limited dependent typing via F# Type Provider
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Dependent Types Provider for Domain-driven Development

The goal of this project is to allow certain primitive types to be contrained according to rules for your domain. Loosely defined this means dependent types for certain primitives (string, number types).

Some code:

open FSharp.DependentTyping

// let's define our product description to be 10-2000 characters
open FSharp.DependentTypes.Strings
type ProductDescription = BoundedString<10, 2000>

// factory methods available for all types
ProductDescription.TryCreate("").IsNone // true
ProductDescription.TryCreate("what a groovy library").IsSome // true

// if you don't care about safety, use the constructor
let ``boom!`` = ProductDescription(null)
ProductDescription.TryCreate(null).IsNone // true

// These are actually System.String!
type TwitterHandle = BoundedString(1, 15)
let b = Name("CAIndy")
let (s  : string) = upcast b // warning "String has no proper subtypes"
Assert.AreEqual("CAIndy", b) // yup!

// It goes to 11
open FSharp.DependentTypes.Numbers
type VolumeLevel = BoundedByte<11> // lower bound defaults to 0

// all number types are supported
[<Literal>] let PerihelionKm = 147.1e+6
[<Literal>] let AphelionKm   = 152.1e+6
type DistanceToSunKm = BoundedDouble<PerihelionKm,AphelionKm>
// yes, Units of measure are coming soon :)


The constructor and factory method are both here because sometimes you trust your inputs and sometimes you don't. This approach should incur less overhead than using a single-case discriminated union, as the type is erased to string (or numeric type).

Request for Feedback

There's already some good guidance in the community on dealing with constrained strings as semantically richer types, but I think this approach is lighter weight.

Is there a feature here you'd like to see? Let me know!


That's all for now. Here's the plan thus far:

  • PatternString accepting RegEx string type parameter

  • Units of measure for bounded numbers

  • NuGet love

  • Cross-targetting

  • C# Interop tests

Copyright 2016 Christopher Atkins