module Miscellaneous(
    if',
    (?),
    selectFrom
    ) where




import Control.Monad as Con
import qualified Data.List as List
import qualified Data.Maybe as Maybe




if' :: Bool -> t -> t -> t
if' a b c = if a then b else c




infixr 1 ?
(?) :: Bool -> t -> t -> t
(?) = if'



--  kinda functions like poor man's sql
--  first argument is the indices of the items you want in the results
--  second argument is index-item pairs to dictate what records are acceptable to select from
--  third argument is the list of items that makes up the record under consideration
--  then if the record was deemed acceptable you get the bits you wanted
--  (note that all indices start from 1)
selectFrom :: (Num t, Eq t, Eq a, Enum t) => [t] -> [(t,a)] -> [a] -> Maybe [a]
selectFrom pick has from =
    let tailFunc r i =
            let check = List.lookup (fst i) has
            in if (Maybe.isNothing check || Maybe.fromJust check == snd i)
                then if (List.elem (fst i) pick)
                        then Just (r ++ [snd i])
                        else Just r
                else Nothing
    in Con.foldM tailFunc [] (zip [1,2..] from)