Add support for default values in struct tags

[arvindh123]

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• [X] I have searched the issue tracker for an issue that matches the one I want to file, without success.

Problem Description

Currently, Viper only supports retrieving values from environment variables and it requires the user to specify the exact environment variable name in the struct tag. It would be useful to have the ability to specify a default value in the struct tag as well, so that if the environment variable is not set, the default value will be used instead.

Proposed Solution

The proposed solution is to add a new field in the struct tag called "default" which will hold the default value. The syntax for the struct tag will be as follows:

`env:"ENV_VAR_NAME,default=DEFAULT_VALUE"`

For example:

type Config struct {
    Port int `env:"PORT,default=8080"`
}

Alternatives Considered

No response

Additional Information

No response

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[tkrop]

I love this proposal, and I definitely would like some support for setting default values using tags, however reusing the env-tag seems to create redundant information that makes the name of environment variables unclear. So using something simpler as go-defaults default-tag during initial creation of the root struct would be more sufficient.

This would result in support for

type Config struct {
    Port int `default=8080"`
}

However, since go-defaults can actually be applied to the input struct before being inserted, it actually turns out that the core issue is to define the environment variable names for default in viper. So one would actually just need to extract a key value list of the base struct to setup defaults.

Viper support for such a feature could therefor just consist of allowing to register a default configuration object that is providing all basic default values. In the end it than does not matter whether this template is provided by go-defaults or any other way to setup a default structure.