Re: [Haskell-cafe] Generic typeclass for converting between types
John Goerzen wrote: Magnus Therning wrote: John Goerzen wrote: It's hard to remember all the functions to use to do these. I often resort to a chart I made for numeric conversions. Not the type of response you want, but would you publish that chart somewhere, please ;-) Sure :-) http://book.realworldhaskell.org/read/using-typeclasses.html#numerictypes.conversion For those who care about correctness or efficiency, note that the (toRational . fromRational) path is not good. It's not correct because of the exceptional values that Float and Double can carry, and it's not efficient because most hardware supports direct conversions for the basic numeric types. See the RealToFrac class in Data.Number.Transfinite in the logfloat[1] package for a better way. There's a generic instance (Real a, Transfinite a, Fractional b, Transfinite b) = RealToFrac a b which Haddock displays as duplicate instances for each of the optimized GHC-only instances. [1] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/logfloat -- Live well, ~wren ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Generic typeclass for converting between types
Hi folks,
I've been thinking today that I frequently need to convert data beween
types:
* Between various numeric types
* Between various calendar types (both within the new calendar
system, and between the old and new)
* Marshalling data back and forth to a database in HDBC
It's hard to remember all the functions to use to do these. I often
resort to a chart I made for numeric conversions.
It occurs to me that it would be nice to be able to
(convert (5.8::Double))::Int
or
(convert calendarTime)::ZonedTime
So, the first question is: does something like this exist already?
I'm not aware of it, but I'm not sure how to search either.
I'm thinking of something like the below. With a little magic, it's
quite possible to make errors easy to generate in the safe fashion
(for instance, when converting from String to Integer using reads).
{-# LANGUAGE MultiParamTypeClasses #-}
import Control.Monad.Error
type ConvertResult a = Either ConvertError a
class Convertible a b where
safeConvert :: a - ConvertResult b
instance Convertible Int Double where
safeConvert = return . fromIntegral
instance Convertible Double Int where
safeConvert = return . truncate -- could do bounds checking here
instance Convertible Integer Double where
safeConvert = return . fromIntegral
instance Convertible Double Integer where
safeConvert = return . truncate
convert :: Convertible a b = a - b
convert inp = case safeConvert inp of
Left e - error (show e)
Right x - x
-- rudimentary error type for this example
data ConvertError = ConvertError {
sourceValue :: String,
errorMessage :: String
}
deriving (Eq, Read, Show)
instance Error ConvertError where
strMsg x = ConvertError (unknown) x
The other option is to use an approach more like I have in HDBC. In
HDBC, there is a direct need to encapsulate data for transport, so
I've got this:
class (Show a) = SqlType a where
toSql :: a - SqlValue
safeFromSql :: SqlValue - FromSqlResult a
data SqlValue = SqlString String
| SqlByteString B.ByteString
| SqlWord32 Word32
| SqlWord64 Word64
... many more
...
instance SqlType Int32 where
sqlTypeName _ = Int32
toSql = SqlInt32
safeFromSql (SqlString x) = read' x
safeFromSql (SqlByteString x) = (read' . byteString2String) x
safeFromSql (SqlInt32 x) = return x
safeFromSql (SqlInt64 x) = return . fromIntegral $ x
The advantage of this is that if you've got a whole slew of types and
you're going to be converting between all of them (for instance,
numeric types), if you turn on -Wall the compiler will help you know
when your safeFromSql instance doesn't convert everything. The
disadvantage is that the type system doesn't enforce whether or not it
is even possible to convert certain things (for instance, a TimeOfDay
to a Char), and so we have to return a Left for those.
Any thoughts?
-- John
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Re: [Haskell-cafe] Generic typeclass for converting between types
On Fri, Jan 23, 2009 at 3:01 PM, John Goerzen jgoer...@complete.org wrote: Hi folks, I've been thinking today that I frequently need to convert data beween types: * Between various numeric types * Between various calendar types (both within the new calendar system, and between the old and new) * Marshalling data back and forth to a database in HDBC It's hard to remember all the functions to use to do these. I often resort to a chart I made for numeric conversions. Not the type of response you want, but would you publish that chart somewhere, please ;-) /M -- Magnus Therning(OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus identi.ca|twitter: magthe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Generic typeclass for converting between types
Magnus Therning wrote: On Fri, Jan 23, 2009 at 3:01 PM, John Goerzen jgoer...@complete.org wrote: Hi folks, I've been thinking today that I frequently need to convert data beween types: * Between various numeric types * Between various calendar types (both within the new calendar system, and between the old and new) * Marshalling data back and forth to a database in HDBC It's hard to remember all the functions to use to do these. I often resort to a chart I made for numeric conversions. Not the type of response you want, but would you publish that chart somewhere, please ;-) Sure :-) http://book.realworldhaskell.org/read/using-typeclasses.html#numerictypes.conversion You might also find these useful: Typeclass instances for numeric types: http://book.realworldhaskell.org/read/using-typeclasses.html#numerictypes.typeclasses Chart of numeric functions: http://book.realworldhaskell.org/read/using-typeclasses.html#numerictypes.funcs -- John /M ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
