[Haskell-cafe] Monad Input/Output and Monad Transformers

Luke Palmer lrpalmer at gmail.com
Thu Jul 2 16:18:21 EDT 2009

On Thu, Jul 2, 2009 at 5:31 AM, Maciej Piechotka <uzytkownik2 at gmail.com>wrote:

> 2. I find writing monad transformers annoying.
> Additionally if package defines transformer A and another transformer B
> they need to be connected 'by hand'.

You have not given any concrete problems or examples, so it's hard for me to
comment.  But at first glance, I would conjecture that you are relying too
heavily on monads and sequential thinking.

Consider what your code would look like without a single monad.  Obviously
you cannot talk to the network without IO, but your program can still be *
modeled* purely, and then toss in IO at the last second to tie it to the
network.  This model may be difficult for you because it requires your brain
to be rewired; feel free to mail this list with concrete modeling problems
and we will help you out.

As for the pure model, throw away Reader, Writer, State -- everything, and
just use pure functions.  Then add monads back in *at small scopes* when
they clean things up.

I used to approach problems by designing a monad for my whole program, using
an appropriate stack of transformers.  I suspect such an approach led to the
claim that "monads are not appropriate for large software systems" in a
popular paper a few months ago.  As I have gained more experience, I found
that this is the *wrong* way to go about using them.  Now my primary use of
monads is within the scope of a single function, to tie together the helper
functions in the where clause.


> I find a simple solution which probably is incorrect as it hasn't been
> used:
> instance (MonadState s n, Monad (m n), MonadTrans m) =>
>         MonadState s (m n) where
>        get = lift get
>        put = lift . put
> (requires FlexibleInstances MultiParamTypeClasses FlexibleContexts
> UndecidableInstances - two last are not extensions used by mtl)
> Regards
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe
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