Eval built-in function: len

src/Monkey/Environment.elm

@@ -1,6 +1,6 @@
 module Monkey.Environment exposing
   ( Environment
-  , empty
+  , empty, fromList
 
   , LookupResult(..)
   , lookup
@@ -23,6 +23,10 @@ empty : Environment k v
 empty = Empty
 
 
+fromList : List (k, v) -> Environment k v
+fromList assocs = Many assocs Empty
+
+
 type LookupResult v
   = NotFound
   | FoundValue v

src/Monkey/Interpreter.elm

@@ -25,6 +25,7 @@ type Value
   | VArray (Array Value)
   | VHash (Hash Value)
   | VFunction Closure
+  | VBuiltInFunction BuiltInFunction
 
 
 type alias Closure =
@@ -34,6 +35,9 @@ type alias Closure =
   }
 
 
+type alias BuiltInFunction = List Value -> Eval RuntimeError Value
+
+
 type alias Env = Env.Environment P.Id Value
 
 
@@ -68,7 +72,7 @@ run input =
   case P.parse input of
     Ok program ->
       evalProgram program
-        |> Eval.run Env.empty
+        |> Eval.run builtInFunctions
         |> Result.map Tuple.second
         |> Result.mapError RuntimeError
 
@@ -402,8 +406,15 @@ computeDiv valueA valueB =
 
 evalCall : Value -> List Value -> Eval RuntimeError Value
 evalCall value args =
-  expectFunction value
-    |> Eval.andThen (\closure -> applyClosure closure args)
+  case value of
+    VFunction closure ->
+      applyClosure closure args
+
+    VBuiltInFunction builtInFunction ->
+      builtInFunction args
+
+    _ ->
+      Eval.fail <| TypeError [TFunction] (typeOf value)
 
 
 applyClosure : Closure -> List Value -> Eval RuntimeError Value
@@ -461,16 +472,6 @@ evalIndex valueA valueB =
       Eval.fail <| TypeError [TArray, THash] (typeOf valueA)
 
 
-expectFunction : Value -> Eval RuntimeError Closure
-expectFunction value =
-  case value of
-    VFunction closure ->
-      Eval.succeed closure
-
-    _ ->
-      Eval.fail <| TypeError [TFunction] (typeOf value)
-
-
 expectInt : Value -> Eval RuntimeError Int
 expectInt value =
   case value of
@@ -534,6 +535,9 @@ typeOf value =
     VFunction _ ->
       TFunction
 
+    VBuiltInFunction _ ->
+      TFunction
+
 
 answerToString : Answer -> String
 answerToString answer =
@@ -585,3 +589,30 @@ valueToString value =
 
     VFunction _ ->
       "<function>"
+
+    VBuiltInFunction _ ->
+      "<built-in-function>"
+
+
+builtInFunctions : Env
+builtInFunctions =
+  [ ("len", builtInLen)
+  ]
+  |> List.map (Tuple.mapSecond VBuiltInFunction)
+  |> Env.fromList
+
+
+builtInLen : BuiltInFunction
+builtInLen args =
+  case args of
+    [VString s] ->
+      Eval.succeed <| VNum <| String.length s
+
+    [VArray a] ->
+      Eval.succeed <| VNum <| Array.length a
+
+    [arg] ->
+      Eval.fail <| TypeError [TString, TArray] (typeOf arg)
+
+    _ ->
+      Eval.fail <| ArgumentError 1 (List.length args)

tests/Test/Monkey/Interpreter.elm

@@ -21,6 +21,7 @@ suite =
     , infixSuite
     , ifSuite
     , functionSuite
+    , builtInFunctionsSuite
     , answerToStringSuite
     ]
 
@@ -510,6 +511,44 @@ functionSuite =
     ]
 
 
+builtInFunctionsSuite : Test
+builtInFunctionsSuite =
+  describe "built-in functions"
+    [ describe "len"
+        [ makeGoodExamples
+            [ ( """
+                len("")
+                """
+              , VNum 0
+              )
+            , ( """
+                len("four")
+                """
+              , VNum 4
+              )
+            , ( """
+                len("hello world")
+                """
+              , VNum 11
+              )
+            ]
+
+        , makeBadExamples
+            [ ( """
+                len(1)
+                """
+              , TypeError [TString, TArray] TInt
+              )
+            , ( """
+                len("one", "two")
+                """
+              , ArgumentError 1 2
+              )
+            ]
+        ]
+    ]
+
+
 answerToStringSuite : Test
 answerToStringSuite =
   describe "answerToString"