Gluon allows correct and consistent automatic serialization and deserialization for a specific universe of datatypes. For every type in this universe, Gluon defines an F# representation, a TypeScript representation, a serialized JSON representation, and automatic converters between in-memory and serialized representations. The grammar for the type set is given below:

type Type =
    | Boolean
    | String
    | Bytes
    | Json
    | Int
    | Double
    | DateTime
    | Record of list<Field * Type>
    | Union of list<Case * list<Field * Type>>
    | Tuple of list<Type>
    | Option of Type
    | Array of Type
    | Sequence of Type
    | List of Type   
    | StringDict of Type

JSON (or other serialized format) representations should be considered Gluon internal implementation detail and may change without notice. Gluon will never need to discriminate on values to infer the type of the represented objects, since type information is always available statically. This allows distinct types use similar JSON representations (such as a string) without confusion.

Boolean, String, Int, Double have intuitive representations. Gluon should preserve the complete range of values (including NaN, Infinity, null strings).


Since version 0.4.2, Gluon supports two semantics for DateTime values: the "physical" interpretation of DateTime is a time-point in universal time coordinates, and the "calendar" interpretation:

x = y <=> ymdhms(x) = ymdhmx(y)

For convenience, both interpretations use the same types (the distinction is not tracked in the type system). On the F# side the type is System.DateTime, and on the TypeScript side it is Date.

Physical datetimes

On CLR, physical datetimes are represented as DateTime object with Kind being either DateTimeKind.Local or DateTimeKind.Utc. In TypeScript, these are represented as vanilla Date objects. During interop, zone conversions happen to preserve the universal coordinate meaning of the datetime. Note that the browser always displays Date objects in user-local zone, which might be different from the server timezone; server timezone information is lost.

Calendar datetimes

Calendar semantics are useful when displaying dates as-is, bypassing timezone conversions. Two dates are considered calendar-equivalent if they have the same year, month, day, hour, minute and second. Time zone information is not tracked and must be implied.

On CLR, calendar datetimes are DateTime objects with DateTimeKind.Unspecified. In TypeScript, these are represented as a Date object that is extended to have an extra tag x.unspecified == true. Browsers ignores the tag and treat the Date objects as if they were in browser-local timezone, however the tag is used by Gluon to make sure these values roundtrip to the server using calendar semantics.

Records and unions

Records and unions use the WebSharper serialization convention, where the internal representation of the type is considered, regardless of public/internal/private declarations. This is unlike some other conventions such as JSON.NET or other CLR serializers, that use public properties or opt-in and opt-out attributes. This limits flexibility but greatly simplifies constructing equivalent types in TypeScript, and ensuring turnaround. Records will map to TypeScript classes in an intuitive manner.

Unions currently generate a TypeScript class for each case and an untagged TypeScript union type alias for the union itself. This scheme has runtime efficiency and allows to discriminate values with the instanceof operator. This representation may change in the future, an alternative here is to provide a match construct that enforces completeness check in the type system.

Since 0.4.x, Gluon also generates a typed pattern-matching helper for unions.


Tuples will map to JavaScript records and anonymous object types in TypeScript.


Array, sequence and list types will all map to TypeScript arrays.


Options will map to a newly introduced Option TypeScript type, and StringDict will use a newly introduced Dict TypeScript type logically equivalent to F# Dictionary, implemented efficiently using object keys in JavaScript.

Unstructured data

Finally, there are some rare situations where absolute type safety is not feasible or counter-productive. In these cases it is nice to have an escape hatch that allows to communicate data without specifying its structure entirely. String is one such escape hatch; Gluon provides two more: Bytes type to transfer byte sequences, represented as packed typed arrays in TypeScript, and a black-box JSON type that allows embedding any value in the JSON grammar - represented as a corresponding JavaScript object, and typed "any" in TypeScript.

type Type =
  | Boolean
  | String
  | Bytes
  | Json
  | Int
  | Double
  | DateTime
  | Record of (obj * Type) list
  | Union of (obj * (obj * Type) list) list
  | Tuple of Type list

Full name: datatypes.Type
union case Type.Boolean: Type
Multiple items
union case Type.String: Type

module String

from Microsoft.FSharp.Core
union case Type.Bytes: Type
union case Type.Json: Type
union case Type.Int: Type
union case Type.Double: Type
union case Type.DateTime: Type
union case Type.Record: (obj * Type) list -> Type
type 'T list = List<'T>

Full name: Microsoft.FSharp.Collections.list<_>
union case Type.Union: (obj * (obj * Type) list) list -> Type
union case Type.Tuple: Type list -> Type
Multiple items
union case Type.Option: Type -> Type

module Option

from Microsoft.FSharp.Core
Multiple items
union case Type.Array: Type -> Type

module Array

from Microsoft.FSharp.Collections
union case Type.Sequence: Type -> Type
Multiple items
union case Type.List: Type -> Type

module List

from Microsoft.FSharp.Collections

type List<'T> =
  | ( [] )
  | ( :: ) of Head: 'T * Tail: 'T list
  interface IEnumerable
  interface IEnumerable<'T>
  member GetSlice : startIndex:int option * endIndex:int option -> 'T list
  member Head : 'T
  member IsEmpty : bool
  member Item : index:int -> 'T with get
  member Length : int
  member Tail : 'T list
  static member Cons : head:'T * tail:'T list -> 'T list
  static member Empty : 'T list

Full name: Microsoft.FSharp.Collections.List<_>
union case Type.StringDict: Type -> Type
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