Fable validation demo 💙

Type something below and try to guess what validate does, knowing that the helper Result.toText is just a one-liner that renders Result<> using different emojis for success and error.

Perhaps you have something like this in mind:

match 'type with
| Text     -> check that it's 1 line & not null
| FreeText -> check that it's not null
| Integer  -> check that it can be parsed to int

Turns out it does nothing of the sort, in fact validate is not even defined anywhere in the code! It’s a generic function from FSharp.Domain.Validation, one that has no awareness of our own custom types Text, FreeText, and Integer.

100% Object-free ✔

You may be thinking “ok then FreeText is an object with a constructor that validates a string”, but it’s in fact much simpler than that, it’s just a combination of a validation rule with a type name. The interface below is only used to identify it as a Validation box, and conveniently enforce the definition of a validation rule with the appropriate signature:

type FreeText = private FreeText of string with
  interface TextBox with
    member _.Validate =
      String.IsNullOrWhiteSpace ==> IsMissingOrBlank
      // 🤯

This simplicity is not just a nicety, if you’re going to replace all your validated strings with similar types, and you definitely should, it’s important that these can be defined with minimal code.

Certified DRY™ ✔

Our other custom type Text also rejects empty strings, but its definition doesn't even declare that rule:

type Text = private Text of FreeText with
  interface TextBox with
    member _.Validate =
      containsControlChars ==> ContainsCtrlChars
      // 🤯🤯

Certified KISS™ ✔

So declaring types requires very little code, but so does validating! Most of the time the function validate doesn’t even have to specify a type like in the code below:

open type FSharp.Domain.Validation.Box<str, TxtErr>
open FsToolkit.ErrorHandling

// this creates a valid(ated) record using
// a validation CE from FsToolkit.ErrorHandling
validation {

  let! text = validate inp1
  and! freeText = validate inp2
  //🤯🤯🤯

  return
  {
    TextProp = text
    FreeTextProp = freeText
  }
}

May contain traces of RTFM 📖

I know it all sounds super easy but do me and yourself a favor, read the project’s README before trying this at home. Not only it’s more up-to-date than this demo, but it also uses examples that don’t make the use of FSharp.Domain.Validation.Operators making them more stock F# and easier to follow.

For instance here it’s not immediately obvious that the _.Validate function returns a list of errors.

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Fable users read this 🚨

With Fable you’ll have reference only the package FSharp.Domain.Validation👉.Fable

Records like MyDomain above are worthless unless they can be used in javascript, in order to properly serialize them with Thoth.Json use extra encoders for each box type:

open FSharp.Domain.Validation.Thoth

let myExtraCoders =
  Extra.empty
  |> Extra.withCustom
      Codec.Encode<Text>
      Codec.Decode<Text>
  |> Extra.withCustom
      Codec.Encode<FreeText>
      Codec.Decode<FreeText>
  // etc…

The function Unchecked.boxof won’t be available in Fable until Fable#2321 is closed, so for now the only way to quickly skip Result<_,_> is with something like:
```
|> function Ok x -> x | _ -> failwith "💣"
```