{- Copyright (C) 2009-2011 John Goerzen All rights reserved. For license and copyright information, see the file LICENSE -} {- | Module : Data.Convertible.Utils Copyright : Copyright (C) 2009-2011 John Goerzen License : BSD3 Maintainer : John Goerzen Stability : provisional Portability: portable -} module Data.Convertible.Utils(boundedConversion, convertVia ) where import Data.Convertible.Base import Data.Typeable {- | Utility function to perform bounds checking as part of a conversion. Does this be examining the bounds of the destination type, converting to the type of the source via 'safeConvert', comparing to the source value. Results in an error if the conversion is out of bounds. -} boundedConversion :: (Ord a, Bounded b, Show a, Show b, Convertible a Integer, Convertible b Integer, Typeable a, Typeable b) => (a -> ConvertResult b) -- ^ Function to do the conversion -> a -- ^ Input data -> ConvertResult b -- ^ Result boundedConversion func inp = do result <- func inp let smallest = asTypeOf minBound result let biggest = asTypeOf maxBound result let smallest' = (convert smallest)::Integer let biggest' = (convert biggest)::Integer let inp' = (convert inp)::Integer if inp' < smallest' || inp' > biggest' then convError ("Input value outside of bounds: " ++ show (smallest, biggest)) inp else return result {- | Useful for defining conversions that are implemented in terms of other conversions via an intermediary type. Instead of: >instance Convertible CalendarTime POSIXTime where > safeConvert a = do r <- safeConvert a > safeConvert (r :: ClockTime) we can now write: >instance Convertible CalendarTime POSIXTime where > safeConvert = convertVia (undefined::ClockTime) which does the same thing -- converts a CalendarTime to a ClockTime, then a ClockTime to a POSIXTime, both using existing 'Convertible' instances. -} convertVia :: (Convertible a b, Convertible b c) => b -- ^ Dummy data to establish intermediate type - can be undefined -> a -- ^ Input value -> ConvertResult c -- ^ Result convertVia dummy inp = do r1 <- safeConvert inp safeConvert (asTypeOf r1 dummy)