[Haskell-cafe] Pointless functors
greenrd at greenrd.org
Thu Mar 12 22:43:39 EDT 2009
On Fri, 13 Mar 2009 14:32:23 +0000
Ross Paterson <ross at soi.city.ac.uk> wrote:
> On Fri, Mar 13, 2009 at 03:18:15PM +0100, Martijn van Steenbergen
> > Are there any functors f for which no point/pure/return :: a -> f a
> > exists?
> No. Choose an arbitrary element shape :: f () and define
> point x = fmap (const x) shape
This suggests a fun way to define a monad for a functor F:
-- Does nothing, has no effect, and returns ()
noOp :: F ()
noOp = ...
instance Monad F where
return x = fmap (const x) noOp
(<<=) f x = ...
I think noOp would be a nice addition to the Monad class, as well.
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