Repository F# setup

open Reified
open Reified.Refinements
open Reified.Result

Customer Id Tutorial

The built-in types cover common shapes. This tutorial defines a domain-specific type and tests whether it warrants a wrapper.

Verified

open Reified
open Reified.Refinements

Reified

Refinements

Decide whether it earns a type

A refined type is worth defining when the invariant does something for the code that receives it. Ask what becomes total, or what branch disappears:

Candidate	Verdict
`CustomerId`, a positive account number	Type. Lookup, ordering, and equality all rely on it, and an id of `0` is a bug you want to catch once.
`EmailAddress`, matches an email pattern	Constraint. Nothing downstream is total because of it; you unwrap it to send mail.
`ShippingWeight`, positive, and summed across a parcel	Constraint. The moment you add two of them, F# cannot carry "positive" through, so the type turns every sum into a `Result`.
`NormalisedName`, trimmed and lower-cased	Neither. That is a transformation, so it belongs in Parse.

Only the first changes what later code can assume. The second is real validation with no downstream consequence; the third is validation that arithmetic immediately undoes. Both stay constraints on the underlying value:

Fragment

field _.Email {
    constrain Constraint.present
    constrain Constraint.email
}

Define the type

A refined type is a private wrapper, one canonical projection, and a refinement:

Verified

type CustomerId =
    private
    | CustomerId of int

    /// The canonical underlying representation.
    member this.Value =
        let (CustomerId value) = this
        value

    override this.ToString() =
        string this.Value

module CustomerId =
    /// Admission and its reverse projection, packaged together.
    let refinement =
        Refinement.define
            (Constraint.greaterThan 0) // the rule over the underlying int
            CustomerId                 // wrap a value that passed
            _.Value                    // unwrap again, always

    let create value = Refinement.create refinement value

    let value (input: CustomerId) = input.Value

FsLiveDocsGeneratedPage37_6BB268B4E740.CustomerId

CustomerId

int

An abbreviation for the CLI type . Basic Types

this: CustomerId

Value: CustomerId -> unit -> int

value: int

ToString: CustomerId -> unit -> string

string: 'T -> string

Converts the argument to a string using ToString. For standard integer and floating point values and any type that implements IFormattableToString conversion uses CultureInfo.InvariantCulture. The input value. The converted string. string 'A' // evaluates to "A" string 0xff // evaluates to "255" string -10 // evaluates to "-10"

Value: int

FsLiveDocsGeneratedPage37_6BB268B4E740.CustomerIdModule

refinement: Refinement<int,CustomerId>

Reified.Refinements.Refinement

Creates and applies reusable refinement definitions.

define: Constraint<'underlying> -> ('underlying -> 'refined) -> ('refined -> 'underlying) -> Refinement<'underlying,'refined>

Defines a refinement from one constraint, a constructor, and the reverse projection. Compose several rules with Constraint.all before defining, and reach for Constraint.custom when the rule is an arbitrary predicate. Both produce an ordinary constraint, so there is no separate plural or check-taking constructor. type RetryCount = RetryCount of int let retryCount = Refinement.define (Constraint.between 0 10) RetryCount (fun (RetryCount value) -> value)

Reified.ConstraintModule

Creates, executes, composes, and inspects constraints.

greaterThan: 'value -> Constraint<'value>

Requires a value strictly greater than the supplied bound. let quantity : Constraint<int> = Constraint.greaterThan 0

_arg1: CustomerId

create: int -> Result<CustomerId,Violation>

create: Refinement<'underlying,'refined> -> 'underlying -> Result<'refined,Violation>

Constructs a refined value, reporting why the raw value was not admitted. type RetryCount = RetryCount of int let retryCount = Refinement.define (Constraint.between 0 10) RetryCount (fun (RetryCount value) -> value) 3 |> Refinement.create retryCount |> Result.mapError Violation.render

value: CustomerId -> int

input: CustomerId

The case is private, so CustomerId.create is the only way in:

Verified

CustomerId.create 42   // Ok
CustomerId.create 0    // Error (Atomic (Expected (RelationAtom (Compared (GreaterThan, Integer 0L)), Some (Integer 0L))))

FsLiveDocsGeneratedPage37_6BB268B4E740.CustomerIdModule

create: int -> Result<CustomerId,Violation>

Use Constraint.all [ ... ] when several constraints describe admission, or Constraint.customWith for an invariant no built-in constraint describes, then hand either to the same Refinement.define. See Define Refined Types for both.

Give it the operations that justify it

CustomerId is a key; its operations support lookup and identity, not arithmetic:

Isolated

open Reified
open Reified.Refinements

type CustomerId =
    private
    | CustomerId of int

    member this.Value =
        let (CustomerId value) = this
        value

module CustomerId =
    let refinement = Refinement.define (Constraint.greaterThan 0) CustomerId _.Value
    let create value = Refinement.create refinement value
    let value (input: CustomerId) = input.Value

    // ... as above

    /// Total: distinct ids stay distinct, so no entry can be lost.
    let index (customers: DistinctList<CustomerId>) = DistinctList.toSet customers

    /// Total: ids are ordered, so a range of them is an interval.
    let range (first: CustomerId) (second: CustomerId) = Interval.between first second

Reified

Refinements

FsLiveDocsGeneratedPage38_6BB268B4E740.CustomerId

CustomerId

int

An abbreviation for the CLI type . Basic Types

this: CustomerId

Value: CustomerId -> unit -> int

value: int

FsLiveDocsGeneratedPage38_6BB268B4E740.CustomerIdModule

refinement: Refinement<int,CustomerId>

Reified.Refinements.Refinement

Creates and applies reusable refinement definitions.

define: Constraint<'underlying> -> ('underlying -> 'refined) -> ('refined -> 'underlying) -> Refinement<'underlying,'refined>

Reified.ConstraintModule

Creates, executes, composes, and inspects constraints.

greaterThan: 'value -> Constraint<'value>

Requires a value strictly greater than the supplied bound. let quantity : Constraint<int> = Constraint.greaterThan 0

_arg1: CustomerId

Value: int

create: int -> Result<CustomerId,Violation>

create: Refinement<'underlying,'refined> -> 'underlying -> Result<'refined,Violation>

value: CustomerId -> int

input: CustomerId

index: DistinctList<CustomerId> -> Set<CustomerId>

customers: DistinctList<CustomerId>

Reified.Refinements.DistinctList`1

A list with no duplicate items, preserving first-seen order.

Reified.Refinements.DistinctListModule

Operations over lists known to hold no duplicates.

toSet: DistinctList<'value> -> Set<'value>

Builds a set. Total and lossless — this is the operation that justifies the type, because distinct items always produce a set of the same size, while Set.ofList on an ordinary list silently collapses duplicates.

range: CustomerId -> CustomerId -> Interval<CustomerId>

first: CustomerId

second: CustomerId

Reified.Refinements.IntervalModule

Operations over inclusive ranges of ordered values.

between: 'a -> 'a -> Interval<'a>

Builds the smallest interval containing both values, ordering them as needed. Total — this is the constructor to reach for first. Requires a total order. With float or float32, a NaN argument cannot be ordered against anything and produces inverted bounds; prefer Interval<FiniteFloat> or create there.

Both work because the invariant is a fact about the value, not about the moment of construction, and neither involves arithmetic, which is where F# stops being able to carry the invariant for you. If no such operation applies, use a constraint instead.

Use it in domain code

Fragment

let loadCustomer (id: CustomerId) =
    // No guard: id is known to be above zero.
    repository.load id.Value

Keep the raw type at input and storage boundaries, and the refined type in between.

Give it a schema

Refinement.constraints exposes the same rule to Schema and other interpreters, so a boundary describes the type without restating it:

Verified

open Reified

let customerIdSchema : Schema<CustomerId> =
    Schema.int |> Schema.refine CustomerId.refinement

Reified

customerIdSchema: Schema<CustomerId>

Reified.Schema`1

Describes a typed value's portable structure and construction for schema interpreters. A schema records shape and construction metadata without tying that metadata to input parsing, diagnostics, validation, codecs, UI generation, or workflow execution. Primitive, collection, optional, union, refined, and record declarations all produce Schema<'value>. Record declarations use the schema<'value> { } computation expression. Each field may contain withSchema, constrain, constraints, refine, and validate operations before the declaration finishes with construct or constructResult.

FsLiveDocsGeneratedPage37_6BB268B4E740.CustomerId

Reified.Schema

Construction, composition, parsing, and checking for universal schemas.

int: Schema<int>

Describes a 32-bit integer.

(|>): 'T1 -> ('T1 -> 'U) -> 'U

Apply a function to a value, the value being on the left, the function on the right The argument. The function. The function result. let doubleIt x = x * 2 3 |> doubleIt // Evaluates to 6

refine: Refinement<'raw,'value> -> Schema<'raw> -> Schema<'value>

Maps a raw schema through a reusable bidirectional refinement. The smart constructor runs during parsing. Inspection supplies the raw representation during checking and encoding.

FsLiveDocsGeneratedPage37_6BB268B4E740.CustomerIdModule

refinement: Refinement<int,CustomerId>

Parsing checks the int, constructs the CustomerId, and reports failures at the field's path. Encoding projects back through Value. The emitted JSON Schema carries exclusiveMinimum: 0 because the constraint travelled with the refinement, you did not write the rule twice.

To resolve the type in a bare field without a withSchema, register it once as described in Refined Schemas, which works the same example through the Schema DSL and shows where schema-local constraints fit alongside it.

Order Totals, the built-in types used in anger.
Define Refined Types, the full Refinement reference.
Schema Integration, applying refinements at structured boundaries.
Refined Schemas, the Schema-side view.

Customer Id Tutorial

Decide whether it earns a type

Define the type

Give it the operations that justify it

Use it in domain code

Give it a schema

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