.NET and F# compatibility

Fable provides support for some classes of .NET BCL (Base Class Library) and most of FSharp.Core library. When possible, Fable translates .NET types and methods to native JavaScript APIs for minimum overhead.

.NET Base Class Library

The following classes are translated to JS and most of their methods (static and instance) should be available in Fable.

.NET	Python
Numeric Types	int
Arrays	array / bytearray
Events	fable-core/Event
System.Boolean	bool
System.Char	str
System.String	str
System.Guid	guid
System.TimeSpan	timedelta
System.DateTime	datetime
System.DateTimeOffset	timedelta
System.Timers.Timer	fable-core/Timer
System.Collections.Generic.List	List
System.Collections.Generic.HashSet	Set
System.Collections.Generic.Dictionary	dict
System.Text.RegularExpressions.Regex	re
System.Lazy	fable-core/Lazy
System.Random	{}
System.Math	math

The following static methods are also available:

• System.Console.WriteLine (also with formatting)

• System.Diagnostics.Debug.WriteLine (also with formatting)

• System.Diagnostics.Debug.Assert(condition: bool)

• System.Diagnostics.Debugger.Break()

• System.Activator.CreateInstance<'T>()

There is also support to convert between numeric types and to parse strings, check the conversion tests.

Caveats

• All numeric types become Python int (64-bit floating type), except for decimal. Check the Numeric Types section to learn more about the differences between .NET and Python.

• Numeric arrays are compiled to array when possible. Numeric arrays of uint8 are compiled to bytearray in order to support libraries that expect e.g bytes as input.

• No bound checks for numeric types (unless you do explicit conversions like byte 500) nor for array indices.

FSharp.Core

Most of FSharp.Core operators are supported, as well as formatting with sprintf, printfn or failwithf (String.Format is also available). The following types and/or corresponding modules from FSharp.Core lib will likewise translate to JS:

.NET	Python
Tuples	tuple
Option	(erased)
Choice	fable-core/Choice
Result	fable-core/Result
String	fable-core/String (module)
Seq	Iterable
List	fable-core/List
Map	fable-core/Map
Set	fable-core/Set
Async	fable-core/Async
Task	Awaitable
Event	fable-core/Event (module)
Observable	fable-core/Observable (module)
Arrays	array / bytearray
Events	fable-core/Event
MailboxProcessor	fable-core/MailboxProcessor (limited support)

Caveats II

• Options are erased in Python (Some 5 becomes just 5 in Python and None translates to None). This is needed for example, to represent Python optional properties. However in a few cases (like nested options) there is an actual representation of the option in the runtime.

• Async.RunSynchronously is not supported.

• MailboxProcessor is single-threaded in Python and currently only Start, Receive, Post and PostAndAsyncReply are implemented (cancellationToken or timeout optional arguments are not supported).

Object Oriented Programming

Most of F# OOP features are compatible with Fable: interfaces and abstract classes, structs, inheritance, overloading, etc. However, please note that instance members are not attached to the prototype, which means they won’t be accessible from native JS code. The exception to this rule are the implementations of interface and abstract members.

Caveats III

• It’s not possible to type test against interfaces or generic types.

Reflection and Generics

There is some reflection support in Fable, you can check the reflection tests to see what is currently possible.

Generics are erased by default in the generated Python code. However, it is still possible to access generic information (like typeof<'T>) at runtime by marking functions with inline:

let doesNotCompileInFable(x: 'T) =
    typeof<'T>.FullName |> printfn "%s"

let inline doesWork(x: 'T) =
    typeof<'T>.FullName |> printfn "%s"

doesWork 5