[Haskell-cafe] Remember the future

Paul Johnson paul at cogito.org.uk
Fri Aug 17 20:10:53 EDT 2007

jerzy.karczmarczuk at info.unicaen.fr wrote:
> Andrew Coppin writes:
>> I've seen comments in various places that monads allow you to "borrow 
>> things from the future".
>> That sounds completely absurd to me... can anybody explain?
> Actually, "borrowing from the future" - in an interpretation which is 
> close
> to my own interests - doesn't need monads, but *laziness*. 

While this is true, the "mdo" and associated MonadFix class do implement 
it in a monadic framework where you can write things like

      x <- f y
      y <- g x

If you interpret do-notation as equivalent to imperative programming 
then this does indeed look like time travel.  Under the covers its more 
equivalent to

      x = f y
      y = g x

which is also known as "tying the knot" or the "credit card transform" 
(both keywords worth looking up).

However I can't say I really have my head around it properly.


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