README.md

FSharp.Validations

FSharp Validations is a validation library designed with a functional first approach.

The library supports:

• The ability to create rules for properties using the RuleFor<'a, 'b> function
• The ability to apply a list of rules to a propery using Rule<'a>
• the ability to reuse validators using ruleSetFor<'a, 'b>
• The creation of RuleSets<'a> -> 'a ValidationResult

The rules can then be evaluated at any time thanks to the magic of currying.

open FSharp.Validations

type Foo = { Bar: string }

let validateFoo =
    ruleSet [
        ruleFor <@ _.Bar @> [
            rule (fun x -> x.Length > 0) (Some "Length must be greater than 0")
            rule (fun x -> x.Length < 10) (Some "Length must be less than 10") ] ] 

let result = validateFoo { Bar = "Hello, World!" }

// result is Failure (map [("Bar", ["Value must be less than 10"])])

Reusing Rule Sets

Rule sets can be reused by creating a RuleSet<'a> and applying it to a property using the ruleSetFor<'a, 'b> function.

type Bar = { Value: string }
type Foo = { Bar: Bar }

let validateBar =
    ruleSet [
        ruleFor <@ _.Value @> [
            rule (fun x -> x.Length > 0) (Some "Length must be greater than 0")
            rule (fun x -> x.Length < 10) (Some "Length must be less than 10") ] ]

let validateFoo =
    ruleSet [
        ruleSetFor <@ _.Bar @> validateBar ]

Any resulting errors for Bar will have a key of Bar.{propertyName}.

ValidationResult<'a>

A Success represents the entity that has passed validations.

A Failure represents a Map<string, string list> where the key is the property that failed and value is a collection of all the messages attached to the failure upon validation.

let result = validateFoo { bar = "Hello, World!" }

match result with
| Success value -> printfn "%A" value
| Failure errors -> errors |> Map.iter (printfn "%s: %A")

Dependency Injection

A function is provided to convert any 'a -> 'a ValidationResult to an 'a IValidator using the provided toValidator function.

The resulting 'a IValidator can be used for Dependency Injection.

open Microsoft.Extensions.DependencyInjection

type Foo = { Bar: string }

let validateFoo =
    ruleSet<Foo> [ ruleFor <@ _.Bar @> [ notEmpty ] ]

let fooValidator = validateFoo |> toValidator

let result = fooValidator.Validate({ Bar = "Hello, World!" })

// Add to Dependency Injection

let services = ServiceCollection() :> IServiceCollection
services.AddScoped<Foo IValidator>(fun _ -> fooValidator) |> ignore

License, Copywright, Etc.

FSharp.Validations is subject to copywright @ 2025 Oneirosoft and other contributors under the MIT License.