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The first paragraph describes what monads are used for instead of describing what they are, contrary to Wikipedia guidelines. |
The first paragraph describes what monads are used for instead of describing what they are, contrary to Wikipedia guidelines. |
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The rest of the articles then constantly mixes definitions referencing unexplained terms, like 'monadic type' with examples written exclusively in Haskell. <small>—Preceding [[Wikipedia:Signatures|unsigned]] comment added by [[Special:Contributions/87.18.173.1|87.18.173.1]] ([[User talk:87.18.173.1|talk]]) 10:03, 6 July 2008 (UTC)</small><!-- Template:UnsignedIP --> <!--Autosigned by SineBot--> |
The rest of the articles then constantly mixes definitions referencing unexplained terms, like 'monadic type' with examples written exclusively in Haskell. <small>—Preceding [[Wikipedia:Signatures|unsigned]] comment added by [[Special:Contributions/87.18.173.1|87.18.173.1]] ([[User talk:87.18.173.1|talk]]) 10:03, 6 July 2008 (UTC)</small><!-- Template:UnsignedIP --> <!--Autosigned by SineBot--> |
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: Absolutely agreed. "Here's a bit of code in a pseudo-language lacking any referents you'll recognise. To add confusion, we use a custom operator, //, which we have to define in detail. The Maybe monad doesn't appear in this code and is only mentioned briefly at the end of the description. From this you can deduce what a monad is, despite the fact that we've not explained what the Maybe monad is, what makes it a monad, why it's appropriate, and why we chose not to define it here." Better still, go to the Maybe monad section for enlightenment, and it says "The Maybe monad has already been defined above". So by implication I have to read 2/3 of the article to understand the lead-in paragraphs. Must Try Harder. --[[Special:Contributions/62.58.152.52|62.58.152.52]] ([[User talk:62.58.152.52|talk]]) 12:21, 27 October 2009 (UTC) |
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== Definition == |
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Even for someone with a bachelor's in computer science/informatics, this information is prohibitively difficult to understand. Yes, it is true that Monads are a difficult concept. However even if one can understand everything this article is saying (bind/>>=, return), the big picture just isn't presented in this article. It is full of convoluted examples.
Some important questions that need to be addressed early and in their own sections:
Someone should really add this article to some list of articles needing revision, or email the Haskell mailing list.
This is actually the same as Monad (category theory). These should be merged. —Ashley Y 22:19, 2004 May 31 (UTC)
Rename (from "Monad (functional programming)") done. —Ashley Y 23:27, 2004 Jun 2 (UTC)
Er, the first sentence of the second para (and possibly more) looks like a copyvio from http://nomaware.com/monads/html/introduction.html#what --- I don't know the topic well enough to rephrase to eliminate this. Could someone who knows the topic look into this? thanks, jdb ❋ 06:00, 27 Jan 2005 (UTC)
The // causes me no discomfort despite C being my first language. It's really obvious that it is an operator, especially given the text defining it as an operator... at least for me. However I *did* get confused by "par" (is it relevant to learning about Monads? oh no, it's an example taken from electrical engineering that I've seen before; it deserved a comment defining it) —Preceding unsigned comment added by 18.202.1.175 (talk) 23:56, 4 May 2009 (UTC)[reply]
Why is the function called par? --MarSch 11:06, 25 October 2005 (UTC)[reply]
For people used to reading C++ all day, "//" is a very difficult operator name to read. It looks like the start of a comment, and it's almost painful to have to ignore such "comments". Perhaps it could be renamed to "./." or even "///"? Kimbly 02:52, 10 January 2006 (UTC)[reply]
//
and /*
in C++, Java etc., #
in Python etc., ;
in Lisp, --
and {-
in Haskell. I noticed I've special-cased them too and skip text in them automatically. C++ programmers are used to <<
etc. meaning many things because they are overloaded already in the standard and most every language has operator overloading. Of course you can get accustomed to any a syntax, but do we want to make it as hard as possible for people to read this article? It's not like //
would be an integral part of the Haskell syntax. --TuukkaH 09:19, 10 January 2006 (UTC)[reply]//
comments wherever possible; i'm not fond of /**/
), but the example still doesn't cause me any discomfort. Besides, even if you limit the symbols to comments, what about Forth (and Forth-like) programmers, who use parentheses for comments (like in English)? The curly brace programming languages are very important and influential, but they're not special enough to start imposing their syntactical meaning on innocent bystanders.//
already has an established meaning of "modified/alternate division" in several languages (including Python): something that thousands of people can recognize immediately. This alone should make it an obvious winner over inventions as novel as ///
and ./.
. --Piet Delport 19:51, 10 January 2006 (UTC)[reply]#
or##
as an operator, while in Python, shell etc. they are used to start comments. As this page isn't strictly about Haskell, if we want to use Haskell as an example, we could still try to remove unnecessary readability problems from the article. --TuukkaH 05:50, 10 January 2006 (UTC)[reply]
I guess my scam is up. I haven't really used Haskell any significant amount, and I barely understand this monad stuff. I just hoped I might be able to push this article away from the two mistakes so many monad tutorials make—presenting them as "how you do I/O" or completely neglecting to give any intuition to the general concept of a monad. I did my rewrite with a copy of Wadler's "Comprehending Monads" in hand, but since I made so many mistakes I'll tag the article for verification. TuukaH is correct above about the naming of par
. Gazpacho 04:13, 29 October 2005 (UTC)[reply]
This page really seems to be more about monads in Haskell, rather than in a generic programming language. The examples given seem to be in Haskell, which for non haskell programmers (i.e. me) are really hard to follow.
Shouldn't this either be renamed to something like "Monads in Haskell" with a link to understanding Haskell, or have the equations written in a common mathematical way?
return 3 = either [3] or Just 3 -- "Wrap" the value 3 with a monad fmap (+1) [3,2,4] = [4,3,5] -- (+1) is the incrementor function, and it is allowed to "peek inside" this list to increment everything inside it. fmap (+1) (Just 5) = Just 6 -- Peeking inside a "Just" value. join Maybe (Maybe 7) = Maybe 7 -- "Joining" these two Maybe's into one join [[2]] = [2] -- Joining a listifier with a listifier
To be useful, (I concur that) this article should be relieved of its Haskell-centricity. This problem appears to have been lost or ignored; however the importance of the article itself should give some priority to fixing it. Timlevin (talk) 21:15, 28 November 2008 (UTC)[reply]
The cleanup tag really only refers to the fact that the first line is not rendered as the following lines are, and despite my attempts to fix it, it stubbornly remained the same... perhaps this is trivial, but still visually not pleasing. —The preceding unsigned comment was added by Laddiebuck (talk • contribs) 02:52, March 2, 2006 (UTC).
The article states that "Since the bind operator is non-associative, this constrains the sequence of execution." Is this really correct? The monad laws require that bind be associative, and the sequencing effect is due to the non-commutative properties of the bind operator. Chuan-kai Lin 17:26, 3 April 2006 (UTC)[reply]
I rewrote large sections of this article because I cited it for someone who wanted to know what monads are, he told me it didn't make any sense, and I agreed. Mainly, I made it less heavy on mathematical formulas. It's now heavy on Haskell, but I think that's appropriate because Haskell is the main promoter of monads, and this lets people download a Haskell system and try the code. Gazpacho 03:59, 9 May 2006 (UTC)[reply]
The current version of the article is extremely Haskell-centric. There are plenty of tutorials on Haskell monads on the web. There's no need for the Wikipedia article to be one as well.
Just to be clear, I don't have a problem with the use of Haskell as the language to illustrate monad concepts. But the current presentation is very much "here's how to use monads in Haskell". In particular, phrases like "To define a monad in Haskell...", and the discussion of Haskell's do-notation seem more appropriate for a Haskell article than a general article on monads in FP. --Allan McInnes (talk) 20:22, 22 June 2006 (UTC)[reply]
Unfortunately, I agree that if it's too abstract people won't actually be able to learn from it. For people coming from a CS rather than a maths background, Monads are rather tricky.—The preceding unsigned comment was added by 203.59.86.86 (talk • contribs) .
I agree too. I'm trying to completely grasp continuations as monads so I can add them to this. In the papers I've looked at thus far, most of them use Haskell. It'd probably be best to keep the explanation here as is, but move some of the Haskell-centric stuff (like the do-notation stuff) to another page --NotQuiteEXPComplete 12:25, 3 August 2006 (UTC)[reply]
I wish there was a way to include comments right in the article on Wikipedia, but we don't have that, so I'll try to explain where I want changes. My comments refere to the version as of http://en.wikipedia.org/w/index.php?title=Monads_in_functional_programming&oldid=84889989
Section Motivation: the first code example: I think this is Haskell code, but it's not apparent that it really is. Please be explicit about that. "par" is a bad function name; it isn't obvious that it has anything to do with parallel connection of resistors. Better to call it foo, or something. The syntax of the two lines of code is not obvious. The meanings of :: and -> are non-obvious.
Second code example: Is ... a part of the code? The meaning of :: and -> are again non-obvious. In an electrical engineering program, I'd like to use // as an operator means 1(1/x + 1/y), i.e., parallel connection, since the operator consists of two parallel lines. Therefore it's a bad choice for the meaning here; a new form of division.
The last sentence of the code example suggest that the example is wrong. Is it?
Section Definition: Shouldn't this be a definition of monads in functional programming, rather than monads in Haskell?
What does | (vertical bar) mean?
Are the words "data", "Just", "Nothing", parts of the Haskell language?
Is the character ≡
a part of the Haskell language?
Section Failure:
Is the word "mzero" a part of the Haskell language, or of functional programming vocabulary?
The Maybe monad example: What does "'", "in" and "let" mean?
The rest of the article is left for a later time.
Shouldn't Monad (functional programming) be the main name of this article? The discussion above doesn't really explain the reason for the name. --Bkkbrad 01:44, 26 December 2006 (UTC)[reply]
My haskell is a bit rusty, but it seems to me that
(return x) >>= f ≡ f x
should be
(return x) >>= f ≡ return (f x)
—The preceding unsigned comment was added by 129.29.227.4 (talk) 13:16, 27 March 2007 (UTC).[reply]
So I don't understand what Monads are, and after reading the article twice, I still don't know. The opening of the article explains what they are used for, but not what they are. The 'maybe' example is semi-understandable, but nothing explicitly states what problem Monads are solving. The definition given is nice and formal but totally inaccessible to anyone that doesn't already know what's being talked about. Is there some analogy that might be used to explain it? I haven't written a lot of code in functional languages (although I have written a little), never used Haskell, but I do have a Ph.D. in software engineering. 68.4.228.48 17:38, 10 June 2007 (UTC)[reply]
One problem I had reading this was that the article seemed to assume familiarity with Haskell syntax. In particular, unless you know something about the precedence and associativity of the Haskell operators used in the code, then some of the discussion can be unnecessarily confusing. For example, is ->
left-associative? commutative? what about >>=
? The use of parentheses in the binding axiom seems to suggest that these issues are important, but the reader has no way of determining the answer. I am not entirely sure how to fix this problem, since certainly this is the most comfortable notation for most of the authors of this page. On the other hand, it runs the risk of being a page that can only be read by people who don't need it. Perhaps I'm over-thinking this and just adding a bunch of extra parentheses would solve the problem (even if the added parentheses would be non-idiomatic in normal use of Haskell). --Rpgoldman (talk) 22:17, 15 April 2009 (UTC)[reply]
The reference "The three fundamental laws of Monad" does not exists anymore. Is the content at a new spot on the net? 87.163.118.84 09:15, 27 July 2007 (UTC)[reply]
First off, whoever wrote the "Maybe" example, thanks; it's been really helpful in getting my head around this. :) That said, there's something in there that doesn't seem right, and I think it may be an error. Specifically, it looks to me like
(Just x) >>= f = Just (f x)
should be
(Just x) >>= f = f x
f should be of type a -> Maybe b
; Just (f x)
would be of type Maybe Maybe b
, which wouldn't be right. (The standard prelude has (Just x) >>= k
defined as k x
.)
This also makes the "4 stages of binding" bit that precedes it seems a little misleading. I don't think the "piercing" of the result of the function being bound happens in Haskell unless you explicitly ask for it to (by, say, pattern-matching the result of the function being bound). In the case of Maybe, it's simply returning the function result straightaway.
I'd love to get some other thoughts on this, since I'm just getting started with Haskell!
Ngroot 19:44, 8 September 2007 (UTC)[reply]
Ironically, I came here to make an edit because the "Just" is missing. Recall that the result value should be of type "M u". I've added the clarification "(and wrapping the result in the monad type)"; i.e. what is done is this: "(Maybe x)" =extract_value=> "x" =apply_function=> "(f x)" =wrap_into_monad_type=> "Just (f x)". It has to be done this way, because at any moment, the computation could fail, resulting in "Nothing". All successful results are wrapped in "Just". ToolmakerSteve (talk) 21:00, 19 April 2008 (UTC)[reply]
I've now also changed 'A Maybe type is just the underlying type' to read 'A Maybe type is the underlying type wrapped with "Just"'. I am hoping this helps clarify that inside the Maybe monad, all values that aren't Nothing are wrapped in "Just". At each computation step, there is a simple test for "Just" or "Nothing". ToolmakerSteve (talk) 21:36, 19 April 2008 (UTC)[reply]
I'm experimenting with the equivalent of monads in F#; now I understand what Ngroot was saying. This needs to be rewritten, as it is unclear whether the "f" here is referring to a function returning a monad type, (which is why Ngroot saw the result as becoming "Just (Just ...)" as written), or the underlying function operating on the underlying value. ToolmakerSteve I am attempting to rewrite by introducing "f'", which is the monadic function that applies the underlying "f" and wraps it with "Just" on success, or returns "Nothing" on failure. (talk) 23:15, 19 April 2008 (UTC)[reply]
I found the initial description of the binding operation, with its four stages very intuitively appealing, but as I carried on in the article, I became confused. The problem for me is that in the third clause of the definition, binding is defined as taking a monad of underlying type t and a function t → (M u) and returning a value of type (M u). This seems clear enough. However, in the axioms section, all of a sudden we are told about binding two functions in succession, without any explanation of what this means. It may be that the use of Haskell notation here is just confusing me --- I would be happier if the axioms were presented in familiar mathematical notation instead of unfamiliar Haskell notation (see my remarks earlier on this page). I may further be confused by the (to me) very odd use of the term "binding," which I am familiar with from other functional programming contexts as being something that happens to variables... Quite possibly a concrete example would make this all crystal clear. --Rpgoldman (talk) 22:29, 15 April 2009 (UTC)[reply]
I've changed the Importance level to Mid. This subject is central to current practice in functional languages, at the same level than Design Patterns in OOP. But feel free to discuss. Diego (talk) 07:35, 26 June 2008 (UTC)[reply]
I can't understand what monads are from this article.
The first paragraph describes what monads are used for instead of describing what they are, contrary to Wikipedia guidelines. The rest of the articles then constantly mixes definitions referencing unexplained terms, like 'monadic type' with examples written exclusively in Haskell. —Preceding unsigned comment added by 87.18.173.1 (talk) 10:03, 6 July 2008 (UTC)[reply]
I just removed the following from the article: A "monad" should not be defined in terms of itself. That's bullshit (philosophy 101?). Please define what "monad" is without using the term "monad" to define it. If you cannot, I venture to say you don't understand it yourself and shouldn't be writing Wikipedia entries. I pray to God whoever wrote this were not a University professor, for it would indicate a sorry state of our education. (byUser:64.236.243.16)
It *is* definitely inappropriate to have a "definition" section that uses "monadic" without first defining what it means, even if the definition occurs later. shreevatsa (talk) 22:31, 15 July 2008 (UTC)[reply]
I removed the following from the bind disambiguation page, so it can be merged with (say) this article. Please feel free to merge.
Widefox (talk) 10:38, 7 November 2008 (UTC)[reply]
Moving this comment from the article's body so that it can be discussed..Diego (talk) 18:13, 6 December 2008 (UTC)[reply]
"to define arbitrary control flows (like handling concurrency, continuations, or exceptions)." Are they really that general? What, then, are monoids and arrows for? My understanding was that they described more general computations but had fewer laws one could reason with.