add support for 'required' tag value

[bonifaido]

Implementation for checking that required fields got filled in map -> struct decoding direction.

A basic test has been added as well.

type Required struct {
	RequiredBar string `mapstructure:"bar,required"`
	Value       string `mapstructure:"foo"`
}

Fixes: #7

[janderland]

Would be nice if this could be set globally as well, for all fields.

[philippfrank]

@bonifaido I was looking exactly for this. What's the status?

[bonifaido]

I need to rebase on master now I guess, since this is almost 1 year old, after that I'm waiting on a review/merge 🙂

[bonifaido]

Rebased it on latest master.

[philippfrank]

@mitchellh Are there any plans to merge/review this? It's a really helpful addition!

[dz0gchen]

need merge

[wang502]

this is feature is really helpful. can we give this a review? thanks :) @mitchellh

[montaro]

Please merge

[wangshuai14]

It seems not working. Are there errors in my code?

[bonifaido]

Are you using my fork@branch @wangshuai14 ?

[wangshuai14]

Are you using my fork@branch @wangshuai14 ?

Got it! thanks

[bonifaido]

Just rebased on the latest master to see its working.

[Etran-H]

@mitchellh
can comment on this?

[cmatthias]

If it's helpful to anyone, I forked mapstructure and merged this PR as well as #225 into my fork. Feel free to use.

https://github.com/cmatthias/mapstructure

[mitchellh]

Hello all. Its been 3 years, and I've sat on this. I've integrated mapstructure into a dozen or more projects by this point. I've certainly run into this issue before. I believe that its better to use a full featured struct validation library rather than support this one feature in mapstructure. I've always reached for something like ozzo-validation or validator and felt that was just the better approach. I'm sorry, but I want to keep this library as focused on the decoding problem rather than data validation (beyond simple type matching -- necessary for decoding).

[mitchellh]

Actually #225 is a great approach to this that I think balances the line between enabling this sort of functionality but not stepping into what in my opinion is validation territory in the same way. I'll merge that.