by eiriktsarpalis

eiriktsarpalis / TypeShape

Practical generic programming for F#

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TypeShape is a small, extensible F# library for practical datatype-generic programming. Borrowing from ideas used in the FsPickler implementation, it uses a combination of reflection, active patterns and F# object expressions to minimize the amount of reflection required by the user in such applications.

TypeShape permits definition of programs that act on specific algebrae of types. The library uses reflection to derive the algebraic structure of a given

instance and then applies a variant of the visitor pattern to provide relevant type information per shape.

TypeShape can provide significant performance improvements compared to equivalent reflection-based approaches. See the performance page for more details and benchmarks.

Please see my article and slides for a more thorough introduction to the concept.


To incorporate TypeShape in your project place the following line in your

github eiriktsarpalis/TypeShape:8.0 src/TypeShape/TypeShape.fs
and in
File: TypeShape.fs TypeShape
TypeShape is also available on NuGet Badge

Example: Implementing a value printer

open System
open TypeShape.Core

let rec mkPrinter () : 'T -> string = let wrap(p : 'a -> string) = unbox string> p match shapeof with | Shape.Unit -> wrap(fun () -> "()") | Shape.Bool -> wrap(sprintf "%b") | Shape.Byte -> wrap(fun (b:byte) -> sprintf "%duy" b) | Shape.Int32 -> wrap(sprintf "%d") | Shape.Int64 -> wrap(fun (b:int64) -> sprintf "%dL" b) | Shape.String -> wrap(sprintf ""%s"") | Shape.FSharpOption s -> s.Element.Accept { new ITypeVisitor string> with member __.Visit () = let tp = mkPrinter() wrap(function None -> "None" | Some t -> sprintf "Some (%s)" (tp t)) }

| Shape.FSharpList s ->
    s.Element.Accept {
        new ITypeVisitor string> with
            member __.Visit () =
                let tp = mkPrinter()
                wrap(fun ts -> ts |> List.map tp |> String.concat "; " |> sprintf "[%s]")

| Shape.Array s when s.Rank = 1 ->
    s.Element.Accept {
        new ITypeVisitor string> with
            member __.Visit () =
                let tp = mkPrinter ()
                wrap(fun ts -> ts |> Array.map tp |> String.concat "; " |> sprintf "[|%s|]")

| Shape.Tuple (:? ShapeTuple as shape) ->
    let mkElemPrinter (shape : IShapeMember) =
       shape.Accept { new IMemberVisitor string> with
           member __.Visit (shape : ShapeMember) =
               let fieldPrinter = mkPrinter()
               fieldPrinter << shape.Get }

    let elemPrinters : ('T -> string) [] = shape.Elements |> Array.map mkElemPrinter

    fun (r:'T) ->
        |> Seq.map (fun ep -> ep r)
        |> String.concat ", "
        |> sprintf "(%s)"

| Shape.FSharpSet s ->
    s.Accept {
        new IFSharpSetVisitor string> with
            member __.Visit () =
                let tp = mkPrinter()
                wrap(fun (s:Set) -> s |> Seq.map tp |> String.concat "; " |> sprintf "set [%s]")

| _ -> failwithf "unsupported type '%O'" typeof

let p = mkPrinter () p (42, Some false, ["string"], [|1;2;3;4;5|]) // val it : string = "(42, Some (false), ["string"], [|1; 2; 3; 4; 5|])"

Records, Unions and POCOs

TypeShape can be used to define generic programs that access fields of arbitrary types: F# records, unions or POCOs. This is achieved using the

type IShapeMember =
    abstract Get : 'DeclaringType -> 'Field
    abstract Set : 'DeclaringType -> 'Field -> 'DeclaringType
An F# record then is just a list of member shapes, a union is a list of lists of member shapes. Member shapes can optionally be configured to generate code at runtime for more performant
operations. Member shapes come with quoted versions of the API for staged generic programming applications.

To make our pretty printer support these types, we first provide a pretty printer for members:

let mkMemberPrinter (shape : IShapeMember) =
   shape.Accept { new IMemberVisitor string> with
       member __.Visit (shape : ShapeMember) =
           let fieldPrinter = mkPrinter()
           fieldPrinter << shape.Get }
Then for F# records: ```fsharp match shapeof<'T> with | Shape.FSharpRecord (:? ShapeFSharpRecord<'T> as shape) -> let fieldPrinters : (string * ('T -> string)) [] = s.Fields |> Array.map (fun f -> f.Label, mkMemberPrinter f)
    fun (r:'T) ->
        |> Seq.map (fun (label, fp) -> sprintf "%s = %s" label (fp r))
        |> String.concat "; "
        |> sprintf "{ %s }"
Similarly, we could also add support for arbitrary F# unions:
    match shapeof with
    | Shape.FSharpUnion (:? ShapeFSharpUnion as shape) ->
        let cases : ShapeFSharpUnionCase [] = shape.UnionCases // all union cases
        let mkUnionCasePrinter (case : ShapeFSharpUnionCase) =
            let fieldPrinters = case.Fields |> Array.map mkMemberPrinter
            fun (u:'T) -> 
                |> Seq.map (fun fp -> fp u) 
                |> String.concat ", "
                |> sprintf "%s(%s)" case.CaseInfo.Name

    let casePrinters = cases |&gt; Array.map mkUnionCasePrinter // generate printers for all union cases
    fun (u:'T) -&gt;
        let tag : int = shape.GetTag u // get the underlying tag for the union case
        casePrinters.[tag] u

Similar active patterns exist for classes with settable properties and general POCOs.


TypeShape can be extended to incorporate new active patterns supporting arbitrary shapes. Here's an example illustrating how TypeShape can be extended to support ISerializable shapes.

Additional examples

See the project samples folder for more implementations using TypeShape:

  • Printer.fs Pretty printer generator for common F# types.
  • Parser.fs Parser generator for common F# types using FParsec.
  • Equality-Comparer.fs Equality comparer generator for common F# types.
  • hashcode-staged.fs Staged generic hashcode generator.
  • Gmap There are set of
    related functions within the
    module in the Nuget package.

Using the Higher-Kinded Type API (Experimental)

As of TypeShape 8 it is possible to avail of an experimental higher-kinded type flavour of the api, which can be used to author fully type-safe programs for most common applications. Please see my original article on the subject for background and motivation.

To use the new approach, we first need to specify which types we would like our generic program to support:

open TypeShape.HKT

type IMyTypesBuilder = inherit IBoolBuilder inherit IInt32Builder inherit IStringBuilder

inherit IFSharpOptionBuilder
inherit IFSharpListBuilder
inherit ITuple2Builder

The interface

denotes a "higher-kinded" generic program builder which supports combinations of boolean, integer, string, optional, list and pair types.

Next, we need to define how interface implementations are to be folded:

let mkGenericProgram (builder : IMyTypesBuilder) =
    { new IGenericProgram with
        member this.Resolve () : App = 
            match shapeof with
            | Fold.Bool builder r -> r
            | Fold.Int32 builder r -> r
            | Fold.String builder r -> r
            | Fold.Tuple2 builder this r -> r
            | Fold.FSharpOption builder this r -> r
            | Fold.FSharpList builder this r -> r
            | _ -> failwithf "I do not know how to fold type %O" typeof }

This piece of boilerplate composes built-in

active patterns, which contain folding logic for the individual builders inherited by the interface. Note that the order of composition can be significant (e.g. folding with

Let's now provide a pretty-printer implementation for our interface:

// Higher-Kinded encoding
type PrettyPrinter =
    static member Assign(_ : App, _ : 'a -> string) = ()

// Implementing the interface let prettyPrinterBuilder = { new IMyTypesBuilder with member __.Bool () = HKT.pack (function false -> "false" | true -> "true") member __.Int32 () = HKT.pack (sprintf "%d") member __.String () = HKT.pack (sprintf ""%s"")

    member __.Option (HKT.Unpack elemPrinter) = HKT.pack(function None -&gt; "None" | Some a -&gt; sprintf "Some(%s)" (elemPrinter a))
    member __.Tuple2 (HKT.Unpack left) (HKT.Unpack right) = HKT.pack(fun (a,b) -&gt; sprintf "(%s, %s)" (left a) (right b))
    member __.List (HKT.Unpack elemPrinter) = HKT.pack(Seq.map elemPrinter &gt;&gt; String.concat "; " &gt;&gt; sprintf "[%s]") }

Putting it all together gives us a working pretty-printer:

let prettyPrint : 't -> string = (mkGenericProgram prettyPrinterBuilder).Resolve () |> HKT.unpack

prettyPrint 42 prettyPrint (Some false) prettyPrint (Some "test", [Some 42; None; Some -1])

Please check the samples/HKT folder for real-world examples of the above.

Projects using TypeShape

Related Work

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