Pure Checks

Check<'value> is the foundation of FsFlow validation. It is a reusable predicate layer that returns Result<'value, unit>.

• If the predicate is true, it returns Ok value.
• If the predicate is false, it returns Error ().

By returning unit on failure, Check stays agnostic about your application’s error types. You decide what the error means at the boundary where you use it.

The Two Shapes of Check

1. Value-Preserving: Check<'value> - returns the input value on success.

   let check = Check.notBlank // string -> Result<string, unit>
   

2. Gate: Check<unit> - returns unit on success (useful for yes/no questions).

   let check = Check.okIf (age > 18) // unit -> Result<unit, unit>
   

Attaching Errors with Check.orError

Use Check.orError to turn a unit failure into a meaningful application error.

type ValidationError =
    | NameRequired
    | InvalidAge

let validateName name =
    name |> Check.notBlank |> Check.orError NameRequired

let validateAge age =
    age |> Check.okIf (age > 0) |> Check.orError InvalidAge

Common Helpers

The Check module includes many built-in helpers:

• Check.notBlank, Check.notNull, Check.notEmpty
• Check.okIf, Check.failIf
• Check.equal, Check.notEqual
• Check.okIfSome, Check.okIfValueSome

Why use Checks?

• Reusability: Write a check once, use it in result {}, validate {}, or any Flow.
• Composition: You can combine checks using Check.and and Check.or.
• Decoupling: Pure logic doesn’t need to know about your logging or your database.