[Haskell-cafe] Sequencing Operations in a Monad

SevenThunders mattcbro at earthlink.net
Fri Sep 14 23:23:48 EDT 2007

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

 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.
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