feature: Add map field name to convert structs dynamically instead of individually with a tag.

[thespags]

Feature to solve #40

MapFieldName = func(s string) {
  return strings.ToLower(s)
}

or #148

MapFieldName = func(s string) {
  return strcase.ToLowerSnakeCase(s)
}

[thespags]

Hi @sagikazarmark , I'm curious about your thoughts on adding this dynamic matching for decodemapfromstruct,
This ends up helping Viper, so I can bind my config value against environment variables FOO_BAR, for an identifier FooBar, without having to write a mapstructure tag for every identifier.

[sagikazarmark]

If I understand correctly, you want to avoid tagging your struct with mapstructure struct tags, correct?

This certainly seems like an easy fix, although there are a couple caveats:

• if IgnoreUntaggedFields is set to true, the field will still be omitted (which I guess could be unexpected to some, since not using tags is the whole point of this feature, still, the current behavior is the right one I believe, especially if you want to mix using tags and the mapper)
• There is an "opposite" direction mapping as well that's supposed to work: from maps to structs. I'm not sure from the top of my head how that would work in this case (but Viper actually does that). So at least a test would be necessary.

Need to think through a couple other edge cases before this can land.

[thespags]

If I understand correctly, you want to avoid tagging your struct with mapstructure struct tags, correct?

Yes the challenge I was having nested into #148 comment:

Specifically, I'm trying to have viper map something like FooBar to be loaded from an environmental variable FOO_BAR without having to create mapstructure for each key. I can't use SetEnvKeyReplacer because by that time the casing has been lost, so I would get FOOBAR.

[thespags]

if IgnoreUntaggedFields is set to true, the field will still be omitted (which I guess could be unexpected to some, since not using tags is the whole point of this feature, still, the current behavior is the right one I believe, especially if you want to mix using tags and the mapper)

Yeah, if you use IgnoreUntaggedFields then it would negate the point of this mapping.

There is an "opposite" direction mapping as well that's supposed to work: from maps to structs. I'm not sure from the top of my head how that would work in this case (but Viper actually does that). So at least a test would be necessary.

I added a test case to demonstrate this, I believe this behavior makes sense. If you specify a tag, we use that, otherwise we will convert the struct's field name for the map.

[thespags]

@sagikazarmark I was wondering if you had a chance to follow up on my updates. Thank you.

[sagikazarmark]

LGTM, thanks @thespags