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
System.Type 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
paket.dependencies file:
github eiriktsarpalis/TypeShape:10.0.0 src/TypeShape/TypeShape.fs
and in paket.references:
File: TypeShape.fs TypeShape
TypeShape is also available on
open System
open TypeShape.Core
let rec mkPrinter<'T> () : 'T -> string =
let wrap(p : 'a -> string) = unbox<'T -> string> p
match shapeof<'T> 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<'T -> string> with
member _.Visit<'a> () =
let tp = mkPrinter<'a>()
wrap(function None -> "None" | Some t -> sprintf "Some (%s)" (tp t))
}
| Shape.FSharpList s ->
s.Element.Accept {
new ITypeVisitor<'T -> string> with
member _.Visit<'a> () =
let tp = mkPrinter<'a>()
wrap(fun ts -> ts |> List.map tp |> String.concat "; " |> sprintf "[%s]")
}
| Shape.Array s when s.Rank = 1 ->
s.Element.Accept {
new ITypeVisitor<'T -> string> with
member _.Visit<'a> () =
let tp = mkPrinter<'a> ()
wrap(fun ts -> ts |> Array.map tp |> String.concat "; " |> sprintf "[|%s|]")
}
| Shape.Tuple (:? ShapeTuple<'T> as shape) ->
let mkElemPrinter (shape : IShapeMember<'T>) =
shape.Accept { new IMemberVisitor<'T, 'T -> string> with
member _.Visit (shape : ShapeMember<'DeclaringType, 'Field>) =
let fieldPrinter = mkPrinter<'Field>()
fieldPrinter << shape.Get }
let elemPrinters : ('T -> string) [] = shape.Elements |> Array.map mkElemPrinter
fun (r:'T) ->
elemPrinters
|> Seq.map (fun ep -> ep r)
|> String.concat ", "
|> sprintf "(%s)"
| Shape.FSharpSet s ->
s.Accept {
new IFSharpSetVisitor<'T -> string> with
member _.Visit<'a when 'a : comparison> () =
let tp = mkPrinter<'a>()
wrap(fun (s:Set<'a>) -> s |> Seq.map tp |> String.concat "; " |> sprintf "set [%s]")
}
| _ -> failwithf "unsupported type '%O'" typeof<'T>
let p = mkPrinter<int * bool option * string list * int []> ()
p (42, Some false, ["string"], [|1;2;3;4;5|])
// val it : string = "(42, Some (false), ["string"], [|1; 2; 3; 4; 5|])"TypeShape can be used to define generic programs that access fields of arbitrary types:
F# records, unions or POCOs. This is achieved using the IShapeMember abstraction:
type IShapeMember<'DeclaringType, 'Field> =
abstract Get : 'DeclaringType -> 'Field
abstract Set : 'DeclaringType -> 'Field -> 'DeclaringTypeAn 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 Get and Set 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<'DeclaringType>) =
shape.Accept { new IMemberVisitor<'DeclaringType, 'DeclaringType -> string> with
member _.Visit (shape : ShapeMember<'DeclaringType, 'Field>) =
let fieldPrinter = mkPrinter<'Field>()
fieldPrinter << shape.Get }Then for F# records:
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) ->
fieldPrinters
|> 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<'T> with
| Shape.FSharpUnion (:? ShapeFSharpUnion<'T> as shape) ->
let cases : ShapeFSharpUnionCase<'T> [] = shape.UnionCases // all union cases
let mkUnionCasePrinter (case : ShapeFSharpUnionCase<'T>) =
let fieldPrinters = case.Fields |> Array.map mkMemberPrinter
fun (u:'T) ->
fieldPrinters
|> Seq.map (fun fp -> fp u)
|> String.concat ", "
|> sprintf "%s(%s)" case.CaseInfo.Name
let casePrinters = cases |> Array.map mkUnionCasePrinter // generate printers for all union cases
fun (u:'T) ->
let tag : int = shape.GetTag u // get the underlying tag for the union case
casePrinters.[tag] uSimilar 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.
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
gmaprelated functions within theTypeShape.Genericmodule in the Nuget package.
As of TypeShape 8 it is possible to avail of a 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<'F> =
inherit IBoolBuilder<'F>
inherit IInt32Builder<'F>
inherit IStringBuilder<'F>
inherit IFSharpOptionBuilder<'F>
inherit IFSharpListBuilder<'F>
inherit ITuple2Builder<'F>The interface MyTypeBuilder<'F> 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<'F>) =
{ new IGenericProgram<'F> with
member this.Resolve<'a> () : App<'F, 'a> =
match shapeof<'a> 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<'a> }This piece of boilerplate composes built-in Fold.* 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 FSharpOption before FSharpUnion).
Let's now provide a pretty-printer implementation for our interface:
// Higher-Kinded encoding
type PrettyPrinter =
static member Assign(_ : App<PrettyPrinter, 'a>, _ : 'a -> string) = ()
// Implementing the interface
let prettyPrinterBuilder =
{ new IMyTypesBuilder<PrettyPrinter> 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 -> "None" | Some a -> sprintf "Some(%s)" (elemPrinter a))
member _.Tuple2 (HKT.Unpack left) (HKT.Unpack right) = HKT.pack(fun (a,b) -> sprintf "(%s, %s)" (left a) (right b))
member _.List (HKT.Unpack elemPrinter) = HKT.pack(Seq.map elemPrinter >> String.concat "; " >> sprintf "[%s]") }Putting it all together gives us a working pretty-printer:
let prettyPrint<'t> : 't -> string = (mkGenericProgram prettyPrinterBuilder).Resolve<'t> () |> 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.
