add tab completion by displaying functions matching prefix

Added a function to State which returns a list of all defined
values/functions matching a prefix. Modify the tab printout to
display 5 of these results.

src/BC/Prompt.hs

@@ -80,10 +80,14 @@ printStatus state str =
 
 printCompletions :: State -> String -> IO ()
 printCompletions state str =
-  let completions = "" -- TODO: get possible completions
+  let tokens = words str
+      completions = if tokens == []
+                    then []
+                    else getCompletions (last tokens) state
+      compString = intercalate "  " $ take 5 completions
       toPrint = "\n\x1b[32m" -- color to green
               ++ replicate (length promptStr) ' '
-              ++ completions
+              ++ compString
               ++ "\x1b[0m\r\x1b[A" -- color to white, move to prev line
   in do putStr toPrint
         putStr $ concat $ replicate (length promptStr + length str) "\x1b[C"

src/BC/State.hs

@@ -9,3 +9,8 @@ type State = Map String Value
 
 newState :: State
 newState = fromList primitives
+
+getCompletions :: String -> State -> [String]
+getCompletions s = foldWithKey addKey []
+  where addKey k _ ks = if isPrefix s k then k:ks else ks
+        isPrefix s1 s2 = length s2 >= length s1 && (all id $ zipWith (==) s1 s2)