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|
{-# LANGUAGE FlexibleInstances #-}
module BizExpr (repl, eval, Context) where
import Data.Maybe (fromMaybe)
import Text.Read (readMaybe)
import qualified Data.Text.Lazy as L
import qualified Data.Map as M
import System.IO (isEOF)
import Control.Monad (when)
type Context = M.Map L.Text L.Text
repl :: IO ()
repl = do
eof <- isEOF
when (not eof) (
getLine >>=
putStrLn
. maybe "Failed to evaluate expression" id
. (eval M.empty :: String -> Maybe String)
>> repl
)
maybeHead (x:_) = Just x
maybeHead _ = Nothing
eval :: CoerceTo a => Context -> String -> Maybe a
eval c x = coerceTo =<< evalAst =<< maybeHead . fst =<< parseLevel c x
data Value = IntVal Integer | StrVal String | BoolVal Bool | FloatVal Float
deriving (Show)
class CoerceTo a where
coerceTo :: Value -> Maybe a
coerceFrom :: a -> Value
instance CoerceTo Integer where
coerceTo (FloatVal n) = Just $ floor n
coerceTo (IntVal n) = Just n
coerceTo v = coerceTo . FloatVal =<< coerceTo v
coerceFrom = IntVal
instance CoerceTo Float where
coerceTo (FloatVal n) = Just n
coerceTo (IntVal n) = Just $ fromIntegral n
coerceTo (StrVal s) = readMaybe s
coerceTo (BoolVal True) = Just 1
coerceTo (BoolVal False) = Just 0
coerceFrom = FloatVal
instance CoerceTo String where
coerceTo (BoolVal v) = Just $ show v
coerceTo (IntVal v) = Just $ show v
coerceTo (StrVal v) = Just v
coerceTo (FloatVal v) = Just $ show v
coerceFrom = StrVal
instance CoerceTo Bool where
coerceTo (BoolVal v) = Just v
coerceTo (IntVal v) = Just $ v /= 0
coerceTo (StrVal "True") = Just True
coerceTo (StrVal "true") = Just True
coerceTo v = coerceTo . IntVal =<< coerceTo v
coerceFrom = BoolVal
type Name = String
data Ast
= Expr Name [Ast]
| Val Value
deriving (Show)
parseLevel :: Context -> String -> Maybe ([Ast], String)
parseLevel c = go "" (Just [])
where
go :: String -> Maybe [Ast] -> String -> Maybe ([Ast], String)
go prev (Just l) ('(' : next) =
case parseLevel c next of
Nothing ->
Nothing
Just (arg, remnant) ->
let r =
if remnant /= "" && head remnant == ','
then tail remnant
else remnant
in go "" (Just (l ++ [Expr prev arg])) r
go prev (Just l) (')' : remnant) =
let l0 = [Val $ newVal c prev | prev /= ""]
in Just (l ++ l0, remnant)
go prev (Just l) (',' : next) =
let l0 = [Val $ newVal c prev]
in go "" (Just (l ++ l0)) next
go prev l (' ' : next) =
go "" l next
go "" (Just l) "" =
Just (l, "")
go prev (Just l) "" =
Just (l ++ [Val $ newVal c prev], "")
go prev l (x : xs) =
go (prev ++ [x]) l xs
newVal :: Context -> String -> Value
newVal c x = case M.lookup (L.pack x) c of
Nothing -> StrVal x
Just n -> StrVal $ L.unpack n
evalAst :: Ast -> Maybe Value
evalAst (Expr name ast) = fromMaybe Nothing (getFunc name <*> evalLevel ast)
evalAst (Val val) = Just val
evalLevel :: [Ast] -> Maybe [Value]
evalLevel = mapM evalAst
getFunc :: String -> Maybe ([Value] -> Maybe Value)
getFunc "sum" = Just eSum
getFunc "any" = Just eAny
getFunc "all" = Just eAll
getFunc "average" = Just eAverage
getFunc _ = Nothing
eSum :: [Value] -> Maybe Value
eSum = eFold (+) (0.0 :: Float)
eAny :: [Value] -> Maybe Value
eAny = eFold (||) False
eAll :: [Value] -> Maybe Value
eAll = eFold (&&) True
eAverage :: [Value] -> Maybe Value
eAverage xs = do
let n = coerceTo =<< eSum xs
d = Just . fromIntegral $ length xs
in FloatVal <$> ((/) <$> n <*> d)
eFold :: (CoerceTo a, CoerceTo b) => (b -> a -> b) -> b -> [Value] -> Maybe Value
eFold fn init [] = Just $ coerceFrom init
eFold fn init (x : xs) =
let b = coerceTo =<< eFold fn init xs
a = coerceTo x
in coerceFrom <$> (fn <$> b <*> a)
|
