Prototype v2

[alexflint]

This PR will introduce a v2 of go-arg, to be imported with

import "github.com/alexflint/go-arg/v2

It should be backwards with v1 for about 95% of users. If you only used arg.MustParse then you can use v2 without modifications to your code. The main changes are:

• add Parser.ProcessCommandLine, Parser.ProcessEnvironment, Parser.ProcessDefaults so that you can control the order in which things are loaded
• add Parser.OverwriteWithCommandLine, Parser.OverwriteWithEnvironment, Parser.OverwriteWithDefaults for further control over which arguments get overwritten when by which other arguments
• Drop the Config struct in favor of a list of ParserOptions to NewParser

Features still to be implemented on this branch:

• Bash autocomplete support
• Custom templates for help and usage text

For users and contributors to go-arg: please leave your feedback and suggestions on this PR.

[codecov-commenter]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage :thumbsup:

Coverage data is based on head (c046f49) compared to base (11f9b62). Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #197   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            5         5           
  Lines          688       690    +2     
=========================================
+ Hits           688       690    +2     

Impacted Files	Coverage Δ
parse.go	100.00% <0.00%> (ø)

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[GibMeMyPacket]

Hope you make #146 as soon as possible. I will migrate to this prototype immediately right after that.

[suntong]

Last commit was 8 months ago, and it is now 19 commits ahead, 20 commits behind master.

IE, I see delaying would be more harmful than helpful.

Maybe we can have v2 now and have the remaining features added later?

[purpleidea]

I took a look, and one design question baffled me:

We have cli values, env values, default values, and config-file values. I would expect that the precedence order be:

default value config-file value env value cli value

IOW, default is what's used when nothing else is given. Which can be overridden by a config-file value. Which can be overridden by an ENV value. Which can be overridden by the cli flag.

I am probably just naive here, but I can't think of a scenario when this is not the desired precedence order. As a result, I believe that allowing the user to set this order makes our API more complicated. Just my personal opinion, but I am happy to hear alternate viewpoints.

[suntong]

Agree 🤦‍♂️, totally agree with your opinion @purpleidea 👍

[jweyrich]

I'm also on the waitlist for the multiple "providers". And agree with @purpleidea rationale above. In the future it would be cool to see something like the chained providers in the AWS SDK for Java, which allows you to pick your provider (env, file, etc), or compose a chain of them in a specific order. More in https://docs.aws.amazon.com/sdk-for-java/latest/developer-guide/credentials-specify.html and https://github.com/aws/aws-sdk-java/blob/master/aws-java-sdk-core/src/main/java/com/amazonaws/auth/DefaultAWSCredentialsProviderChain.java