Examples of creating constrained types in F#

ConstrainedTypesExamples.fsx

// General hints on defining types with constraints or invariants
// 
// Just as in C#, use a private constructor 
// and expose "factory" methods that enforce the constraints
//
// In F#, only classes can have private constructors with public members. 
//
// If you want to use the record and DU types, the whole type becomes
// private, which means that you also need to provide:
// * a constructor function ("create").
// * a function to extract the internal data ("value").
//
// This is marginally annoying, but you can always use the C# approach if you like!
//
// OTOH, your opaque types really ARE opaque! 
//
// The other alternative is to use signature files (which are not discussed here)

module ConstrainedTypes =
    open System

    // ---------------------------------------------
    // Constrained String50 (FP-style)
    // ---------------------------------------------

    /// Type with constraint that value must be non-null
    /// and <= 50 chars.
    type String50 = private String50 of string

    /// Module containing functions related to String50 type
    module String50 =

        // NOTE: these functions can access the internals of the
        // type because they are in the same scope (namespace/module)
        
        /// constructor
        let create str = 
            if String.IsNullOrEmpty(str) then
                None
            elif String.length str > 50 then
                None
            else
                Some (String50 str)

        // function used to extract data since type is private
        let value (String50 str) = str

    // ---------------------------------------------
    // Constrained String50 (object-oriented style)
    // ---------------------------------------------

    /// Class with constraint that value must be non-null
    /// and <= 50 chars.
    type OOString50 private(str) =
        /// constructor
        static member Create str = 
            if String.IsNullOrEmpty(str) then
                None
            elif String.length str > 50 then
                None
            else
                Some (OOString50(str))
        /// extractor
        member this.Value = str

    // ---------------------------------------------
    // Constrained AtLeastOne (FP style)
    // ---------------------------------------------

    /// Type with constraint that at least one of the fields
    /// must be set.
    type AtLeastOne = private {
        A : int option
        B : int option
        C : int option
        }

    /// Module containing functions related to AtLeastOne type
    module AtLeastOne =
        
        /// constructor
        let create aOpt bOpt cOpt =
            match aOpt,bOpt,cOpt with
            | (Some a,_,_) -> Some <| {A = aOpt; B=bOpt; C=cOpt}
            | (_,Some b,_) -> Some <| {A = aOpt; B=bOpt; C=cOpt}
            | (_,_,Some c) -> Some <| {A = aOpt; B=bOpt; C=cOpt}
            | _ -> None
            // This might fail, so return option -- caller must test for None

        // These three always succeed, no need to test for None
        let createWhenAExists a bOpt cOpt = 
            {A = Some a; B=bOpt; C=cOpt}
        let createWhenBExists aOpt b cOpt = 
            {A = aOpt; B=Some b; C=cOpt}
        let createWhenCExists aOpt bOpt c = 
            {A = aOpt; B=bOpt; C=Some c}

        // function used to extract data since type is private
        let value atLeastOne = 
            let a = atLeastOne.A
            let b = atLeastOne.B
            let c = atLeastOne.C
            (a,b,c) 

    // ---------------------------------------------
    // Constrained AtLeastOne (object-oriented style)
    // ---------------------------------------------

    /// Class with constraint that at least one of the fields
    /// must be set.
    type OOAtLeastOne private (aOpt:int option,bOpt:int option,cOpt:int option) = 
        
        /// constructor
        static member create(aOpt,bOpt,cOpt) =
            match aOpt,bOpt,cOpt with
            | (Some a,_,_) -> Some <| OOAtLeastOne(aOpt,bOpt,cOpt )
            | (_,Some b,_) -> Some <| OOAtLeastOne(aOpt,bOpt,cOpt )
            | (_,_,Some c) -> Some <| OOAtLeastOne(aOpt,bOpt,cOpt )
            | _ -> None

        // These three always succeed, no need to test for None
        static member createWhenAExists(a,bOpt,cOpt) = 
            OOAtLeastOne(Some a,bOpt,cOpt)
        static member createWhenBExists(aOpt,b,cOpt) = 
            OOAtLeastOne(aOpt,Some b,cOpt)
        static member createWhenCExists(aOpt,bOpt,c) = 
            OOAtLeastOne(aOpt,bOpt,Some c)

        member this.Value = 
            (aOpt,bOpt,cOpt) 

    // ---------------------------------------------
    // Constrained DU (FP style)
    // ---------------------------------------------

    /// DU with constraint that classification must be done correctly
    type NumberClass = 
        private
        | IsPositive of int // int must be > 0
        | IsNegative of int // int must be < 0
        | Zero

    /// Module containing functions related to NumberClass type
    module NumberClass =
        let create i = 
            if i > 0 then IsPositive i
            elif i < 0 then IsNegative i
            else Zero 

        // active pattern used to extract data since type is private
        let (|IsPositive|IsNegative|Zero|) numberClass = 
            match numberClass with
            | IsPositive i -> IsPositive i 
            | IsNegative i -> IsNegative i 
            | Zero -> Zero 

// ======================================================
// This client attempts to use the types defined above
// ======================================================

module Client = 
    open ConstrainedTypes 


    // ---------------------------------------------
    // Constrained String50 (FP-style)
    // ---------------------------------------------

    let s50Bad = String50 "abc" 
    // ERROR: The union cases or fields of the type 'String50' are not accessible from this code location
    
    let s50opt = String50.create "abc" 
    s50opt 
    |> Option.map String50.value 
    |> Option.map (fun s -> s.ToUpper())
    |> Option.iter (printfn "%s")


    // ---------------------------------------------
    // Constrained String50 (object-oriented style)
    // ---------------------------------------------

    let ooS50Bad = OOString50("abc")   
    // ERROR: This type has no accessible object constructors

    let ooS50opt = OOString50.Create "abc" 
    ooS50opt 
    |> Option.map (fun s -> s.Value)
    |> Option.map (fun s -> s.ToUpper())
    |> Option.iter (printfn "%s")

    // ---------------------------------------------
    // Constrained AtLeastOne (FP style)
    // ---------------------------------------------
    let atLeastOneBad = {A=None; B=None; C=None}  
    // ERROR: The union cases or fields of the type 'AtLeastOne' are not accessible from this code location 

    let atLeastOne_BOnly = AtLeastOne.create None (Some 2) None
    match atLeastOne_BOnly with
    | Some x -> x |> AtLeastOne.value |> printfn "%A" 
    | None -> printfn "Not valid"
    
    let atLeastOne_AOnly = AtLeastOne.createWhenAExists 1 None None
    let atLeastOne_AB = AtLeastOne.createWhenAExists 1 (Some 2) None
    atLeastOne_AB |> AtLeastOne.value |> printfn "%A" 

    

    // ---------------------------------------------
    // Constrained AtLeastOne (OO style)
    // ---------------------------------------------

    let ooAtLeastOneBad = OOAtLeastOne(None, None, None)  //  This type has no accessible object constructors

    let atLeastOne_BOnly = OOAtLeastOne.create(None,Some 2,None)
    match atLeastOne_BOnly with
    | Some x -> printfn "%A" x.Value
    | None -> printfn "Not valid"
    
    let ooAtLeastOne_AOnly = OOAtLeastOne.createWhenAExists(1,None,None)
    let ooAtLeastOne_AB = OOAtLeastOne.createWhenAExists(1,Some 2,None)
    ooAtLeastOne_AB.Value |> printfn "A=%A" 
    

    // ---------------------------------------------
    // Constrained DU (FP style)
    // ---------------------------------------------

    // attempt to create a bad value
    let numberClassBad = IsPositive -1  
    // ERROR: The union cases or fields of the type 'NumberClass' are not accessible from this code location 

    let numberClass = NumberClass.create -1
    // this fails because the DU cases are not accessible
    match numberClass with
    | IsPositive i -> printfn "%i is positive" i
    | IsNegative i -> printfn "%i is negative" i
    | Zero -> printfn "is zero" 

    open NumberClass // bring active pattern into scope
    // this works because the active pattern is being used.
    match numberClass with
    | IsPositive i -> printfn "%i is positive" i
    | IsNegative i -> printfn "%i is negative" i
    | Zero -> printfn "is zero" 

@gdennie see further discussions on this in the fs-lang suggestions repo here

When dealing with record types, you might need to add public getters like here.

I really wish F# had something built-in for this, because this is really complicated. So many ways to try and do it, and many of those ways have subtle issues. Let's just look at one from here.

/// Type with constraint that at least one of the fields
/// must be set.
...

Looks great, and absolutely seems to do what you want, until...

let ao = AtLeastOne.create None None (Some 1)
let oops = { ao with C = None }

We've just created invalid state. :(

Here's a pattern for this that hopefully addresses all concerns. I'm going to use FsToolkit to do validation during creation.

open FsToolkit.ErrorHandling

// Use a module for constrained type
module User =
    // How to describe an "unvalidated"/unconstrained type. This can be a simple type like string or int as well.
    type Descriptor = { First: String; Last: String }
    // The "opaque" constrained type
    type T = private T of Descriptor
    
    // validation helper
    let private validateName name n =
        if String.IsNullOrWhiteSpace(n) then Error <| sprintf "%s name can't be empty" name
        else Ok n

    // "Constructor" that may fail validation
    let tryMk desc =
        validation {
            let! first = validateName "First" desc.First
            and! last = validateName "Last" desc.Last
            return T desc
        }

    // "Constructor" that throws when validation fails.
    let mk desc =
        match tryMk desc with
        | Error e -> failwith (["Invalid user:"] @ e |> String.concat "\n   ")
        | Ok t -> t

    // Extract the Descriptor from the opaque type
    let desc (T user) = user

    // Other functions that work with the type can be declared here

// Active pattern for our type
let (|User|) user = User.desc user

let scott = User.mk { First = "Scott"; Last = "Wlaschin" }
let msg =
     match scott with
     | User { First = "Scott" } -> "It's a Scott!"
     | _ -> "Not a Scott. :("
printfn "%s" msg

Here's a pattern for this that hopefully addresses all concerns. I'm going to use FsToolkit to do validation during creation.

open FsToolkit.ErrorHandling

// Use a module for constrained type
module User =
    // How to describe an "unvalidated"/unconstrained type. This can be a simple type like string or int as well.
    type Descriptor = { First: String; Last: String }
    // The "opaque" constrained type
    type T = private T of Descriptor
    
    // validation helper
    let private validateName name n =
        if String.IsNullOrWhiteSpace(n) then Error <| sprintf "%s name can't be empty" name
        else Ok n

    // "Constructor" that may fail validation
    let tryMk desc =
        validation {
            let! first = validateName "First" desc.First
            and! last = validateName "Last" desc.Last
            return T desc
        }

    // "Constructor" that throws when validation fails.
    let mk desc =
        match tryMk desc with
        | Error e -> failwith (["Invalid user:"] @ e |> String.concat "\n   ")
        | Ok t -> t

    // Extract the Descriptor from the opaque type
    let desc (T user) = user

    // Other functions that work with the type can be declared here

// Active pattern for our type
let (|User|) user = User.desc user

let scott = User.mk { First = "Scott"; Last = "Wlaschin" }
let msg =
     match scott with
     | User { First = "Scott" } -> "It's a Scott!"
     | _ -> "Not a Scott. :("
printfn "%s" msg

Hello!
I saw your remark about the with keyword circumventing constructor validation. "We've just created invalid state."
I'm wondering whether your implementation above has a way to get around this issue?

Kind regards,

Julius

@juliusdeblaaij Interestingly, it appears to.

let changed = { scott with First = "Bill" }

The record label 'First' is not defined.

If it didn't, you could make User.T entirely opaque using a signature file, but it doesn't appear to be necessary.