Yet Another Monad Tutorial

[Bayley, Alistair]

> -----Original Message-----
> From: Jeff Newbern [mailto:jnewbern@nomaware.com]
> 
> Alistair,
> 
> Hhhmmm.  That is an interesting issue, and I am unsure how to treat it
> in the tutorial.  I did attempt to explain the ability of the monad to
> isolate impure computations, but I think I need to make a better
> explanation of what an action is and how it is used.
> 
> As for the question of whether using a monad in this way casts Haskell
> out of the ranks of the functionally pure, I don't know.  
> This feels to
> me like a debate over personal interpretation, with decent 
> arguments on
> both sides.  I think the most accurate thing to say is that it enables
> Haskell to incorporate non-pure features without destroying the purity
> of the core of the language.


The interesting point for me is: you can't escape from the IO monad, but you
can from others. You can use non-IO functions from IO functions, but you
can't use IO functions from non-IO functions. AFAICT, this is not true of
all monads; as Wolfgang said, you can write a function that uses monadic
values but returns a non-monadic ("pure") value.

You (sort of) explain this in the first two paragraphs of "No way out", but
things that might help are:
 - two example lists: of one-way monads (IO), and others (List, Maybe)
 - non-monadic function using monadic values (
http://www.nomaware.com/monads/html/listmonad.html#example )


> Any suggestions for how best to present this issue to a reader who may
> be new to the world of FP?  Should we mention it at all?

I think you should mention it in a monad tutorial, hence the request. It
will help (has helped) me understand a little more about monads.


*****************************************************************
The information in this email and in any attachments is 
confidential and intended solely for the attention and use 
of the named addressee(s). This information may be 
subject to legal professional or other privilege or may 
otherwise be protected by work product immunity or other 
legal rules.  It must not be disclosed to any person without 
our authority.

If you are not the intended recipient, or a person 
responsible for delivering it to the intended recipient, you 
are not authorised to and must not disclose, copy, 
distribute, or retain this message or any part of it.
*****************************************************************