Re: [Haskell-cafe] Re: Profiling nested case

18 Jul 2008

      ben.franksen:
...
Mitar wrote:
...
On Fri, Jul 18, 2008 at 3:54 PM, Chaddaï Fouché
 wrote:
...
...
So that I can easily change the type everywhere. But it would be much
nicer to write:
data Quaternion a = Q !a !a !a !a deriving (Eq,Show)
Only the performance of Num instance functions of Quaternion is then
quite worse.
You can probably use a specialization pragma to get around that.
But why is this not automatic? If I use Quaternions of only one type
in the whole program then why it does not make specialized version for
it? At least with -O2 switch.
You could try jhc: it does whole program optimization. Ghc compiles each
module separately.
No need to switch compilers. GHC is able to do a pretty good job.
Consider,

    data Q a = Q !a !a !a !a deriving (Eq,Show)

    -- yeah, polymorphic
    go :: Num a => Q a -> Q a
    go (Q 0 0 0 0) = Q 1 2 3 4
    go (Q a b c d) = go $! Q (a * a) (b * b) (c * c) (d * d)

    -- ah, but we fix it.
    type QI = Q Int

    -- and try that:
    main = print (go (Q 2 3 7 13 :: QI))

GHC specialises and gives us,

    $wgo :: Int# -> Int# -> Int# -> Int# -> Q Int

So just use ghc-core to check what you're getting.

-- Don

Re: [Haskell-cafe] Re: Profiling nested case

Don Stewart