On Tue, 19 Aug 2003 14:09:16 +0100 (BST) C T McBride <[EMAIL PROTECTED]> wrote:
> Hi > > > > As an example, I'll use the Maybe monad. Suppose I want to write > > > code to handle experimental data, in which there might be missing > > > values. I might then decide to represent measurements by data of > > > type "Maybe Double", with missing values represented by "Nothing". > > > I could then go on to define functions on missing values, which > > > would return "Nothing" when their argument is "Nothing", and I > > > could string these functions together via the monad mechanism. > > > Fine. But how would I handle e.g. addition of two such values? > > > The result should be "Nothing" when either of its arguments > > > is"Nothing". Is there any mechanism to handle that? > > > > Yes, liftM2. Defined in module Monad (or Data.Monad resp.). > > > > > Konrad. > > > > Wolfgang > > Or, more generally, > > infixl 9 <$> > > (<$>) :: Monad m => m (s -> t) -> m s -> m t > mf <$> ms = > do f <- mf > s <- ms > return (f s) or just liftM2 ($) or just ap _______________________________________________ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
