CRLF.hs

{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE DuplicateRecordFields #-}
{-# LANGUAGE FlexibleContexts #-}

module Data.CRLF where

import GHC.Generics

import Control.Monad.Except
import Data.Binary
import Data.Bits

import qualified Control.Monad.State as S
import qualified Data.Text as T

data ColumnTypeError a = TooManyValues Int | TooFewValues | LoNotLessThanHi a a | RepeatedValue a
  deriving (Eq, Show, Read)

data ColumnType a b = Categorical { missingVal :: Maybe b,
                                    dict :: [(b, a)] } |
                      Discrete { missingValue :: Maybe b,
                                 lo :: Maybe (b, a),
                                 hi :: Maybe (b, a) } |
                      Continuous { missingValue :: Maybe b,
                                   lo :: Maybe (b, a),
                                   hi :: Maybe (b, a) }
    deriving (Eq, Show, Read, Generic)

instance (Binary a, Binary b) => Binary (ColumnType a b)

mkCategorical :: (Eq a, FiniteBits b, Enum b, MonadError (ColumnTypeError a) m) => [a] -> m (ColumnType a b)
mkCategorical xs
  | length xs < 3 = throwError TooFewValues
  {- This case gives an error like this:
  
  src/Data/CRLF.hs|56 col 21 error| error: • Could not deduce (FiniteBits b0) arising from a use of ‘finiteBitSize’ from the context: (Eq a, FiniteBits b, Enum b, MonadError (ColumnTypeError a) m) bound by the type signature for: mkCategorical :: forall a b (m :: * -> *). (Eq a, FiniteBits b, Enum b, MonadError (ColumnTypeError a) m) => [a] -> m (ColumnType a b) at /home/koz/documents/uni/research/papers/case-study/code/crlf/src/Data/CRLF.hs:53:1-108 The type variable ‘b0’ is ambiguous These potential instances exist: instance FiniteBits Int16 -- Defined in ‘GHC.Int’ instance FiniteBits Int32 -- Defined in ‘GHC.Int’ instance FiniteBits Int64 -- Defined in ‘GHC.Int’ ...plus 8 others ...plus two instances involving out-of-scope types (use -fprint-potential-instances to see them all) • In the second argument of ‘(^)’, namely ‘finiteBitSize (undefined :: b)’ In the second argument of ‘(>)’, namely ‘2 ^ finiteBitSize (undefined :: b)’ In the expression: length xs > 2 ^ finiteB
  
  Why am I getting this? This is a copy/paste from a (much) larger file, so line numbers may not match up -}
  | length xs > 2 ^ finiteBitSize (undefined :: b) = throwError (TooManyValues (length xs))
  | otherwise = case duplicated xs of
                  Just x -> throwError (RepeatedValue x)
                  Nothing -> return (Categorical Nothing (zip [zeroBits ..] xs))
                  
duplicated :: (Eq a) => [a] -> Maybe a
duplicated [] = Nothing
duplicated xs = S.evalState (foldM go Nothing xs) []
  where go res@(Just _) _ = return res
        go Nothing x = do history <- S.get
                          if x `elem` history
                          then return (Just x)
                          else S.put (x : history) >> return Nothing