scheme-fsharp

eval-apply.fs

let rec eval (env:Map<string,expression> list) expr =
  match expr with
  | NullExpr              ->  failwith "invalid interpreter state"
  | Combination([])       ->  failwith "invalid combination"
  | Combination(h::t)     -> 
     match eval env h with
     | (nenv, Procedure(f))          -> apply f t env
     | (nenv, Function(args, code))  -> 
        let newenv =
           try List.fold2 bindArg (expandEnv nenv) args t
           with ex -> failwith "invalid number of arguments"
        evalExprs newenv code
     | (nenv, expr)                  -> (nenv, expr)
  | Procedure(f)          -> (env, Procedure(f))
  | Function(args, code)  -> failwith "invalid function call"
  | Value(v)              -> (env, Value(v))
  | List(v)               -> (env, List(v))
  | Symbol(s)             ->
     match lookup env s with
     | Some(e)   -> (env, e)
     | None      -> failwith (sprintf "unbound symbol '%A'" s)
 
and apply f args env = f env args

repl-example

mtscheme v0.1
null
> (define (factorial x) (if (= x 0) 1 (* x (factorial (- x 1)))))
null
> (factorial 9)
362880.00
> 

repl.fs

let rec commandLoop (env, res) = 
   match res with
   | Value(Number(v))  -> printfn "%4.2f\n> " v
   | Value(Name(v))    -> printfn "%s\n> " v
   | Value(Boolean(v)) -> printfn "%b\n> " v
   | List(l)           -> printfn "%s" (listToStr l)
   | _                 -> printfn "null\n> "
   try Console.ReadLine() 
      |> List.ofSeq 
      |> parse 
      |> List.head 
      |> (eval env)
      |> commandLoop
   with ex -> commandLoop (env, Value(Name(ex.Message)))

types.fs

type Expression =
| NullExpr
| Combination of Expression list
| List of ListType list
| Function of ListType list * Expression list
...