TypeShape is a small, extensible F# library for practical 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.
See my 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:2.10 src/TypeShape/TypeShape.fs
and in paket.references:
File: TypeShape.fs TypeShape
TypeShape is also available on
open System
open TypeShape
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.Accept {
new IFSharpOptionVisitor<'T -> string> with
member __.Visit<'a> () =
let tp = mkPrinter<'a>()
wrap(function None -> "None" | Some t -> sprintf "Some (%s)" (tp t))
}
| Shape.Tuple2 s ->
s.Accept {
new ITuple2Visitor<'T -> string> with
member __.Visit<'t1, 't2> () =
let tp = mkPrinter<'t1>()
let sp = mkPrinter<'t2>()
wrap(fun (t : 't1, s : 't2) -> sprintf "(%s, %s)" (tp t) (sp s))
}
| Shape.FSharpList s ->
s.Accept {
new IFSharpListVisitor<'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.Accept {
new IArrayVisitor<'T -> string> with
member __.Visit<'a> _ =
let tp = mkPrinter<'a> ()
wrap(fun ts -> ts |> Array.map tp |> 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 list * string option) * (bool * unit)> ()
p (([1 .. 5], None), (false, ())) // "(([1; 2; 3; 4; 5], None), (false, ()))"Let's see how the value printer compares to sprintf:
#time "on"
type TestType = (int list * string option * string) * (bool * unit)
let value : TestType = (([1 .. 5], None, "42"), (false, ()))
let p1 = sprintf "%A" : TestType -> string
let p2 = mkPrinter<TestType>()
// Real: 00:00:00.442, CPU: 00:00:00.437, GC gen0: 31, gen1: 0, gen2: 0
for i = 1 to 1000 do ignore <| p1 value
// Real: 00:00:00.006, CPU: 00:00:00.000, GC gen0: 2, gen1: 1, gen2: 0
for i = 1 to 1000 do ignore <| p2 valueTypeShape can be used to define generic programs that target arbitrary types:
F# records, unions or POCOs. This is achieved using the IShapeMember abstraction:
type IShapeMember<'DeclaringType, 'Field> =
inherit IShapeMember<'DeclaringType>
abstract Project : 'DeclaringType -> 'Field
abstract Inject : '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 Project and Inject 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 IShapeMemberVisitor<'DeclaringType, 'DeclaringType -> string> with
member __.Visit (shape : ShapeMember<'DeclaringType, 'Field>) =
let fieldPrinter = mkPrinter<'Field>()
fieldPrinter << shape.Project }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> [] = s.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 = s.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.fsx Pretty printer generator for common F# types.
- Parser.fsx Parser generator for common F# types using FParsec.
- Equality-Comparer.fsx Equality comparer generator for common F# types.
- Gmap.fsx A gmap implementation.
- hashcode-staged.fsx Staged generic hashcode generator.
Head (branch master), Build & Unit tests
- Windows/.NET
- Linux/Mono 5.0
