Register Extern Functions

Externs are host functions exposed to scripts.

Each extern defines:

• name,
• type scheme,
• arity,
• implementation.

Minimal pattern

{ Name = "My.add"
  Scheme = Forall([], TFun(TInt, TFun(TInt, TInt)))
  Arity = 2
  Impl = fun ctx args -> ... }

Why schemes matter

Type schemes are injected into type inference, so script calls are type-checked before evaluation.

Real function injection example

open FScript.Language

let slugifyExtern =
    { Name = "Host.slugify"
      Scheme = Forall([], TFun(TString, TString))
      Arity = 1
      Impl =
        fun _ args ->
            match args with
            | [ VString s ] ->
                s.Trim().ToLowerInvariant().Replace(" ", "-")
                |> VString
            | _ ->
                failwith "Host.slugify expects one string argument" }

Use it with runtime externs:

open FScript.Runtime

let hostContext =
    { HostContext.RootDirectory = "."
      DeniedPathGlobs = [] }

let externs = slugifyExtern :: Registry.all hostContext

Higher-order externs (callback into script)

When an extern receives a script function as argument, use ExternalCallContext.Apply to invoke it.

let applyTwiceExtern =
    { Name = "Host.applyTwice"
      Scheme = Forall([], TFun(TFun(TInt, TInt), TFun(TInt, TInt)))
      Arity = 2
      Impl =
        fun ctx args ->
            match args with
            | [ fn; VInt n ] ->
                let once = ctx.Apply fn (VInt n)
                ctx.Apply fn once
            | _ ->
                failwith "Host.applyTwice expects (int -> int) and int" }

Design tips

• Keep extern APIs small and explicit.
• Prefer pure externs when possible.
• Return option-like outcomes for recoverable failures.
• Choose stable names; extern names are script-level API surface.