SevenThunders wrote:
Ryan Ingram wrote:
As long as the FFI calls don't make destructive updates to existing matrices, you can do what you want.
For example, assuming you have:
-- adds the second matrix to the first & overwrites the first matrixAddIO :: MatrixIO -> MatrixIO -> IO ()
-- creates a new copy of a matrix matrixCopyIO :: MatrixIO -> IO MatrixIO ...
Well as you point out there is an efficiency issue if we need to copy matrices all of the time in order to insure 'referential transparency'. Moreover I manage my matrices on a stack in C, since it makes it easy to handle memory allocation and deallocation. The stack configuration tends to be highly fluid so there are always side effects going on. Right now my Matrix type wraps the index from the bottom of the Matrix stack into the IO monad.
If you need destructive updates, you indeed need a monad. Otherwise, I'd use ForeignPtrs and import the matrix operations as pure functions (~ unsafePerformIO).
I was just wondering if there was any obvious way to force an IO action to execute only once, since now each reference to the action IO causes it to execute again.
Isn't that simply do x <- onlyOnce mult x x with onlyOnce :: IO Int mult :: Int -> Int -> IO Int ? If you want mult = liftM2 something :: IO Int -> IO Int -> IO Int you can do x' <- onlyOnce let x = return x' mult x x which is do x <- return `liftM` onlyOnce mult x x for short. Regards, apfelmus