Difference between revisions of "What a Monad is not"
(→Warning: update "warm fuzzy" link, originally referenced article seemed to be named "understanding monad", too) 

(41 intermediate revisions by 9 users not shown)  
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==Warning== 
==Warning== 

−  This page is currently an unprocessed braindump. Feel free to dump additional stuff or massage stuff into didactic pleasures. 

+  Don't be surprised if you leave this page more confused than before. That just means that it has successfully destroyed your false assumptions, or that you've fallen for some horrible inside joke. Beware of [[Zygohistomorphic prepromorphisms]]. Go for [http://en.wikibooks.org/wiki/Haskell/Understanding_monads warm and fuzzy], instead. 

−  
−  Also, don't be surprised if you leave this page more confused than before. That just means that it has successfully destroyed your false assumptions, or that you've fallen for some horrible inside joke. Beware of [[Zygohistomorphic prepromorphisms]]. Go for [http://ertes.de/articles/monads.html warm and fuzzy], instead. 

==Monads are not a good choice as topic for your first Haskell blog entry== 
==Monads are not a good choice as topic for your first Haskell blog entry== 

−  ...just accept that they're [http:// 
+  ...just accept that they're [http://blog.plover.com/prog/burritos.html burritos], and wait until later. 
==Monads are not a language feature== 
==Monads are not a language feature== 

Line 11:  Line 9:  
==Haskell doesn't need Monads== 
==Haskell doesn't need Monads== 

−  ...well, apart from the Haskell standard defining the way IO is done in terms of Monads: It could be done differently and still work. 
+  ...well, apart from the current Haskell standard defining the way IO is done in terms of Monads: [http://donsbot.wordpress.com/2009/01/31/revivingthegoferstandardpreludecirca1994/ It could be done differently and still work]. 
==Monads are not impure== 
==Monads are not impure== 

Line 17:  Line 15:  
==Monads are not about state== 
==Monads are not about state== 

−  While it is certainly possible to abstract away explicit state passing by using a Monad, that's not what a monad is. 
+  While it is certainly possible to abstract away explicit state passing by using a Monad, that's not what a monad is. Some examples for monads that are not about state: Identity monad, Reader monad, List monad, Continuation monad, Exception monad. 
==Monads are not about strictness== 
==Monads are not about strictness== 

−  Monad operations (bind and return) have to be 
+  Monad operations (bind and return) have to be [[Nonstrict semanticsnonstrict]] in fact, always! However 
other operations can be specific to each monad. 
other operations can be specific to each monad. 

−  For instance some are strict (like IO), and some are 
+  For instance some are strict (like IO), and some are nonstrict (like []). Then there are some that come in multiple flavours, like State. 
+  
+  Try the following: 

+  
+  <haskell> 

+  runState (sequence . repeat $ state (\x > (x,x+1))) 0 

+  </haskell> 

+  
+  Having a look at the implementation of fixIO might be helpful, too. 

==Monads are not values== 
==Monads are not values== 

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Monads are not values in the same sense that addition and multiplication are not numbers: They capture a  very specific  relationship between values of a specific domain into a common abstraction. We're going to call these values monads manage ''mobits'', somewhat like this: 
Monads are not values in the same sense that addition and multiplication are not numbers: They capture a  very specific  relationship between values of a specific domain into a common abstraction. We're going to call these values monads manage ''mobits'', somewhat like this: 

+  <haskell> 

type Mobit m a = Monad m => m a 
type Mobit m a = Monad m => m a 

+  </haskell> 

The IO monad manages mobits representing sideeffects ("IO actions"). 
The IO monad manages mobits representing sideeffects ("IO actions"). 

Line 37:  Line 37:  
The Reader monads manages mobits that are pure computations that use asks to propagate information instead of explicit arguments 
The Reader monads manages mobits that are pure computations that use asks to propagate information instead of explicit arguments 

−  ...and while addition and multiplication are both monoids over the positive natural numbers, a monad is a monoid in a category of endofunctors. It' 
+  ...and while addition and multiplication are both monoids over the positive natural numbers, a monad is a monoid object in a category of endofunctors: return is the unit, and join is the binary operation. It couldn't be more simple. If that confuses you, it might be helpful to see a Monad as a lax functor from a terminal bicategory. 
−  
−  
==Monads are not a replacement for applicative functors== 
==Monads are not a replacement for applicative functors== 

Instead, every monad ''is'' an applicative functor (as well as a functor). It is considered good practice not to use >>= if all you need is <*>, or even fmap. 
Instead, every monad ''is'' an applicative functor (as well as a functor). It is considered good practice not to use >>= if all you need is <*>, or even fmap. 

−  Not confusing 
+  Not confusing which features of monads are specific to monads only and which stem from applicative functors is vitally important for a deeper understanding of monads. As an example, the applicative functor interface of parser libraries can parse contextfree languages (modulo hacks abusing open recursion), while the monadic interface can parse contextsensitive grammars: Monads allow you to influence further processing by inspecting the result of your parse. To understand why, have a look at the type of >>=. To understand why applicative functors by themselves are sufficient to track the current parsing position and express sequencing, have a look at the [http://www.cs.uu.nl/research/techreps/repo/CS2008/2008044.pdf uuparsinglib tutorial (pdf)]. 
The exact differences are elaborated in even greater detail in Brent Yorgey's excellent [[Typeclassopedia]]. 
The exact differences are elaborated in even greater detail in Brent Yorgey's excellent [[Typeclassopedia]]. 

−  ==Monads are not about ordering== 
+  ==Monads are not about ordering/sequencing== 
−  +  Monads are commonly used to order sequences of computations. But this is misleading. Just as you can use monads for state, or strictness, you can use them to order computations. But there are also commutative monads, like Reader, that don't order anything. So ordering is not in any way essential to what a monad is. 

−  +  Let's have a look at what's meant by ordering. Consider an expression like 

−  let x = a 

+  <haskell> 

−  y = b 

+  let x = a 

−  +  y = b 

+  in f x y 

+  </haskell> 

That gives the same result as 
That gives the same result as 

−  let y = b 

+  <haskell> 

−  x = a 

+  let y = b 

−  +  x = a 

+  in f x y 

+  </haskell> 

−  It doesn't matter what order we write the two 
+  It doesn't matter what order we write the two bindings. But for doing I/O we'd like ordering. Monads allow us to express 
−  do 

+  <haskell> 

−  x < getChar 

+  do 

−  +  x < getChar 

−  +  y < getChar 

+  return (x,y) 

+  </haskell> 

−  and have it be different 
+  and have it be different from 
−  do 

+  <haskell> 

−  y < getChar 

+  do 

−  +  y < getChar 

−  +  x < getChar 

+  return (x,y) 

+  </haskell> 

−  +  Unlike the first, the second example returns a pair of characters in the opposite order to which they were entered. 

−  However, there are monads for which swapping the order of lines like this makes no difference. For example the Maybe monad. 

+  It might help to meditate about the difference between 'assignment' and 'binding', right now. 

+  
+  However, just to spoil the enlightenment you just attained, there are monads for which swapping the order of lines like this makes no difference: For example, the Reader monad. 

So while it is correct to say that monads can be used to order operations, it would be wrong to say that monads are a mechanism for ordering operations. 
So while it is correct to say that monads can be used to order operations, it would be wrong to say that monads are a mechanism for ordering operations. 

−  This notion of commutativity 
+  This notion of commutativity looks superficially very different from the familiar one in vanilla algebra where a+b=b+a. It doesn't mean that 
+  <haskell> 

+  m >> n === n >> m 

+  </haskell> 

+  which doesn't hold in general for ''any'' nontrivial monad, as for the most part 

+  <haskell> 

+  return 1 >> return 2 === return 2 =/= return 1 === return 2 >> return 1 

+  </haskell> 

+  
+  This shouldn't be too surprising, though, as >> isn't the binary operation of a monoid. The [http://ncatlab.org/nlab/show/commutative+algebraic+theory categorytheoretic definition] of a commutative monad is rather more abstract. 

+  
+  ==See also== 

−  (Just 2 >> Just 3) == Just 3 

+  * [[Do notation considered harmful]] 

−  (It took long to explain that, maybe someone can edit this down.) 

+  [[Category:FAQ]] 

+  [[Category:Monad]] 
Latest revision as of 17:17, 26 July 2014
Contents
 1 Warning
 2 Monads are not a good choice as topic for your first Haskell blog entry
 3 Monads are not a language feature
 4 Haskell doesn't need Monads
 5 Monads are not impure
 6 Monads are not about state
 7 Monads are not about strictness
 8 Monads are not values
 9 Monads are not a replacement for applicative functors
 10 Monads are not about ordering/sequencing
 11 See also
Warning
Don't be surprised if you leave this page more confused than before. That just means that it has successfully destroyed your false assumptions, or that you've fallen for some horrible inside joke. Beware of Zygohistomorphic prepromorphisms. Go for warm and fuzzy, instead.
Monads are not a good choice as topic for your first Haskell blog entry
...just accept that they're burritos, and wait until later.
Monads are not a language feature
Really. They are defined in terms of Haskell, not Haskell in terms of them. Conversely,
Haskell doesn't need Monads
...well, apart from the current Haskell standard defining the way IO is done in terms of Monads: It could be done differently and still work.
Monads are not impure
...In no way whatsoever. You don't even need flexible morals to claim it. To be more specific, it's IO that's impure. That makes the IO monad impure. But that's not a general property of monads  just IO. And even then, we can pretend that Haskell is a purely functional description language for imperative programs. But we didn't want to employ flexible morals, now did we?
Monads are not about state
While it is certainly possible to abstract away explicit state passing by using a Monad, that's not what a monad is. Some examples for monads that are not about state: Identity monad, Reader monad, List monad, Continuation monad, Exception monad.
Monads are not about strictness
Monad operations (bind and return) have to be nonstrict in fact, always! However other operations can be specific to each monad. For instance some are strict (like IO), and some are nonstrict (like []). Then there are some that come in multiple flavours, like State.
Try the following:
runState (sequence . repeat $ state (\x > (x,x+1))) 0
Having a look at the implementation of fixIO might be helpful, too.
Monads are not values
This point might be driven home best by pointing out that instance Monad Foo where ... is not a data type, but a declaration of a typeclass instance. However, to elaborate:
Monads are not values in the same sense that addition and multiplication are not numbers: They capture a  very specific  relationship between values of a specific domain into a common abstraction. We're going to call these values monads manage mobits, somewhat like this:
type Mobit m a = Monad m => m a
The IO monad manages mobits representing sideeffects ("IO actions").
The List monad manages mobits representing multiple values ("[a]")
The Reader monads manages mobits that are pure computations that use asks to propagate information instead of explicit arguments
...and while addition and multiplication are both monoids over the positive natural numbers, a monad is a monoid object in a category of endofunctors: return is the unit, and join is the binary operation. It couldn't be more simple. If that confuses you, it might be helpful to see a Monad as a lax functor from a terminal bicategory.
Monads are not a replacement for applicative functors
Instead, every monad is an applicative functor (as well as a functor). It is considered good practice not to use >>= if all you need is <*>, or even fmap.
Not confusing which features of monads are specific to monads only and which stem from applicative functors is vitally important for a deeper understanding of monads. As an example, the applicative functor interface of parser libraries can parse contextfree languages (modulo hacks abusing open recursion), while the monadic interface can parse contextsensitive grammars: Monads allow you to influence further processing by inspecting the result of your parse. To understand why, have a look at the type of >>=. To understand why applicative functors by themselves are sufficient to track the current parsing position and express sequencing, have a look at the uuparsinglib tutorial (pdf).
The exact differences are elaborated in even greater detail in Brent Yorgey's excellent Typeclassopedia.
Monads are not about ordering/sequencing
Monads are commonly used to order sequences of computations. But this is misleading. Just as you can use monads for state, or strictness, you can use them to order computations. But there are also commutative monads, like Reader, that don't order anything. So ordering is not in any way essential to what a monad is.
Let's have a look at what's meant by ordering. Consider an expression like
let x = a
y = b
in f x y
That gives the same result as
let y = b
x = a
in f x y
It doesn't matter what order we write the two bindings. But for doing I/O we'd like ordering. Monads allow us to express
do
x < getChar
y < getChar
return (x,y)
and have it be different from
do
y < getChar
x < getChar
return (x,y)
Unlike the first, the second example returns a pair of characters in the opposite order to which they were entered.
It might help to meditate about the difference between 'assignment' and 'binding', right now.
However, just to spoil the enlightenment you just attained, there are monads for which swapping the order of lines like this makes no difference: For example, the Reader monad.
So while it is correct to say that monads can be used to order operations, it would be wrong to say that monads are a mechanism for ordering operations.
This notion of commutativity looks superficially very different from the familiar one in vanilla algebra where a+b=b+a. It doesn't mean that
m >> n === n >> m
which doesn't hold in general for any nontrivial monad, as for the most part
return 1 >> return 2 === return 2 =/= return 1 === return 2 >> return 1
This shouldn't be too surprising, though, as >> isn't the binary operation of a monoid. The categorytheoretic definition of a commutative monad is rather more abstract.