Porting the Dark backend

The big project at the moment is the replatform, moving the Dark backend from OCaml to F#. The motivation for this is to give us access to a richer ecosystem of libraries to build Dark on, especially cloud libraries. This is described more in a set of blog posts:

Porting standard library functions

Dark's standard library functions need to be ported from OCaml to F#. At last count there were 267 functions that needed porting, in addition to the 207 already ported.

You can get a list of the standard library functions that need porting by running:

cat fsharp-backend/src/*/StdLib/*.fs | grep "//" | perl -n -e 'foreach ($_ =~ /name = fn "([A-Za-z0-9]+)" "([A-Za-z0-9]+)" (\d)/ ) { print "$1::$2_v$3\n" } ' | sort | uniq

When porting a function, start by finding the commented out version of it (in fsharp-backend/src/LibExecution/StdLib or fsharp-backend/src/LibBackend/StdLib). The original OCaml code has been preprocessed in bulk to closely match the intended F# code, so once you uncomment it, there should only be a small amount of work to port it over.


The functions in OCaml were written to not function concurrently, while the F# version is designed to run concurrently. The major difference here is that the OCaml backend returns a Dval (a Dark value), while the F# version returns a TaskOrValue<Dval>.

For most simple functions that do no IO, a TaskOrValue just wraps a Dval. For example, compare the original OCaml version of Int::add_v0:

; { prefix_names = ["Int::add"]
; infix_names = ["+"]
; parameters = [par "a" TInt; par "b" TInt]
; return_type = TInt
; description = "Adds two integers together"
; func =
| _, [DInt a; DInt b] ->
DInt (Dint.( + ) a b)
| args ->
fail args)
; preview_safety = Safe
; deprecated = false }

to the F# version

{ name = fn "Int" "add" 0
parameters = [ Param.make "a" TInt ""; Param.make "b" TInt "" ]
returnType = TInt
description = "Adds two integers together"
fn =
| _, [ DInt a; DInt b ] -> Value(DInt(a + b))
| _ -> incorrectArgs ())
sqlSpec = SqlBinOp "+"
previewable = Pure
deprecated = NotDeprecated }

As you can see, the major change that was needed for the return value was to wrap the Dval in a Value.


For functions that perform IO, you'll need to use the taskv "computation expression". A "computation expression" is a special F# language feature for writing abstractions with a nice syntax. The taskv CE allows using TaskOrValues easily, and can best be illustrated with an example:

| state, [ DObj value; DStr key; DDB dbname ] ->
taskv {
let db = state.dbs.[dbname]
let! _id = UserDB.set state true db key value
return DObj value
| _ -> incorrectArgs ())

Let's break this down line by line:

  • taskv {: this creates the CE, whose return value will be a TaskOrValue<'any>.
  • let db = state.dbs.[dbname] - this is just regular F# code
  • let! _id = UserDB.set state true db key value - the special thing here is the let! - this line calls UserDB.set, a function which returns a TaskOrValue, and unwraps the TaskOrValue. This means that _id can be treated as a normal value for the rest of this taskv.
  • return DObj value - return takes an ordinary value and turns it into a TaskOrValue, in this case a TaskOrValue<Dval>.

Why do we go through all this trouble? Because this is an async runtime, and let! and return are the enablers of the asyncronicity. A TaskOrValue is a promise, and let! waits for the promise and then continues (running other code while the IO is still pending). This is the exact same as the async keyword in JS, Rust, C# or Python.

Callbacks to the Interpreter

You'll also need this for first-class functions like List::map_v0, as they may be used to call functions which return Tasks. Here is an example of this:

{ name = fn "List" "map" 0
parameters =
[ Param.make "list" (TList varA) "The list to be operated on"
(TFn([ varA ], varB))
"Function to be called on each member" ]
description =
"Returns a list created by the elements of `list` with `fn` called on each of them in order"
returnType = TList varB
fn =
| state, [ DList l; DFnVal b ] ->
taskv {
let! result =
(fun dv ->
(id 0)
[ dv ]
return Dval.list result
| _ -> incorrectArgs ())
sqlSpec = NotYetImplementedTODO
previewable = Pure
deprecated = NotDeprecated }

This runs map_s, which applies the function to the list of arguments (map_s is short for map sequentially, which completes each previous task before starting the next one). applyFnVal calls the interpreter to run the passed in function, which might return a Task.

New types

The F# port also creates the ability to add type checking to Dark. Rather than using a non-nested type like TResult, we use TResult(DType, DType) to indicate the types used in the Ok and Error constructors used by Result. The definition of a type has switched from this in OCaml:

module RuntimeT = struct
type tipe = Serialization_format.tipe =
| TAny
| TInt
| TFloat
| TBool
| TNull
| TDeprecated1
| TStr
| TList
| TObj
| TIncomplete
| TError
| TBlock
| TResp
| TDate
| TDbList of tipe
| TPassword
| TUuid
| TOption
| TErrorRail
| TCharacter
| TResult
| TUserType of string * int
| TBytes

to this in F#:

and DType =
| TAny
| TInt
| TFloat
| TBool
| TNull
| TStr
| TList of DType
| TDict of DType
| TIncomplete
| TError
| TLambda
| THttpResponse of DType
| TDB of DType
| TDate
| TChar
| TPassword
| TUuid
| TOption of DType
| TErrorRail
| TUserType of string * int
| TBytes
| TResult of DType * DType
| TVariable of string
| TFn of List<DType> * DType
| TRecord of List<string * DType>

All of the polymorphic types like List, Option, and Result now take type arguments.

Note in particular that it is possible to accurately provide types for first-class functions, as opposed to TLambda which we used before. For example, (TFn([ TVariable "a" ], TVariable "b")) means "this parameter takes a function with one argument, which can be any type (we'll call it "a") and returns any type (we'll call it "b"). Any name in a TVariable can then be reused in the return type, such as TList (TVariable "b"), above.

Concerns when porting

The most important thing when porting code over is to keep the exact same behaviour between the old code and the new code. For example, that's why we're porting deprecated functions, including those with known bugs or bad behaviour.

In particular, we often have multiple copies of a function because the old one handled DIncomplete incorrectly and the new one handles it properly. If you see something like Dval.of_list, pay special care to replace it with the new equivalent (in this case, Dval.list), so as to handle these edge cases properly.


Many of the tests have been ported over already, and are commented out waiting for their function to be implemented. You can find commented out tests for functions using:

cat fsharp-backend/tests/testfiles/*.tests | grep -v '^\[' | grep "^\/\/" | perl -n -e 'foreach ($_ =~ /([A-Za-z0-9]+)\.([A-Za-z0-9]+_v\d)/ ) { print "$1::$2\n" } ' | grep -v Test | sort | uniq

That said, functions can never have enough tests, so if you think the existing tests aren't adequate, feel free to add more. If you aren't sure of the expected behaviour, run it on your canvas on darklang.com.

Porting APIs

There are many APIs that need to be ported - these are used by the editor. The APIs that need to still be implemented are commented out at the bottom of ApiServer/Api.fs.

These are implemented in two parts:

  • adding types to Api.fs which have the expected shape (you can find the shape in Analysis.ml)
  • directly porting the OCaml code that back those functions

Look at existing examples for how to do this, especially for how to port SQL calls from OCaml to Dark.

Other work to be done

The remainder of the work is bring tracked in the issue tracker under the F# label.