One of the unique features of dry-schema is the ability to define rules that are applied before coercion. This means you can ensure that the input has the right format for the coercion to work, or maybe you just want to restrict certain values entering your system.

Here's a common example - your system supports only one specific date format, and you want to validate that the input value has this format before trying to coerce it into a date object. In order to do that, you can use filter macro, which works just like value, but its predicates are applied before value coercion:

schema = Dry::Schema.Params do
  required(:birthday).filter(format?: /\d{4}-\d{2}-\d{2}/).value(:date)
end'email' => '', 'birthday' => '1981-1-1')
#<Dry::Schema::Result{:email=>"", :birthday=>"1981-1-1"} errors={:birthday=>["is in invalid format"]}>'email' => '', 'birthday' => '1981-01-01')
#<Dry::Schema::Result{:email=>"", :birthday=>#<Date: 1981-01-01 ((2444606j,0s,0n),+0s,2299161j)>} errors={}>

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