[Haskell-cafe] Martin Odersky on "What's wrong with Monads"
Alberto G. Corona
agocorona at gmail.com
Thu Jun 28 16:29:33 CEST 2012
The "problem" of monads is that it defines different execution models,
besides the funcional,/lazy/declarative mode. There is no such problem in
imperative languages, which work ever in an hardwired IO monad. But this
means that the programmer has to code the extra behaviour needed in each
application to do the same.
I summarized this here:
http://haskell-web.blogspot.com.es/2012/06/intuitive-explanation-of-algorithms-in.html
It pretend to be intuitive, not accurate. (See disclaimer ;) . Comments
welcome
2012/6/27 Yves Parès <yves.pares at gmail.com>
> > I'm not happy with any of these options.
>
> Why are you unhappy with the ImplicitParams option?
>
> It's pretty much like resorting to a newtype, as it's been suggested
> before.
>
>
> 2012/6/27 Tillmann Rendel <rendel at informatik.uni-marburg.de>
>
>> Hi Rico,
>>
>> Rico Moorman wrote:
>>
>>> data Tree = Leaf Integer | Branch (Tree Integer) (Tree Integer)
>>>>
>>>> amount:: Tree -> Integer
>>>> amount (Leaf x) = x
>>>> amount (Branch t1 t2) = amountt1 + amountt2
>>>>
>>>> [...] additional requirement: "If the command-line flag --multiply is
>>>> set,
>>>>
>>>> the function amount computes the product instead of the sum."
>>>>
>>>> How would you implement this requirement in Haskell without changing the
>>>> line "amount (Leaf x) = x"?
>>>>
>>>
>> The (for me at least) most obvious way to do this would be, to make the
>>> operation to be applied to determine the amount (+ or *) an explicit
>>> parameter in the function's definition.
>>>
>>>
>>> data Tree a = Leaf a
>>> | Branch (Tree a) (Tree a)
>>> amount :: (a -> a -> a) -> Tree a -> a
>>> amount fun (Leaf x) = x
>>> amount fun (Branch t1 t2) = amount fun t1 `fun` amount fun t2
>>>
>>
>> I agree: This is the most obvious way, and also a very good way. I would
>> probably do it like this.
>>
>> Which drawbacks do you see besides increased verbosity?
>>>
>>
>> Well, you did change the equation "amount (Leaf x) = x" to "amount fun
>> (Leaf x) = x". In a larger example, this means that you need to change many
>> lines of many functions, just to get the the value of fun from the point
>> where it is known to the point where you need it.
>>
>> [...] I am wondering which ways of doing this in Haskell you mean.
>>>
>>
>> I thought of the following three options, but see also Nathan Howells
>> email for another alternative (that is related to my option (1) below):
>>
>>
>> (1) Implicit parameters:
>>
>> {-# LANGUAGE ImplicitParams #-}
>> data Tree = Leaf Integer | Branch Tree Tree
>>
>> amount :: (?fun :: Integer -> Integer -> Integer) => Tree -> Integer
>>
>> amount (Leaf x) = x
>> amount (Branch t1 t2) = ?fun (amount t1) (amount t2)
>>
>>
>> (2) Lexical Scoping:
>>
>> data Tree = Leaf Integer | Branch Tree Tree
>>
>> amount :: (Integer -> Integer -> Integer) -> Tree -> Integer
>> amount fun = amount where {
>>
>> amount (Leaf x) = x
>> ; amount (Branch t1 t2) = fun (amount t1) (amount t2) }
>>
>>
>> (3) UnsafePerformIO:
>>
>> import System.IO.Unsafe (unsafePerformIO)
>>
>> data Tree = Leaf Integer | Branch Tree Tree
>>
>>
>> amount :: Tree -> Integer
>> amount (Leaf x) = x
>> amount (Branch t1 t2) = fun (amount t1) (amount t2)
>> where fun = unsafePerformIO ...
>>
>>
>> I'm not happy with any of these options. Personally, I would probably go
>> ahead and transform the whole program just to get the value of fun to where
>> it is needed. Nevertheless, having actually done this before, I understand
>> why Martin Odersky doesn't like doing it :)
>>
>>
>> Tillmann
>>
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>
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