[Dry-struct] .meta(omittable: true) vs. .optional.default(nil)

Support
1

I would like to ask for clarification about optional attributes in structs. What is the difference between

.meta(omittable: true)

and

.optional.default(nil) ?

Which is recommended in which situation? Are they different concepts or do they solve the same (or similar) problem? Or does one API supersede the other, possibly?

2

These are different things. One describes a possibly missing key and makes sense only in the context of a hash or a struct. The second one is about values that can be nil. You won’t see a difference when access an attribute with struct.attribute_name but missing keys won’t appear in the .attributes and .to_h output, have a look:

 :001 > Dry.Struct(name: 'string', age?: 'integer').new(name: 'John').to_h
 => {:name=>"John"} 
 :002 > Dry.Struct(name: 'string', age: Types::Integer.optional.default(nil)).new(name: 'John').to_h
 => {:name=>"John", :age=>nil}

FYI age? is a shortcut for omittable: true aka required: false

1 Like
3

I try to summarize again to confirm.

Optional is a type definition detail. The type will allow both any of it’s values, but nil in addition.

Not required (aka. omittable) is a struct (schema?) definition detail. It makes it ‘optional’ (in common parlance, but I see how saying that might be confusing because of how this library names things).

I think I got it. Thanks for your answer.