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Recursive Monadic Bindings
Hugs supports the recursive-do notation, an extension to the do-notation that
allows recursive bindings. This is based on the work by the same title [EL00].
The do-notation in Haskell is not recursive. This means that the variables
bound in the do-notation enter into scope only after their bindings take
place. Compare this to a let expression in Haskell, in which let-bound
variables are visible in the entire binding group. It turns out that several
applications can benefit from recursive bindings. A simple example of it's use
is:
import MonadRec
justOnes = mdo xs <- Just (1:xs)
return xs
As expected, justOnes evaluates to Just [1,1,1,1,1,1,1,1,1,1,.....
There are three important points in using the recursive-do notation:
1. The recursive version of the do-notation uses the keyword mdo (rather
than do).
2. The scripts using mdo should import MonadRec
3. Hugs must be started with -98 flag
The MonadRec library introduces the MonadRec class. It's definition is:
class Monad m => MonadRec m where
mfix :: (a -> m a) -> m a
The MonadRec class declares the function mfix, which dictates how the
recursion should behave. If recursive bindings are required for a monad, then
that monad must be declared an instance of the MonadRec class. For details,
see the above mentioned reference.
The MonadRec library automatically declares List and Maybe monads as instances
of the MonadRec class. So, no special care is needed for these two instances.
The Hugs IOExts library defines fixIO and ST library defines fixST functions.
These are exactly the mfix functions corresponding to the internal IO and ST
monads. To use recursive-bindings with IOExts and ST libraries, just say:
import IOExts
instance MonadRec IO where mfix = fixIO
and,
import ST
instance MonadRec (ST s) where mfix = fixST
respectively. (And similarly for the LazyST library.) Then, the mdo-notation
will become automatically available for the IO and ST monads.
The web page: http://www.cse.ogi.edu/PacSoft/projects/muHugs/ contains
upto-date information on recursive-monadic-bindings.
------------------------------------------------------------------------------
[EL00] Levent Erkk and John Launchbury. Recursive Monadic Bindings. In the
Proceedings of the International Conference on Functional Programming,
ICFP'00, pp. 174-185, 2000.
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