This regexp will be computed each time you call the method, it uses a lot of resources

Move it out the method, especially because you are using must compile. It would panic quickly if you update the regexp to something invalid, not when this feature is used

[syntaqx]

Good find, I've moved the MustCompiles to variables in main but will wait to do a release until we've got through any other feedback.