Re: [Haskell-cafe] Library API design: functional objects VS type classes
Hi Rob,
I usually prefer type class approach for early stage of development.
Type class approach is more flexible, less works required.
One might get a function with lots of constraints, and quite a lot of
language extensions may appear, though it works.
Once things got settled down, I reconsider API.
The type signatures shown in your example::
class FooC a where
mkFooC :: IO a
readFooC :: a - IO Int
incrFooC :: a - IO ()
and:
data FooT a = FooT {
readFooT :: IO a
, incrFooT :: IO ()
}
Resulting type of 'readFooC' is fixed to 'Int' within the type class.
On the other hand, resulting type of 'readFooT' is type variable 'a'.
Made slight modification to the type class shown in your
example. Changed result type of 'readFooC' to take associated
type:
http://hpaste.org/83507
Once criteria for comparison I can think is performance.
For compilation time, I guess functional object approach give better
performance, since some of the works done by compiler are already done
manually. Though, I haven't done benchmark of compilation time, and
not sure how much interest exist in performance of compilation.
For runtime performance, one can do benchmark in its concrete usecase.
I suppose, generally, functions defined with type class are slower
than functions having concrete type. See SPECIALIZE pragam in GHC[1].
Another criteria I can think is extensibility.
Suppose that we want to have new member function, 'incrTwice'. If we
have chance to change the source of 'FooC', adding new member function
to 'FooC' type class directly is possible, with default function body
filled in.
class FooC a where
type FooCVal a :: *
mkFooC :: IO a
readFooC :: a - IO (FooCVal a)
incrFooC :: a - IO ()
incrTwiceC :: a - IO ()
incrTwiceC a = incrFooC a incrFooC a
Though, having reasonable default is not always possible.
For additional source of inspiration, might worth looking the
classic[2], and scrap your type classes article[3].
[1]:
http://www.haskell.org/ghc/docs/7.6.1/html/users_guide/pragmas.html#specialize-pragma
[2]: http://homepages.inf.ed.ac.uk/wadler/papers/class/class.ps
[3]: http://www.haskellforall.com/2012/05/scrap-your-type-classes.html
Hope these help.
Regards,
--
Atsuro
On Tue, Mar 5, 2013 at 7:50 AM, Rob Stewart robstewar...@gmail.com wrote:
Hi,
I have a question about API design for Haskell libraries. It is a simple
one:
functional object data structures encapsulating mutable state VS type
classes encapsulating mutable state
Here is a simple example. I present an API: using a type class `FooC`,
and aso as a data structure `FooT`. Both are stateful, in the form of
an MVar holding an Integer, with an operation `incrFoo` to increment
this value by one, and another `readFoo` to read the Integer value.
-
import Control.Concurrent
-- API approach 1: Using type classes
class FooC a where
mkFooC :: IO a
readFooC :: a - IO Int
incrFooC :: a - IO ()
newtype Bar = Bar (MVar Int)
instance FooC Bar where
mkFooC = newMVar 0 = \i - return $ Bar i
readFooC (Bar mv) = readMVar mv
incrFooC (Bar mv) =
modifyMVar_ mv $ \i - return (i+1)
-- API approach 2: Using direct field records
data FooT a = FooT {
readFooT :: IO a
, incrFooT :: IO ()
}
mkBar :: IO (FooT Int)
mkBar = do
mv - newMVar 0
return FooT {
readFooT = readMVar mv
, incrFooT = modifyMVar_ mv $ \i - return (i+1)
}
-- Tests the type class API
testTypeClass :: IO ()
testTypeClass = do
bar - mkBar
incrFooT bar
incrFooT bar
i - readFooT bar
print i -- prints 2
-- Tests the direct data structure API
testDataStruct :: IO ()
testDataStruct = do
bar - (mkFooC :: IO Bar)
incrFooC bar
incrFooC bar
i - readFooC bar
print i -- prints 2
With that, I now ask: which is more common? Which is the better API
design for a library? The APIs are almost identical. Under what
conditions is the type classes preferred over the mutable object
style data structure? There are two related resources that provides
additional context here, that favour the functional objects approach:
- Section 3.4 Mutable Objects in Haskell's Overlooked Object
System http://goo.gl/gnZXL
- A similar question (data structures vs type classes) in Haskell
Antipattern: Existential Typeclass
http://lukepalmer.wordpress.com/2010/01/24/haskell-antipattern-existential-typeclass/
Thanks!
--
Rob
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Re: [Haskell-cafe] Compiling dph package with ghc-7.4.0.20111219
I don't know so much about ghc-7.4, but if loading dph codes in ghci is the main matter here, below might help: http://warmfuzzything.posterous.com/loading-dph-codes-in-ghci Best, -- Atsuro Hoshino On Sun, Jan 22, 2012 at 2:58 AM, Brandon Allbery allber...@gmail.com wrote: On Sat, Jan 21, 2012 at 12:50, mukesh tiwari mukeshtiwari.ii...@gmail.com wrote: Hi Brandon Thank you for reply. Could you please tell me how to install dph because cabal install is not working with ghc-7.4.0.20111219 and I have issue with ghc-7.2.1 and dph Sorry, but I don't know. I don't follow or use DPH, I just know how the GHC repo works, and that they had a problem with including additional library sub-repos which just happened to include (probably out of date, since DPH is developed separately) DPH libraries. As such, I would not expect DPH to be included in the official 7.4 release unless the DPH folks submit an updated version (and probably not even then since GHC is in the release candidates phase, so it's a bit too late to add a newer DPH into it). -- brandon s allbery allber...@gmail.com wandering unix systems administrator (available) (412) 475-9364 vm/sms ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Error in converting List of Lists into PArray ( Parray a )
Hi Mukesh, Below is a naive implementation of converting `[[(Int,Double)]]' to `PArray (PArray (Int, Double))' . There's no instance for `PA [a]', I've explicitly separated the inner and outer conversion. Though, when reading data from a file and converting, it might be better to use `hGet' in: http://hackage.haskell.org/packages/archive/dph-prim-par/0.5.1.1/doc/html/Data-Array-Parallel-Unlifted.html -- module Main where import Data.Array.Parallel import Data.Array.Parallel.PArray () import qualified Data.Array.Parallel.PArray as P mat_li :: [[(Int, Double)]] mat_li = [ zip [1..] [ 1, 2, 3, 0, 0, 0, 0, 0, 4 ] , zip [1..] [ 0, 0, 0, 0, 0, 0, 0, 0, 0 ] , zip [1..] [ 0, 0, 0, 0, 0, 0, 1, 2, 0 ] ] mat_pa_1 :: PArray (PArray (Int, Double)) mat_pa_1 = P.fromList (fmap P.fromList mat_li) mat_pa_2 :: PArray (PArray (Int, Double)) mat_pa_2 = conv_outer (conv_inner mat_li) conv_inner :: P.PA a = [[a]] - [PArray a] conv_inner = map P.fromList conv_outer :: P.PA a = [PArray a] - PArray (PArray a) conv_outer xs = case xs of [] - P.empty (x:xs) - P.singleton x P.+:+ conv_outer xs main :: IO () main = -- Printing `mat_pa_1' shows an error: -- -- No instance nor default method for class operation -- Data.Array.Parallel.PArray.PData.fromListPR -- -- print mat_pa_1 print mat_pa_2 Hope this well help. Regards, -- Atsuro Hoshino On Thu, Jan 19, 2012 at 4:47 AM, mukesh tiwari mukeshtiwari.ii...@gmail.com wrote: Hello all I am trying to convert List of Lists ( [[(Int , Double )]] ) into PArray ( PArray ( Int , Double )) but getting run time error. This code works fine and print list of PArray ( Int , Double ) but when i put print $ P.fromList ( map P.fromList c ) then i am getting runtime error. It says Main: Data/Array/Parallel/PArray/PDataInstances.hs:337:10-30: No instance nor default method for class operation Data.Array.Parallel.PArray.PData.fromListPR. Could some one please tell me how to resolve this issue. Thank you --import ParallelMat import Data.List import System.Environment import Data.Array.Parallel import qualified Data.Array.Parallel.PArray as P processMatrix :: [ [ Double ] ] - [ [ Double ] ] - [ ( [ ( Int , Double ) ] , [ ( Int , Double ) ]) ] processMatrix [] [] = [] processMatrix ( x : xs ) ( y : ys ) | ( all ( == 0 ) x ) Prelude.|| ( all ( == 0 ) y ) = processMatrix xs ys | otherwise = ( filter ( \( x , y ) - y /= 0 ) . zip [ 1..] $ x ,filter ( \( x , y ) - y /= 0 ) . zip [1..] $ y ) : processMatrix xs ys main = do [ first , second ] - getArgs a - readFile first b - readFile second let a' = transpose . tail . map ( map ( read :: String - Double ) . words ) . lines $ a b' = tail . map ( map ( read :: String - Double ) . words ) . lines $ b ( c , d ) = unzip $ processMatrix a' b' print $ ( map P.fromList c ) --print d Macintosh-0026bb610428:Haskell mukesh$ ghc --make -Odph -fdph-par Main.hs [1 of 1] Compiling Main ( Main.hs, Main.o ) Linking Main ... Macintosh-0026bb610428:Haskell mukesh$ ./Main A.in A.in [fromListPArray [(1,1.0),(6,1.0)],fromListPArray [(4,11.0),(9,11.0)],fromListPArray [(1,4.0),(4,2.0),(6,4.0),(9,2.0)]] Putting print $ P.fromList ( map P.fromList c ) Macintosh-0026bb610428:Haskell mukesh$ ghc --make -Odph -fdph-par Main.hs [1 of 1] Compiling Main ( Main.hs, Main.o ) Linking Main ... Macintosh-0026bb610428:Haskell mukesh$ ./Main A.in A.in Main: Data/Array/Parallel/PArray/PDataInstances.hs:337:10-30: No instance nor default method for class operation Data.Array.Parallel.PArray.PData.fromListPR Input file A.in 10 10 1 2 3 0 0 0 0 0 0 4 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 2 11 2 0 1 2 0 0 0 0 0 0 0 1 2 3 0 0 0 0 0 0 4 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 2 11 2 0 1 2 0 0 0 0 0 0 0 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
