step 4 scaffolding

impls/sml/Makefile

@@ -1,4 +1,4 @@
-STEP_BINS = step0_repl step1_read_print step2_eval step3_env
+STEP_BINS = step0_repl step1_read_print step2_eval step3_env step4_if_fn_do
 
 all: $(STEP_BINS)
 
@@ -14,6 +14,9 @@ step2_eval: step2_eval.mlb step2_eval.sml env.sml reader.sml printer.sml types.s
 step3_env: step3_env.mlb step3_env.sml env.sml reader.sml printer.sml types.sml util.sml main.sml
 	mlton -output $@ $<
 
+step4_if_fn_do: step4_if_fn_do.mlb step4_if_fn_do.sml env.sml reader.sml printer.sml types.sml util.sml main.sml
+	mlton -output $@ $<
+
 clean:
 	rm -f $(STEP_BINS)
 

impls/sml/step4_if_fn_do.mlb

@@ -0,0 +1,11 @@
+local
+    $(SML_LIB)/basis/basis.mlb
+    util.sml
+    types.sml
+    printer.sml
+    reader.sml
+    env.sml
+    step4_if_fn_do.sml
+in
+    main.sml
+end

impls/sml/step4_if_fn_do.sml

@@ -0,0 +1,79 @@
+exception NotDefined of string
+exception NotApplicable of string
+
+fun read s =
+    readStr s
+
+fun eval e (LIST (SYMBOL "def!"::args)) = evalDef e args
+  | eval e (LIST (SYMBOL "let*"::args)) = evalLet e args
+  | eval e (LIST (a::args))             = (e, evalApply e (eval' e a) args)
+  | eval e (SYMBOL s)                   = (e, evalSymbol e s)
+  | eval e ast                          = (e, ast)
+
+and eval' e ast = (#2 o eval e) ast
+
+and evalDef e [SYMBOL s, ast] = let val v = eval' e ast in (def s v e, v) end
+  | evalDef _ _               = raise NotApplicable "def! needs a symbol and a form to evaluate"
+
+and evalLet e [LIST bs, ast] = (e, eval' (bind bs e) ast)
+  | evalLet _ _              = raise NotApplicable "let* needs a list of bindings and a form to evaluate"
+
+and evalApply e (FN f) args = f (map (eval' e) args)
+  | evalApply _ a      args = raise NotApplicable (prStr a ^ " is not applicable on " ^ prStr (LIST args))
+
+and evalSymbol e s = valOrElse (lookup e s)
+                               (fn _ => raise NotDefined ("symbol '" ^ s ^ "' not found"))
+
+and bind (SYMBOL s::v::rest) e = def s (eval' e v) e |> bind rest
+  | bind []                  e = e
+  | bind _ _ = raise NotApplicable "bindings must be a list of symbol/form pairs"
+
+fun print f =
+    prStr f
+
+fun rep e s =
+    s |> read |> eval e |> (fn (e, v) => (e, print v))
+    handle Nothing           => (e, "")
+         | SyntaxError msg   => (e, "SYNTAX ERROR: " ^ msg)
+         | NotApplicable msg => (e, "CANNOT APPLY: " ^ msg)
+         | NotDefined msg    => (e, "NOT DEFINED: " ^ msg)
+
+fun malPlus  (INT a, INT b) = INT (a + b)
+  | malPlus _ = raise NotApplicable "can only add integers"
+fun malTimes (INT a, INT b) = INT (a * b)
+  | malTimes _ = raise NotApplicable "can only multiply integers"
+fun malMinus (INT b, INT a) = INT (a - b)
+  | malMinus _ = raise NotApplicable "can only subtract integers"
+fun malDiv   (INT b, INT a) = INT (a div b)
+  | malDiv _ = raise NotApplicable "can only divide integers"
+
+val initEnv = ENV [] |> bind [
+    SYMBOL "+",
+    FN (foldl malPlus (INT 0)),
+    SYMBOL "*",
+    FN (foldl malTimes (INT 1)),
+    SYMBOL "-",
+    FN (fn [x]   => malMinus (x, INT 0)
+         | x::xs => foldr malMinus x xs
+         | _     => raise NotApplicable "'-' requires arguments"),
+    SYMBOL "/",
+    FN (fn [x]   => malDiv (x, INT 1)
+         | x::xs => foldr malDiv x xs
+         | _     => raise NotApplicable "'/' requires arguments")
+]
+
+fun repl () = repl' initEnv
+
+and repl' e =
+    let open TextIO
+    in (
+        print("user> ");
+        case inputLine(stdIn) of
+            SOME(line) =>
+                let val (e', s) = rep e line
+                    val _ = print(s ^ "\n")
+                in
+                    repl' e'
+                end
+            | NONE => ()
+    ) end