They provide elegant solutions to a great many problems, some quite
simple. Here's how I would compute subobjects (subseqs, subsets),
partitions and permutations monadically:
(def true-or-false (constantly (either true false)))
;; computes subseqs in the seq monad and subsets in the set monad
(defn subs [s]
(m-filter true-or-false s)))
(defn partitions [s]
(m-partition-by true-or-false s))
(def permutations [s]
(when (seq s)
(>> (permutations (rest s))
[[left right] (m-split-with true-or-false %)]
(concat left [(first s)] right))))
You could respond that most of the heavy lifting is done by the
monadic transliterations of commonplace seq functions like filter,
partition-by and split-with. You would be right; that is the whole
point.
Anyway, you certainly don't need to know any of this stuff to code
Clojure, and I do agree that is a strength of the language compared
to, say, Haskell.
-Per
On Thu, Mar 18, 2010 at 3:13 AM, David Nolen <[email protected]> wrote:
> On Wed, Mar 17, 2010 at 4:09 PM, Konrad Hinsen <[email protected]>
> wrote:
>>
>> On 17 Mar 2010, at 20:54, David Nolen wrote:
>>
>>> But seriously, in my personal opinion Monads are relatively useless in
>>> the context of Clojure.
>>
>> I'd say that in any impure functional language, monads are useful in the
>> context of specific applications or algorithmic approaches. When working
>> with continuations, or when writing complex parsers, monads are useful. It's
>> just Haskell that has a language-specific relation to monads, because it
>> needs them for fundamental tasks such as I/O.
>
> James, and Konrad sorry if I sounded overly dismissive of Monads ;) Yes for
> parsers, continuations, other tricky problems they provide a very elegant
> solution.
>
> David
>
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