Data strucutre of Form Validation error message

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5

Personally, I’m a big fan of a data structure that looks roughly like this when modeling data that can be validated:

type Validated a
  = Initial -- Field is neither valid nor invalid on initial render
  | Valid a -- We can turn the valid raw string into a custom value `a`
  | Invalid String String -- We track both the raw string and the error message

You can wrap any field in it, including fields that get transformed to fancier types:

type Model =
  { name : Validated String
  , phone : Validated PhoneNumber
  , email : Validated Email
  }

I walked through data modeling a multi-step form with validation in this discourse thread. I also created an executable example on Ellie that uses this.

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