Re:Ask HN: What is a monad?
by danieldk on Sunday December 12, 2010 @04:11PM
(#1998030)
A monad puts you into a box and only gives you a few interaction points with the outside world. You can do whatever you want in a monad, but you are never allowed to leave the box.
This is not true for many commonly used monads, except for the IO monad. You can extract values from the list monad, Maybe monad, state monad, etc.
Put simply, a monad specifies how values can be wrapped in a 'box', and how such boxes are combined. For instance, Maybe is a box that can contains either a variable or nothing. In Haskell:
data Maybe a = Nothing | Just a
The Maybe monad specifies how expressions resulting in a Maybe are combined. Expressions are evaluated until/unless one of the expressions returns 'Nothing', and the value of the monad becomes Nothing.
Comparably, the IO monad specifies how IO actions are combined, the State monad combines expressions in such a manner to create 'state', etc.
We have tried to illustrate Monads with a Haskell-ish example here:
This is not true for many commonly used monads, except for the IO monad. You can extract values from the list monad, Maybe monad, state monad, etc.
Put simply, a monad specifies how values can be wrapped in a 'box', and how such boxes are combined. For instance, Maybe is a box that can contains either a variable or nothing. In Haskell:
data Maybe a = Nothing | Just a
The Maybe monad specifies how expressions resulting in a Maybe are combined. Expressions are evaluated until/unless one of the expressions returns 'Nothing', and the value of the monad becomes Nothing.
Comparably, the IO monad specifies how IO actions are combined, the State monad combines expressions in such a manner to create 'state', etc.
We have tried to illustrate Monads with a Haskell-ish example here:
http://www.nlpwp.org/book/chap-words.xhtml#id3390030