Re: prompts: any example ?

[Ian Price]

Ian Price <address@hidden> writes:

> The trick comes from, I think, Filinski's "Representing Monads",
> although it has been quite a while since I've read it. Instead of monads
> being represented by the usual 'bind' and 'unit' functions, or the
> (categorical?) definition of 'unit', 'fmap', 'join', they are instead
> represented by two operators 'reflect' and 'reify'. 
>
> reify : (() -> a) -> m a
> reflect : m a -> a
>
> reify takes a function that returns a value, and returns a monadic
> value i.e. it lifts a pure expression to an effectful one.
>
> reflect takes a monadic value and returns the value. i.e. it lowers the
> effectful value into the pure layer.

One thing I forgot to mention is that there is a simple way to turn the
unit/bind representation into the reify/reflect representation, and you
see it at the top of the gist.

(define (reify thunk)
  (reset (return (thunk))))

(define (reflect m)
  (shift k (>>= m k)))

only I used tagged variants of shift/reset instead to allow mixing (as
already mentioned)

-- 
Ian Price

"Programming is like pinball. The reward for doing it well is
the opportunity to do it again" - from "The Wizardy Compiled"