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A reusable description of valid values, coupled to the closures that execute it. One constraint value serves direct checking, refined-value admission, Schema, documentation, and export. There is no separate check type: check is the operation, Constraint is the noun. Both closures are retained deliberately. They are not duplicates of one rule: test over a conjunction may stop at the first failing child, while check must run every child to accumulate. Interpreted atoms and custom predicates therefore have a Boolean path that does no violation work, and combinators preserve that property when every child has it. A customWith constraint supplies only a violation-returning callback, so its test runs that callback and discards the error. The description is never interpreted during execution. Closures are composed once, at construction.

An abbreviation for the CLI type . Basic Types

Creates, executes, composes, and inspects constraints.

Requires every constraint to hold, evaluating each in declaration order and accumulating failures. The empty list is the satisfied identity. F# visits list elements left to right, so annotate the binding when the first element is a type-directed value: let requiredName : Constraint<string> = Constraint.all [ Constraint.present; Constraint.lengthBetween 2 40 ]. let requiredName : Constraint<string> = Constraint.all [ Constraint.present; Constraint.lengthBetween 2 40 ]

Requires a value to be inhabited according to its shape. Whitespace-only text is blank, as are null text, a null or empty collection or map, None, ValueNone, and an empty Nullable. Blankness means .NET's whitespace set plus U+FEFF, which is what lets the rule be exported; see nonBlankPattern. The shape is selected from the return type, so a reusable binding needs its annotation: let requiredName : Constraint<string> = Constraint.present. Applied where the type is already known — inside an annotated rule, or to a schema — no annotation is needed. let requiredName : Constraint<string> = Constraint.present

Requires text or a collection to have at most the supplied size. let summary : Constraint<string> = Constraint.maxLength 280

Creates, executes, composes, and inspects constraints.

Runs a constraint, returning why the value failed. let retryCount = Constraint.between 0 10 42 |> Constraint.check retryCount |> Result.mapError Violation.render

Helper type for error handling without exceptions. Choices and Results

An abbreviation for the CLI type . Basic Types

Why a value failed its constraint. A diagnostic contract, not an application error union. Domain code maps a whole violation once with Result.mapError; Schema adds the path at which it occurred. Violations are plain comparable data. No closure and no constraint description is reachable from one, so structural equality holds and a violation can be retained and compared long after the constraint that produced it went out of scope. There is no promised wire format. Reified-produced groups are never empty and never unary: a single failing child is returned directly rather than wrapped. The first * rest shape encodes non-emptiness only; non-unarity is a normalization invariant.

Creates, executes, composes, and inspects constraints.

Runs a constraint and returns the unchanged value after success. let requiredName : Constraint<string> = Constraint.present "Alice" |> Constraint.guard requiredName |> Result.mapError Violation.render

Creates, executes, composes, and inspects constraints.

Runs a constraint, returning why the value failed. let retryCount = Constraint.between 0 10 42 |> Constraint.check retryCount |> Result.mapError Violation.render

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

Fail-fast helpers over the standard F# Result type.

Maps the error value of a result.

Inspects, traverses, and renders violations.

Renders a violation as an English sentence fragment with no trailing punctuation, keeping conjunction and alternative groups distinct. Violation.render (Atomic (Expected (PresenceAtom Present, None))) // "value must be present"