Re: in Clojure I rarely find myself reaching for something like the state monad, as I would in Haskell

2014-05-20 Thread Phillip Lord
Julian juliangam...@gmail.com writes:
 My question is - have other Clojure/Haskell programmers had this 
 experience? (ie I rarely find myself reaching for something like the state 
 monad). I'm interested to hear if so, and why. 


I find myself reaching for the state monad all the time; then I realise
that, still, no one has been able to explain what a monad is in a way
that I understand, so I stop.

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Re: in Clojure I rarely find myself reaching for something like the state monad, as I would in Haskell

2014-05-19 Thread Bob Hutchison

On May 16, 2014, at 8:49 PM, Julian juliangam...@gmail.com wrote:

 A quick shoutout to the Clojure Community - thanks for the way you've all 
 contributed to make my life (mentally) richer. 
 
 James Reeves (author of Compojure and many other wonderful libraries) made 
 this interesting comment on Hacker News:
  Clojure has libraries that implement monads, but these aren't often used 
  for threading state. I can't quite place my finger on why, but in Clojure I 
  rarely find myself reaching for something like the state monad, as I would 
  in Haskell.
 Clojure tends to view mutability as a concurrency problem, and the tools it 
 provides to deal with mutability, such as atoms, refs, agents, channels and 
 so forth, are not mechanisms to avoid mutation, as to provide various 
 guarantees that restrict it in some fashion.
 It might be that in the cases where I'd use a state monad in Haskell, in 
 Clojure I might instead use an atom. They're in no way equivalent, but they 
 have some overlapping use-cases.
 https://news.ycombinator.com/item?id=7751424
 My question is - have other Clojure/Haskell programmers had this experience? 
 (ie I rarely find myself reaching for something like the state monad). I'm 
 interested to hear if so, and why. 
 

I'm perhaps an atypical specimen (and I'm not what I'd call an expert Haskell 
programmer) but... I've written a fairly substantial prototype in Haskell 
(maybe 25k sloc, whatever that means) that made heavy use of the state monad. 
I've re-written the same application in Clojure and it never, even once, 
occurred to me to use the state monad. As James Reeves pointed out, Clojure 
programmers normally use atoms or refs to manage this kind of state. Looking at 
my two code bases you'd notice that the state monad would have been replaced in 
Clojure by a bunch of refs and atoms. If the state had been less complex and 
the application less concurrent I'd have used a single atom.

In Haskell you'd not be able to use this technique nearly as frequently, if at 
all, and certainly not in the same way. 

Please excuse the imprecision in the following... 

In Haskell, all IO is marked in the type signatures, and so if you want to 
perform any IO you have to carry the IO monad with you (IO appears in your type 
signature) into all 'sub' computations. The IO monad exists at the top level of 
your program, and so must be 'carried' all the way from the top level, you 
can't just suddenly start using IO deep within some computation. And this is 
exactly what you want in Haskell -- it's a *good* thing. In Clojure you can 
perform IO wherever you wish, and in Clojure this is exactly what you want, and 
is also a *good* thing. 

Haskell's STM transactions can be thought of as a form of IO action (like 
reading a file is an IO action) that modify refs (there are no atoms in 
Haskell, only refs). A transaction must be started in the IO monad and then, 
like IO, the STM monad is 'carried' in type signatures through all intervening 
computations that could take part in the transaction. The STM type/monad 
'blocks' the IO type/monad (you can't do other IO actions if you might take 
part in an STM transaction (IO action), this is an effect of, and enforced by, 
Haskell's type system (i.e. it's a compilation not a runtime error)). In 
Clojure the STM isn't part of the IO system, and you can start or take part in 
a transaction anywhere you want to, even nest dosyncs within a single 
transaction, and intermingle transactional code with IO (no matter how bad an 
idea that is).

That's a lot of talk to get to the point that using the STM has an 
insignificant impact on the structure of your Clojure programs, while in 
Haskell the impact is huge (of course, it's possible to argue that that's a 
*good* thing). In Haskell the state monad is pretty flexible, well supported by 
the language, and allows you to sidestep a lot of this impact (for one thing, 
you can introduce it anywhere). In Clojure the state monad would buy you 
nothing (in my opinion) while using it would have an impact on your programs 
structure. In Haskell there are forces pushing you to use the state monad, 
while in Clojure there are forces pushing you away from the state monad.

In my opinion.



I've 'simplified' my explanation, and obscured some of the actual 
issues/powers/advantages of Haskell's type system. For example, I hand-wavingly 
using the phrase 'you can/can't introduce...' -- it isn't quite like that, but 
that's the effect, so, I think, close enough for this discussion.

 

As an aside, a Clojure programmer might get a feel for what Haskell's state 
monad is like by considering the - and - macros. Within the - macro you 
start by defining your initial state then applying a sequence of operations to 
an updated state. You don't have to mention the state in your code again. The 
functions called in the - and - macros have either their first or last 
argument 'receiving' the state and returns the updated 

Re: in Clojure I rarely find myself reaching for something like the state monad, as I would in Haskell

2014-05-19 Thread Ben Wolfson
I wouldn't say that I *often* find myself reaching for monads, or the state
monad in particular, but I certainly have found them useful on occasion
(and would have sometimes refrained from using them where I'd naturally
lean to doing so solely to avoid creating an dependency). For instance,
whenever there's a couple of functions that return either a success value
or an error message that have to be threaded together---an error monad to
do the plumbing makes this a lot nicer.

We've got a library at ReadyForZero for walking though json and xml
structures and doing transformations based on the values found there, or a
bit of configuration, using a combined reader-writer-state monad, and a
bunch of code that uses it. The state that's held is actually just a zipper
into the structure, the configuration at this point is only a keyword, and
the writer log holds reports of unexpected values. This could all be done
with other machinery---pass the zipper around directly (or hold it in an
atom), pass the log around directly (or hold it in an atom), use a dynamic
variable + binding for the configuration (since the reader monad amounts to
that anyway). However, I think the monadic code is easier to work with,
partly because nothing does need to be managed or passed around explicitly
(so it's easier to put together lots of little pieces), and partly because
it enables the use of generic tools. Also, traversing the the structures
has a fairly imperative feel---go here, go there, do this
transformation---with occasional variable binding, and the macro for
monadic computations the monad library we're using provides makes
expressing that fairly convenient. (Though I may be biased, since I wrote
it.)

It's true that there doesn't seem to be much need for introducing a monad
library and using the state monad if you *only* had the state monad, since
Clojure has other ways to deal with mutation (incidentally, I don't think
it's true to say that Haskell only has refs, not atoms; there are functions
to modify IORefs, which live outside the STM system, atomically).



On Fri, May 16, 2014 at 5:49 PM, Julian juliangam...@gmail.com wrote:

 A quick shoutout to the Clojure Community - thanks for the way you've all
 contributed to make my life (mentally) richer.

 James Reeves (author of Compojure and many other wonderful libraries) made
 this interesting comment on Hacker News:
  Clojure has libraries that implement monads, but these aren't often
 used for threading state. I can't quite place my finger on why, but in
 Clojure I rarely find myself reaching for something like the state monad,
 as I would in Haskell.

 Clojure tends to view mutability as a concurrency problem, and the tools
 it provides to deal with mutability, such as atoms, refs, agents, channels
 and so forth, are not mechanisms to avoid mutation, as to provide various
 guarantees that restrict it in some fashion.

 It might be that in the cases where I'd use a state monad in Haskell, in
 Clojure I might instead use an atom. They're in no way equivalent, but they
 have some overlapping use-cases.

 https://news.ycombinator.com/item?id=7751424

 My question is - have other Clojure/Haskell programmers had this
 experience? (ie I rarely find myself reaching for something like the
 state monad). I'm interested to hear if so, and why.

 JG

 PS If this post is unhelpful, could be worded better - please let me know.
 I'm asking out of curiosity, not with intent to troll.

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-- 
Ben Wolfson
Human kind has used its intelligence to vary the flavour of drinks, which
may be sweet, aromatic, fermented or spirit-based. ... Family and social
life also offer numerous other occasions to consume drinks for pleasure.
[Larousse, Drink entry]

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Re: in Clojure I rarely find myself reaching for something like the state monad, as I would in Haskell

2014-05-19 Thread Ben Wolfson
On Mon, May 19, 2014 at 7:48 AM, Bob Hutchison hutch-li...@recursive.cawrote:


 Haskell's STM transactions can be thought of as a form of IO action (like
 reading a file is an IO action) that modify refs (there are no atoms in
 Haskell, only refs). A transaction must be started in the IO monad and
 then, like IO, the STM monad is 'carried' in type signatures through all
 intervening computations that could take part in the transaction. The STM
 type/monad 'blocks' the IO type/monad (you can't do other IO actions if you
 might take part in an STM transaction (IO action), this is an effect of,
 and enforced by, Haskell's type system (i.e. it's a compilation not a
 runtime error)). In Clojure the STM isn't part of the IO system, and you
 can start or take part in a transaction anywhere you want to, even nest
 dosyncs within a single transaction, and intermingle transactional code
 with IO (no matter how bad an idea that is).


You can use regular IO in an STM action with unsafeUItoSTM: 
http://hackage.haskell.org/package/base-4.7.0.0/docs/GHC-Conc-Sync.html#v:unsafeIOToSTM.
IMO it's advantageous that you have to explicitly say that you want to do
IO inside a transaction, given that (in general) you probably shouldn't be
doing it.

-- 
Ben Wolfson
Human kind has used its intelligence to vary the flavour of drinks, which
may be sweet, aromatic, fermented or spirit-based. ... Family and social
life also offer numerous other occasions to consume drinks for pleasure.
[Larousse, Drink entry]

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Re: in Clojure I rarely find myself reaching for something like the state monad, as I would in Haskell

2014-05-19 Thread Bob Hutchison

On May 19, 2014, at 1:44 PM, Ben Wolfson wolf...@gmail.com wrote:

 I wouldn't say that I *often* find myself reaching for monads, or the state 
 monad in particular, but I certainly have found them useful on occasion (and 
 would have sometimes refrained from using them where I'd naturally lean to 
 doing so solely to avoid creating an dependency). For instance, whenever 
 there's a couple of functions that return either a success value or an error 
 message that have to be threaded together---an error monad to do the plumbing 
 makes this a lot nicer.

I badly miss the Maybe and Either monads, but would want the syntactic support 
Haskell provides (which I can't see will ever be available in Clojure)

 
 We've got a library at ReadyForZero for walking though json and xml 
 structures and doing transformations based on the values found there, or a 
 bit of configuration, using a combined reader-writer-state monad, and a bunch 
 of code that uses it. The state that's held is actually just a zipper into 
 the structure, the configuration at this point is only a keyword, and the 
 writer log holds reports of unexpected values. This could all be done with 
 other machinery---pass the zipper around directly (or hold it in an atom), 
 pass the log around directly (or hold it in an atom), use a dynamic variable 
 + binding for the configuration (since the reader monad amounts to that 
 anyway). However, I think the monadic code is easier to work with, partly 
 because nothing does need to be managed or passed around explicitly (so it's 
 easier to put together lots of little pieces), and partly because it enables 
 the use of generic tools. Also, traversing the the structures has a fairly 
 imperative feel---go here, go there, do this transformation---with occasional 
 variable binding, and the macro for monadic computations the monad library 
 we're using provides makes expressing that fairly convenient. (Though I may 
 be biased, since I wrote it.)
 
 It's true that there doesn't seem to be much need for introducing a monad 
 library and using the state monad if you *only* had the state monad, since 
 Clojure has other ways to deal with mutation (incidentally, I don't think 
 it's true to say that Haskell only has refs, not atoms; there are functions 
 to modify IORefs, which live outside the STM system, atomically).

Which is why I didn't call them atoms :-) There's also a ref in the ST monad 
(which is a bells-and-whistles state monad that has fallen into a bit of disuse 
since the IO monad appeared)

 
 
 
 On Fri, May 16, 2014 at 5:49 PM, Julian juliangam...@gmail.com wrote:
 A quick shoutout to the Clojure Community - thanks for the way you've all 
 contributed to make my life (mentally) richer. 
 
 James Reeves (author of Compojure and many other wonderful libraries) made 
 this interesting comment on Hacker News:
  Clojure has libraries that implement monads, but these aren't often used 
  for threading state. I can't quite place my finger on why, but in Clojure I 
  rarely find myself reaching for something like the state monad, as I would 
  in Haskell.
 Clojure tends to view mutability as a concurrency problem, and the tools it 
 provides to deal with mutability, such as atoms, refs, agents, channels and 
 so forth, are not mechanisms to avoid mutation, as to provide various 
 guarantees that restrict it in some fashion.
 It might be that in the cases where I'd use a state monad in Haskell, in 
 Clojure I might instead use an atom. They're in no way equivalent, but they 
 have some overlapping use-cases.
 https://news.ycombinator.com/item?id=7751424
 My question is - have other Clojure/Haskell programmers had this experience? 
 (ie I rarely find myself reaching for something like the state monad). I'm 
 interested to hear if so, and why. 
 JG
 PS If this post is unhelpful, could be worded better - please let me know. 
 I'm asking out of curiosity, not with intent to troll. 
 
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 -- 
 Ben Wolfson
 Human kind has used its intelligence to vary the flavour of drinks, which 
 may be sweet, aromatic, fermented or spirit-based. ... Family and social life 
 also offer numerous other occasions to consume drinks for pleasure. 
 [Larousse, Drink entry]
 
 
 -- 
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Re: in Clojure I rarely find myself reaching for something like the state monad, as I would in Haskell

2014-05-19 Thread Bob Hutchison

On May 19, 2014, at 1:50 PM, Ben Wolfson wolf...@gmail.com wrote:

 On Mon, May 19, 2014 at 7:48 AM, Bob Hutchison hutch-li...@recursive.ca 
 wrote:
 
 Haskell's STM transactions can be thought of as a form of IO action (like 
 reading a file is an IO action) that modify refs (there are no atoms in 
 Haskell, only refs). A transaction must be started in the IO monad and then, 
 like IO, the STM monad is 'carried' in type signatures through all 
 intervening computations that could take part in the transaction. The STM 
 type/monad 'blocks' the IO type/monad (you can't do other IO actions if you 
 might take part in an STM transaction (IO action), this is an effect of, and 
 enforced by, Haskell's type system (i.e. it's a compilation not a runtime 
 error)). In Clojure the STM isn't part of the IO system, and you can start or 
 take part in a transaction anywhere you want to, even nest dosyncs within a 
 single transaction, and intermingle transactional code with IO (no matter how 
 bad an idea that is).
 
 You can use regular IO in an STM action with unsafeUItoSTM: 
 http://hackage.haskell.org/package/base-4.7.0.0/docs/GHC-Conc-Sync.html#v:unsafeIOToSTM.
  IMO it's advantageous that you have to explicitly say that you want to do IO 
 inside a transaction, given that (in general) you probably shouldn't be doing 
 it.

You're right. I didn't want to bring that up though. It's well named. I used 
the various unsafe* functions regularly while debugging, and even then only for 
writing to the console. I would be *very* *very* reluctant to use any of them 
in production, and certainly not intentionally design something that required 
their use. So, for the purposes of this discussion, I figured I'd just pretend 
they didn't exist.

Cheers,
Bob

 
 -- 
 Ben Wolfson
 Human kind has used its intelligence to vary the flavour of drinks, which 
 may be sweet, aromatic, fermented or spirit-based. ... Family and social life 
 also offer numerous other occasions to consume drinks for pleasure. 
 [Larousse, Drink entry]
 
 
 -- 
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Re: in Clojure I rarely find myself reaching for something like the state monad, as I would in Haskell

2014-05-19 Thread Ben Wolfson
On Mon, May 19, 2014 at 11:28 AM, Bob Hutchison hutch-li...@recursive.cawrote:


 On May 19, 2014, at 1:44 PM, Ben Wolfson wolf...@gmail.com wrote:

 I wouldn't say that I *often* find myself reaching for monads, or the
 state monad in particular, but I certainly have found them useful on
 occasion (and would have sometimes refrained from using them where I'd
 naturally lean to doing so solely to avoid creating an dependency). For
 instance, whenever there's a couple of functions that return either a
 success value or an error message that have to be threaded together---an
 error monad to do the plumbing makes this a lot nicer.


 I badly miss the Maybe and Either monads, but would want the syntactic
 support Haskell provides (which I can't see will ever be available in
 Clojure)


I've been pretty happy with:
https://github.com/bwo/monads/blob/master/src/monads/core.clj#L65



 We've got a library at ReadyForZero for walking though json and xml
 structures and doing transformations based on the values found there, or a
 bit of configuration, using a combined reader-writer-state monad, and a
 bunch of code that uses it. The state that's held is actually just a zipper
 into the structure, the configuration at this point is only a keyword, and
 the writer log holds reports of unexpected values. This could all be done
 with other machinery---pass the zipper around directly (or hold it in an
 atom), pass the log around directly (or hold it in an atom), use a dynamic
 variable + binding for the configuration (since the reader monad amounts to
 that anyway). However, I think the monadic code is easier to work with,
 partly because nothing does need to be managed or passed around explicitly
 (so it's easier to put together lots of little pieces), and partly because
 it enables the use of generic tools. Also, traversing the the structures
 has a fairly imperative feel---go here, go there, do this
 transformation---with occasional variable binding, and the macro for
 monadic computations the monad library we're using provides makes
 expressing that fairly convenient. (Though I may be biased, since I wrote
 it.)

 It's true that there doesn't seem to be much need for introducing a monad
 library and using the state monad if you *only* had the state monad, since
 Clojure has other ways to deal with mutation (incidentally, I don't think
 it's true to say that Haskell only has refs, not atoms; there are functions
 to modify IORefs, which live outside the STM system, atomically).


 Which is why I didn't call them atoms :-) There's also a ref in the ST
 monad (which is a bells-and-whistles state monad that has fallen into a bit
 of disuse since the IO monad appeared)


Well, my point was that IORefs seem to provide in Haskell what atoms
provide in Clojure. The refs in ST don't do the the same work because the
existential type parameter keeps them from being shared across distinct
runST invocations.


-- 
Ben Wolfson
Human kind has used its intelligence to vary the flavour of drinks, which
may be sweet, aromatic, fermented or spirit-based. ... Family and social
life also offer numerous other occasions to consume drinks for pleasure.
[Larousse, Drink entry]

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Re: in Clojure I rarely find myself reaching for something like the state monad, as I would in Haskell

2014-05-19 Thread Bob Hutchison

On May 19, 2014, at 2:45 PM, Ben Wolfson wolf...@gmail.com wrote:

 On Mon, May 19, 2014 at 11:28 AM, Bob Hutchison hutch-li...@recursive.ca 
 wrote:
 
 I badly miss the Maybe and Either monads, but would want the syntactic 
 support Haskell provides (which I can't see will ever be available in Clojure)
 
 I've been pretty happy with: 
 https://github.com/bwo/monads/blob/master/src/monads/core.clj#L65
 

Hmm. I'm going to have to take another look at that. Thanks!

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in Clojure I rarely find myself reaching for something like the state monad, as I would in Haskell

2014-05-16 Thread Julian
A quick shoutout to the Clojure Community - thanks for the way you've all 
contributed to make my life (mentally) richer. 

James Reeves (author of Compojure and many other wonderful libraries) made 
this interesting comment on Hacker News:
 Clojure has libraries that implement monads, but these aren't often used 
for threading state. I can't quite place my finger on why, but in Clojure I 
rarely find myself reaching for something like the state monad, as I would 
in Haskell.

Clojure tends to view mutability as a concurrency problem, and the tools 
it provides to deal with mutability, such as atoms, refs, agents, channels 
and so forth, are not mechanisms to avoid mutation, as to provide various 
guarantees that restrict it in some fashion.

It might be that in the cases where I'd use a state monad in Haskell, in 
Clojure I might instead use an atom. They're in no way equivalent, but they 
have some overlapping use-cases.

https://news.ycombinator.com/item?id=7751424

My question is - have other Clojure/Haskell programmers had this 
experience? (ie I rarely find myself reaching for something like the state 
monad). I'm interested to hear if so, and why. 

JG

PS If this post is unhelpful, could be worded better - please let me know. 
I'm asking out of curiosity, not with intent to troll. 

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Re: in Clojure I rarely find myself reaching for something like the state monad, as I would in Haskell

2014-05-16 Thread Timothy Baldridge
When I first wrote the core.async go macro I based it on the state monad.
It seemed like a good idea; keep everything purely functional. However,
over time I've realized that this actually introduces a lot of incidental
complexity. And let me explain that thought.

What are we concerned about when we use the state monad, we are shunning
mutability. Where do the problems surface with mutability? Mostly around
backtracking (getting old data or getting back to an old state), and
concurrency.

In the go macro transformation, I never need old state, and the transformer
isn't concurrent. So what's the point? Recently I did an experiment that
ripped out the state monad and replaced it with mutable lists and lots of
atoms. The end result was code that was about 1/3rd the size of the
original code, and much more readable.

So more and more, I'm trying to see mutability through those eyes: I should
reach for immutable data first, but if that makes the code less readable
and harder to reason about, why am I using it?

Timothy


On Fri, May 16, 2014 at 6:49 PM, Julian juliangam...@gmail.com wrote:

 A quick shoutout to the Clojure Community - thanks for the way you've all
 contributed to make my life (mentally) richer.

 James Reeves (author of Compojure and many other wonderful libraries) made
 this interesting comment on Hacker News:
  Clojure has libraries that implement monads, but these aren't often
 used for threading state. I can't quite place my finger on why, but in
 Clojure I rarely find myself reaching for something like the state monad,
 as I would in Haskell.

 Clojure tends to view mutability as a concurrency problem, and the tools
 it provides to deal with mutability, such as atoms, refs, agents, channels
 and so forth, are not mechanisms to avoid mutation, as to provide various
 guarantees that restrict it in some fashion.

 It might be that in the cases where I'd use a state monad in Haskell, in
 Clojure I might instead use an atom. They're in no way equivalent, but they
 have some overlapping use-cases.

 https://news.ycombinator.com/item?id=7751424

 My question is - have other Clojure/Haskell programmers had this
 experience? (ie I rarely find myself reaching for something like the
 state monad). I'm interested to hear if so, and why.

 JG

 PS If this post is unhelpful, could be worded better - please let me know.
 I'm asking out of curiosity, not with intent to troll.

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zero–they had no way to indicate successful termination of their C
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