[Haskell-cafe] Monadic vs "pure" style (was: pros and cons of sta tic typing and side effects)

Yitzchak Gale gale at sefer.org
Tue Aug 30 11:28:49 EDT 2005

There seems to be a misconception in this thread
that there is something "non-functional" or
"imperative" about using monads.

That is simply not true. When what you are trying
to write is most naturally and clearly expressed
as a series of steps - there is no reason not to
use a monad.

Even when a function is most naturally written as
purely recursive, in real-life code you often add
"Monad m =>" to the type signature and wrap the
function in a "return". This is primarily so that
you can propagate exceptions.  (I personally am
not so impressed by the refactoring fears in the
referenced paper, but yes, that is another

True, there are a few rare monads that have
non-lazy semantics; IO and ST come to mind. So I
avoid those unless absolutely necessary.

On Tue, Aug 30, 2005 at 12:40:27PM +0100, Thomas Davie wrote:
> On Aug 30, 2005, at 12:13 PM, Bayley, Alistair wrote:

> >Something else I noticed about my non-monadic
> >code was the way I was threading state through
> >functions.

That was the classical way of doing state in
functional languages, but in my opinion it is very
bad style in modern Haskell.

> >I was tempted to introduce a State monad...


> >I was also wondering what the disadvantages of
> >monadic style are?
> Both the advantage and the disadvantage is that
> you break lazy evaluation.

Not true. Only if you use a non-lazy monad, like
IO or ST.

There is no inherent advantage or disadvantage
to monads. If the idea is most clearly expressed
as a monad, use a monad. If the idea is most
clearly expressed recursively, write it recursively
(but perhaps wrap it in "return").

Using that philosophy, I find that quite a bit of
my code is monadic - most commonly State and
StateT - and still perfectly functional and lazy.

> 90% of the time lazyness is your friend...  but
> there is an occasional 10% of the time when it's
> useful to break lazyness.

I find the percentage much higher than 90%.

> ...monadifying your code can break some nice
> features,...

I do not know of any features it breaks.

> On a side note, whenever I find myself tempted
> to pass state around, I consider whether using
> CPS is better...

I do not know how it could ever make sense to use
CPS, except for a research project that explicitly
requires it.

(Yes, I know about callCC. I use a much simpler
and clearer Exit monad instead.)


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