[Haskell-cafe] Re: Monads aren't evil
bugfact at gmail.com
Sat Jan 10 15:19:58 EST 2009
> For example, which of these is easier to read?
> f,g :: Int -> [Int]
> h1 :: Int -> [Int]
> h1 x = do
> fx <- f x
> gx <- g x
> return (fx + gx)
> h2 :: Int -> [Int]
> h2 x = (+) <$> f x <*> g x
> h3 :: Int -> [Int]
> h3 x = f x + g x -- not legal, of course, but wouldn't it be nice if it
Yes, all that lifting is something that takes away lot of the beauty and
simplicity of Haskell, but as far as my limited knowledge goes, I don't
think this problem is easily solved :)
Anyway, for your particular example, for newbies I guess the clearest would
h0 x = [ fx+gx | fx <- f x, gx <- g x ]
since one must recognize that a list monad exists and what it does...
Now, for binary operators, Thomas Davie made a nice pair of combinators on
Hackage (InfixApplicative) that would allow this to become:
h3 x = f x <^(+)^> g x
But in general, I guess you have a good point...
> Of course this raises problems of order of evaluation, etc, but as
> long as such things were well-defined, that seems fine. If you want
> finer control, you can always go back to more verbose syntax. These
> cases are dominated by the cases where you simply don't care!
> This said, I don't want to sound overly negative; all of this pain is
> *worth* the correctness guarantees that I get when writing in Haskell.
> I just wish I could get the same guarantees with less pain!
> -- ryan
> 2009/1/10 Peter Verswyvelen < bugfact at gmail.com>:
> > Related to this issue, I have a question here.
> > I might be wrong, but it seems to me that some Haskellers don't like
> > writing monads (with do notation) or arrows (with proc sugar) because of
> > fact they have to abandon the typical applicative syntax, which is so
> > to the beautiful lambda calculus core. Or is it maybe because some people
> > choose monads were the less linear applicative style could be used
> > so the choice of monads is not always appropriate.
> > Haskell is full of little hardcoded syntax extensions: list notation
> > syntactic, list comprehensions, and even operator precedence that reduces
> > the need for parentheses, etc...
> > Of course IMHO the syntactic sugar is needed, e.g. a Yampa game written
> > without the arrows syntax would be incomprehensible for the average
> > programmer. But one could claim that this syntactic sugar hides what is
> > really going on under the hood, so for newcomers these extensions might
> > it harder. It could also feel like a hack, a failure to keep things as
> > simple as possible yet elegant.
> > Some people I talked with like that about the Scheme/ & LISP languages:
> > syntax remains ultimately close to the core, with very little hardcoded
> > syntactic extensions. And when one wants to add syntactic extensions, one
> > uses syntactic macros.
> > I'm not sure what others feel about the hardcoded syntax extensions in
> > Haskell...
> > Personally I'm not that much of a purist, I'm an old school hacker that
> > mainly needs to get the job done. I like the syntax extensions in Haskell
> > (and even C#/F# ;) because they let me write code shorter and clearer...
> > On Fri, Jan 9, 2009 at 4:07 AM, Neal Alexander <wqeqweuqy at hotmail.com>
> > wrote:
> >> Ertugrul Soeylemez wrote:
> >>> Hello fellow Haskellers,
> >>> When I read questions from Haskell beginners, it somehow seems like
> >>> try to avoid monads and view them as a last resort, if there is no easy
> >>> non-monadic way. I'm really sure that the cause for this is that most
> >>> tutorials deal with monads very sparingly and mostly in the context of
> >>> input/output. Also usually monads are associated with the do-notation,
> >>> which makes them appear even more special, although there is really
> >>> nothing special about them.
> >> Yea, i was the same way when i started learning Haskell. I understood
> >> Monads worked, and what the motivation was for them, but not why i would
> >> want to restructure code to use them in specific instances.
> >> "Why should i care about monads when i can use Arrows or (.)" was also a
> >> factor.
> >> Its kinda like getting advice from an adult as a child. You have no
> >> particular reason to distrust the advice, but the value of it doesn't
> set in
> >> until something happens to you first hand to verify it.
> >> For me the turning point was writing some code that needed to handle
> >> running code locally/remotely in a transparent manner.
> >> Maybe having a list of real-world usage scenarios or exercises on the
> >> may help.
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