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
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