On Sat, Mar 29, 2008 at 2:23 AM, Don Stewart <dons@galois.com> wrote:

conal:

> I'd like to know if it's possible to get GHC to perform some simple CSE
> for function-level programming. Here's a simple example:
>
> liftA2 (*) sin sin :: Double -> Double
>
> which inlines and simplifies to
>
> \ t -> sin t * sin t
>
> A more realistic, equivalent, example:
>
> let b = sin <$> id in liftA2 (*) b b
>
> Can GHC be nudged into computing 'sin t' once rather than twice?
>

So GHC does do some light CSE, but not in this case, as far as I could
see.

I had a go with a rewrite rule that I felt should have matched, but it
didn't seem to want to fire. Possibly the dictionaries were getting in
the way.

import System.Environment
import Prelude hiding (sin)
import qualified Prelude

sin :: Double -> Double
sin x = Prelude.sin x
{-# NOINLINE sin #-}

times :: Double -> Double -> Double
times x y = x * y
{-# NOINLINE times #-}

{-# RULES

"sin/cse" forall x.
times (sin x) (sin x) = case Prelude.sin x of y -> y * y

#-}

main = do
[x] <- getArgs
let n = read x
print $ sin n `times` sin n

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