[Haskell-cafe] monads with take-out options
jonathanccast at fastmail.fm
Wed Nov 26 14:16:53 EST 2008
On Wed, 2008-11-26 at 19:09 +0000, Jules Bean wrote:
> Greg Meredith wrote:
> > Haskellians,
> > Some monads come with take-out options, e.g.
> > * List
> > * Set
> > In the sense that if unit : A -> List A is given by unit a = [a], then
> > taking the head of a list can be used to retrieve values from inside the
> > monad.
> > Some monads do not come with take-out options, IO being a notorious example.
> > Some monads, like Maybe, sit on the fence about take-out. They'll
> > provide it when it's available.
> To amplify other people's comments:
> List A is just as on the fence as Maybe. "" plays the role of "Nothing".
> Some monads require that you put something in, before you take anything
> out [r -> a, s -> (a,s), known to their friends as reader and state]
> Error is similar to Maybe, but with a more informative Nothing.
> Most monads provide some kind of
> runM :: ## -> m a -> ## a
runM :: f (m a) -> g a
Where f and g are usually functors.
Maybe, of course, has the nice property that g = m.
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