[Haskell-cafe] Haskell showcase in 5 minutes - follow-up

[Arnaud Bailly]

Hello Cafe,
Thanks to all of you who provided ideas and suggestions for introducing
Haskell. I finally went the simple way, presenting the knapsack problem
slightly reframed using tapas (spanish dishes). This problem is small
enough to fit in the 5 minutes time-frame and it actually illustrates some
of the salient points of the langauge: laziness of course,
list-comprehensions, monadic computations (replicateM), first-class and
higher-order functions, the REPL, integration with emacs, command-line
compiling.

Here is the code https://gist.github.com/1948301

I got a lot of positive feedback during the evening as I did some more live
coding on simple problems (fizz-buzz, fibonacci) and I ended up showing
some code I wrote doing real stuff, notably the simple music synthesizer
I wrote (https://github.com/abailly/haskell-synthesizer) for presenting
Haskell to Paris Scala user Group and inspired of course by Paul Hudak's
book and Joao Pizani's blog (
http://joaopizani.hopto.org/en/2012/01/haskell-synth/). The meeting ended
with an impromptu panel on the disadvantages of the languages presented by
each speaker which was really hard for me given the sheer perfection of
Haskell.

One advantage I mentionned somewhat indirectly which I should stress here
is the Haskell community: reactive, vibrant and kind are a few adjectives
that spring to my mind.

Thanks again for your support,
Arnaud Bailly
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Re: [Haskell-cafe] Records in Haskell

[AntC]

Evan Laforge qdunkan at gmail.com writes:

 [ ccing the list because the wiki page was flawed and I made a bunch
 of changes, hope you don't mind ]
 

Thanks Evan, I've had a quick read through.

It's a bit difficult to compare to the other proposals.

I can't see discussion of extracting higher-ranked functions and applying them 
in polymorphic contexts. (This is SPJ's `rev` example.)

Putting h-r fields into records is the standard way of emulating object-
oriented style. SPJ's view is that requirement is very important in practice.

(No proposal has a good answer to updating h-r's, which you do discuss.)


Re the cons 1. Still can't have two records with the same field name in the 
same module since it relies on modules for namespacing.

Did you see the DORF precursor page ? 
http://hackage.haskell.org/trac/ghc/wiki/Records/DeclaredOverloadedRecordFields
/NoMonoRecordFields

I tried to figure out if that would help, but I suspect not. (Looking at the 
desugar for `deriving (Lens)`, you need the H98 field selector functions.) 
Then for me, cons 1. is a show-stopper. (I know you think the opposite.)


I also don't see whether you can 'hide' or make abstract the representation of 
a record type, but still allow read-access to (some of) its fields. Suppose a 
malicious client declares a record with field #a. Can you stop them reading 
and/or updating your field #a whilst still letting them see field #b of your 
record type?


With SDNR, is it possibly to define a polymorphic field selector function? I 
suspect no looking at the desugar for `deriving (Lens)`, but perhaps I've mis-
understood. I mean:
get_a r = ?? #a r -- gets the #a field from any record r

This mechanism then supports the idea of 'virtual' fields -- SPJ's example of 
fullName, built from polymorphic firstName and lastName.


[By the way, did you mean to post to the cafe only? Most of the discussion is 
going on on ghc-users.]


AntC


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Re: [Haskell-cafe] need help with monad transformers

[Dmitry Olshansky]

 If I'm running register outside runWriterT everything will work.

Maybe just

 lift $ register $ print freed2

or I didn't catch something?



2012/3/1 Alexander V Vershilov alexander.vershi...@gmail.com

 Hello.

 I'm trying to add monad stack into network-conduit, and everything
 works except some details [1].

 I've run runReaderT $ runTCPServer (wrapper around runResourceT) and
 inside conduit I want to run writer to gather results of inner computation.
 In inner computation I want to use IO, data from outter stack (ReaderT)
 so I'm running {-1-}:

  (k,t) - lift $ runWriterT $ ask = \x - tell [x] {- 1 -}

 and {-2-}

  (k,t) - lift $ runWriterT $ do {- 2 -}
x - ask
liftIO $ print $x+1
tell [x]

 and that will work (except I've thought I should not lift runWriterT, but
 calling functions inside.

 And finally in computation that will run once I want to register cleaning
 function (for example register $ putStrLn cleaned) ({-3-})

  (k,t) - lift $ runWriterT $ do {- 3 -}
x - ask
liftIO $ print $x+1
register $ print freed2
tell [x]

 but I've got type error. If I'm running register outside runWriterT
 everything
 will work.

 I would apperated if there will be any suggestions how to make this code
 better or use register in internal computation (runWriterT)

 [1] https://gist.github.com/1941151
 --
 Best regards,
  Alexander V Vershilov

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Re: [Haskell-cafe] need help with monad transformers

[Alexander V Vershilov]

It will not work due:

gistfile1.hs:29:36:
Could not deduce (m ~ ResourceT m0)
from the context (Base m ~ IO,
  MonadReader SInit m,
  MonadIO m,
  IsSource src,
  Resource m)
  bound by the type signature for
 app :: (Base m ~ IO,
 MonadReader SInit m,
 MonadIO m,
 IsSource src,
 Resource m) =
src m a1 - Sink a1 m b - ResourceT m b
  at gistfile1.hs:(29,1)-(59,27)
  `m' is a rigid type variable bound by
  the type signature for
app :: (Base m ~ IO,
MonadReader SInit m,
MonadIO m,
IsSource src,
Resource m) =
   src m a1 - Sink a1 m b - ResourceT m b
  at gistfile1.hs:29:1
Expected type: Conduit a1 m a1
  Actual type: Conduit a1 (ResourceT m0) a1
In the second argument of `(=$=)', namely `process2'
In the second argument of `($=)', namely `(process1 =$= process2)'


Thu, Mar 01, 2012 at 02:19:29PM +0400, Dmitry Olshansky wrote
  If I'm running register outside runWriterT everything will work.
 
 Maybe just
 
  lift $ register $ print freed2
 
 or I didn't catch something?
 
 
 
 2012/3/1 Alexander V Vershilov alexander.vershi...@gmail.com
 
 Hello.
 
 I'm trying to add monad stack into network-conduit, and everything
 works except some details [1].
 
 I've run runReaderT $ runTCPServer (wrapper around runResourceT) and
 inside conduit I want to run writer to gather results of inner 
 computation.
 In inner computation I want to use IO, data from outter stack (ReaderT)
 so I'm running {-1-}:
 
  (k,t) - lift $ runWriterT $ ask = \x - tell [x]     {- 1 -}
 
 and {-2-}
 
  (k,t) - lift $ runWriterT $ do {- 2 -}
                    x - ask
                    liftIO $ print $x+1
                    tell [x]
 
 and that will work (except I've thought I should not lift runWriterT, but
 calling functions inside.
 
 And finally in computation that will run once I want to register cleaning
 function (for example register $ putStrLn cleaned) ({-3-})
 
  (k,t) - lift $ runWriterT $ do {- 3 -}
                    x - ask
                    liftIO $ print $x+1
                    register $ print freed2
                    tell [x]
 
 but I've got type error. If I'm running register outside runWriterT
 everything
 will work.
 
 I would apperated if there will be any suggestions how to make this code
 better or use register in internal computation (runWriterT)
 
 [1] https://gist.github.com/1941151
 --
 Best regards,
  Alexander V Vershilov
 
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Re: [Haskell-cafe] FreeSect -- generalised sections syntax extension

[Ras Far]

For anyone who tried building the implementation linked off the
fremissant page (see above email), there was a glitch or two and this
is fixed.  The problem being simply the target path -- I build to a
ramdisk because, as is often the case, GHC produces a rather large
binary and I don't like to thrash my HDD during development.  The
executable is now produced in the root directory of the distro.

Cheers,
Andrew

On Wed, Feb 29, 2012 at 9:27 PM, Ras Far ras...@gmail.com wrote:
 Hello,

 I bit premature perhaps but I wanted to post it on a leap day...

 http://fremissant.net/freesect

 Thanks for eyebloom on #haskell for motivating me to finally implement
 an old idea.  Thanks to the rest on #haskell for doing their best to
 talk me out of it. ;)

 I make no claims regarding the usefulness of the extension, but some
 folks might find it interesting, or may just appreciate additional
 examples of using HSE and SYB.  I regret that I am not a better
 Haskell coder, but it is what it is!

 Kind Reg'ds,
 Andrew Seniuk (rasfar)

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Re: [Haskell-cafe] Does somebody know about these functions?

[Johan Holmquist]

So, these two functions do not appear to be defined, perhaps because
many of their potential uses could be expressed using the functions
from Data.Applicative and Data.Arrow instead.

You may have noticed that the words and lines examples where defunct,
but not difficult to fix:

words = go . dropWhile isSpace where
go [] = []
go s  = withPair (:) id words (break isSpace s)


lines [] = []
lines s  = withPair (:) id (lines . safeTail) (break (== '\n') s)

safeTail [] = []
safeTail l  = tail l


Ofcourse RHS of 'lines s' can be written (and likewise for words):

uncurry (:) $ second (lines . safeTail) $ break (== '\n') s

which looks rather nice to me.

/Johan

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Re: [Haskell-cafe] Fwd: Now Accepting Applications for Mentoring Organizations for GSoC 2012

[Ganesh Sittampalam]

FYI, Edward Kmett has volunteered to do it again.

On 28/02/2012 16:23, Johan Tibell wrote:
 Hi all,
 
 Anyone interested in acting as an admin for haskell.org
 http://haskell.org this year? I'm afraid I won't have time. It's not
 that much work (filling in some information, sending out some emails,
 making sure things happen in time.)
 
 -- Forwarded message --
 From: *Carol Smith* car...@google.com mailto:car...@google.com
 Date: Mon, Feb 27, 2012 at 11:47 AM
 Subject: Now Accepting Applications for Mentoring Organizations for GSoC
 2012
 To: Google Summer of Code Announce
 google-summer-of-code-annou...@googlegroups.com
 mailto:google-summer-of-code-annou...@googlegroups.com
 
 
 Hi all,
 
 We're pleased to announce the applications for mentoring organizations
 for GoogleSummer of Code 2012 are now being accepted [1]. Please go
 Melange [2] to apply on behalf of your organization. Please note that
 the application period [3] closes on 9 March at 23:00 UTC. We will not
 accept any late applications for any reason.
 
 [1]
 - 
 http://google-opensource.blogspot.com/2012/02/mentoring-organization-applications-now.html
 [2] - http://www.google-melange.com
 [3] - http://www.google-melange.com/gsoc/events/google/gsoc2012
 
 Cheers,
 Carol
 
 --
 You received this message because you are subscribed to the Google
 Groups Google Summer of Code Announce group.
 To post to this group, send email to
 google-summer-of-code-annou...@googlegroups.com
 mailto:google-summer-of-code-annou...@googlegroups.com.
 To unsubscribe from this group, send email to
 google-summer-of-code-announce+unsubscr...@googlegroups.com
 mailto:google-summer-of-code-announce%2bunsubscr...@googlegroups.com.
 For more options, visit this group at
 http://groups.google.com/group/google-summer-of-code-announce?hl=en.
 
 
 
 
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Re: [Haskell-cafe] need help with monad transformers

[Alexander V Vershilov]

I've found a solution, I should not use lift for runWriterT, and should
explicilty lift all computation of level I need, i.e. (lift.lift) for ask
and lift for register.

Thu, Mar 01, 2012 at 02:19:29PM +0400, Dmitry Olshansky wrote
  If I'm running register outside runWriterT everything will work.
 
 Maybe just
 
  lift $ register $ print freed2
 
 or I didn't catch something?
 
 
 
 2012/3/1 Alexander V Vershilov alexander.vershi...@gmail.com
 
 Hello.
 
 I'm trying to add monad stack into network-conduit, and everything
 works except some details [1].
 
 I've run runReaderT $ runTCPServer (wrapper around runResourceT) and
 inside conduit I want to run writer to gather results of inner 
 computation.
 In inner computation I want to use IO, data from outter stack (ReaderT)
 so I'm running {-1-}:
 
  (k,t) - lift $ runWriterT $ ask = \x - tell [x]     {- 1 -}
 
 and {-2-}
 
  (k,t) - lift $ runWriterT $ do {- 2 -}
                    x - ask
                    liftIO $ print $x+1
                    tell [x]
 
 and that will work (except I've thought I should not lift runWriterT, but
 calling functions inside.
 
 And finally in computation that will run once I want to register cleaning
 function (for example register $ putStrLn cleaned) ({-3-})
 
  (k,t) - lift $ runWriterT $ do {- 3 -}
                    x - ask
                    liftIO $ print $x+1
                    register $ print freed2
                    tell [x]
 
 but I've got type error. If I'm running register outside runWriterT
 everything
 will work.
 
 I would apperated if there will be any suggestions how to make this code
 better or use register in internal computation (runWriterT)
 
 [1] https://gist.github.com/1941151
 --
 Best regards,
  Alexander V Vershilov
 
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Re: [Haskell-cafe] Records in Haskell

[Evan Laforge]

 Thanks Evan, I've had a quick read through.

Thanks for reading and commenting!

 It's a bit difficult to compare to the other proposals.

 I can't see discussion of extracting higher-ranked functions and applying them
 in polymorphic contexts. (This is SPJ's `rev` example.)

 Putting h-r fields into records is the standard way of emulating object-
 oriented style. SPJ's view is that requirement is very important in 
 practice.

 (No proposal has a good answer to updating h-r's, which you do discuss.)

Yeah, I've never wanted that kind of thing.  I've written in
object-oriented languages so it's not just that I'm not used to the
feature so I don't feel its lack.  And if I did want it, I would
probably not mind falling back to the traditional record syntax,
though I can see how people might find that unsatisfying.  But my
suggestion is meant to solve only the problem of composed record
updates and redundant things in 'Thing.thing_field thing'.  Not
supporting higher-ranked function record fields *only* means that you
can't use this particular convenience to compose updates to a
higher-ranked field.  If you happen to have that particular
intersection of requirements then you'll have to fall back to typing
more things for that particular update.

My motivation is to solve an awkward thing about writing in haskell as
it is, not add a new programming style.

 Re the cons 1. Still can't have two records with the same field name in the
 same module since it relies on modules for namespacing.

 Did you see the DORF precursor page ?
 http://hackage.haskell.org/trac/ghc/wiki/Records/DeclaredOverloadedRecordFields
 /NoMonoRecordFields

 I tried to figure out if that would help, but I suspect not. (Looking at the
 desugar for `deriving (Lens)`, you need the H98 field selector functions.)
 Then for me, cons 1. is a show-stopper. (I know you think the opposite.)

Yeah, I don't think the DORF precursor stuff is related, because it's
all based on typeclasses.  I think there are two places where people
get annoyed about name clashes.  One is where they really want to have
two records with the same field name defined in one module.  The other
is where they are using unqualified imports to shorten names and get a
clash from records in different modules.  Only the former is a
problem, the latter should work just fine with my proposal because ghc
lets you import clashing names as long as you don't call them
unqualified, and SDNR qualifies them for you.

So about the former... I've never had this problem, though the point
about circular imports forcing lots of things into the same module is
well taken, I have experienced that.  In that case: nested modules.
It's an orthogonal feature that can be implemented and enabled
separately, and can be useful in other ways too, and can be
implemented separately.  If we are to retain modules as *the* way to
organize namespaces and visibility then we should think about
fancying-up modules when a namespacing problem comes up.

Otherwise you're talking about putting more than one function into one
symbol, and that's typeclasses, and now you have to think of something
clever to counteract typeclasses' desire to be global (e.g. type
proxies).  Maybe that's forcing typeclasses too far beyond their
power/weight compromise design?

 I also don't see whether you can 'hide' or make abstract the representation of
 a record type, but still allow read-access to (some of) its fields.

If you want a read-only field, then don't export the lens for 'a',
export a normal function for it.  However, it would mean you'd use it
as a normal function, and couldn't pass it to 'get' because it's not a
lens, and couldn't be composed together with lenses.  I'd think it
would be possible to put 'get' and 'set' into different typeclasses
and give ReadLenses only the ReadLens dictionary.  But effectively
we'd need subtyping, so a Lens could be casted automatically to a
ReadLens.  I'm sure it's possible to encode with clever rank2 and
existentials and whatnot, but at that point I'm inclined to say it's
too complicated and not worth it.  Use plain functions.  Since 'get'
turns a lens into a plain function, you can still compose with
'#roField . get (#rwField1 . #rwField2)'.

We could easily support 'get (#roField1 . #roField2)' by doing the
ReadLens thing and putting (-) into ReadLens, it's just combining rw
fields and ro fields into the same composition that would require type
gymnastics.

 Suppose a
 malicious client declares a record with field #a. Can you stop them reading
 and/or updating your field #a whilst still letting them see field #b of your
 record type?

I don't think it's worth designing to support malicious clients, but
if you don't want to allow access to a function or lens or any value,
then don't export it.  #a can't resolve to M.a if M doesn't export
'a'.

 With SDNR, is it possibly to define a polymorphic field selector function? I
 suspect no looking at the desugar for `deriving (Lens)`, but perhaps 

Re: [Haskell-cafe] Fwd: Now Accepting Applications for Mentoring Organizations for GSoC 2012

[Johan Tibell]

On Thu, Mar 1, 2012 at 12:54 PM, Ganesh Sittampalam gan...@earth.li wrote:

 FYI, Edward Kmett has volunteered to do it again.


That's great since he's the most experienced GSoC admin we have. :)

There's still room for a replacement for me. I had a few people show
interest so far.
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Re: [Haskell-cafe] Fwd: Now Accepting Applications for Mentoring Organizations for GSoC 2012

[Ganesh Sittampalam]

On 01/03/2012 21:37, Johan Tibell wrote:
 On Thu, Mar 1, 2012 at 12:54 PM, Ganesh Sittampalam gan...@earth.li
 mailto:gan...@earth.li wrote:
 
 FYI, Edward Kmett has volunteered to do it again.
 
 
 That's great since he's the most experienced GSoC admin we have. :)
 
 There's still room for a replacement for me. I had a few people show
 interest so far.

Maybe I'm confused about the roles, then. Were you co-admins previously,
or something else?

Ganesh


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[Haskell-cafe] Conduits and large ConduitResult chunks

[Nathan Howell]

I'm porting lzma-enumerator over to conduits and I've run into a snag.
The output chunks from lzma can be quite large, so I'd like to stream
the results out in smaller chunks instead of tens (or hundreds) of
megabytes at a time. It seems as though this should be possible, as it
is with enumeratees, but I can't see how. Is there a way to get this
working without unsafeInterleaveIO?

https://github.com/alphaHeavy/lzma-conduit/blob/2b1451f5596b2db5a23df9a0fb128c7444e112b2/src/Data/Conduit/Lzma.hs#L165

thanks,
-n

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Re: [Haskell-cafe] Fwd: Now Accepting Applications for Mentoring Organizations for GSoC 2012

[Johan Tibell]

On Thu, Mar 1, 2012 at 1:42 PM, Ganesh Sittampalam gan...@earth.li wrote:

 On 01/03/2012 21:37, Johan Tibell wrote:
  On Thu, Mar 1, 2012 at 12:54 PM, Ganesh Sittampalam gan...@earth.li
  mailto:gan...@earth.li wrote:
 
  FYI, Edward Kmett has volunteered to do it again.
 
 
  That's great since he's the most experienced GSoC admin we have. :)
 
  There's still room for a replacement for me. I had a few people show
  interest so far.

 Maybe I'm confused about the roles, then. Were you co-admins previously,
 or something else?


Edward and I were co-admins last year. That worked out great in my opinion.

I want to make sure that we have at least one admin this year, otherwise we
won't get any GSoC slots this year, hence my original email. I didn't know
if Edward was interested or not. Now we have a few candidates, including
Edward. My preference is to have 2+ admins, preferably with Edward as one
of them as he has experience in the matter.

Sorry about the confusions.

Cheers,
  Johan
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Re: [Haskell-cafe] FreeSect -- generalised sections syntax extension

[John Lask]

On Wed, Feb 29, 2012 at 9:27 PM, Ras Farras...@gmail.com  wrote:

Hello,

I bit premature perhaps but I wanted to post it on a leap day...

http://fremissant.net/freesect

Thanks for eyebloom on #haskell for motivating me to finally implement
an old idea.  Thanks to the rest on #haskell for doing their best to
talk me out of it. ;)

I make no claims regarding the usefulness of the extension, but some
folks might find it interesting, or may just appreciate additional
examples of using HSE and SYB.  I regret that I am not a better
Haskell coder, but it is what it is!

Kind Reg'ds,
Andrew Seniuk (rasfar)





why couldn't you use standard brackets ( to delimit the extent ? I 
suppose that would have added complexity to the syntax analysis, however 
I think it would have been (in my mind) neater.



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Re: [Haskell-cafe] need help with monad transformers

[Dmitry Olshansky]

Did you try to use transformers instead of mtl? I am just in doubt about it
in my work.

I've found that Conduit use it. So you remove extra dependency.
I hope that transformers have more readable messages (without FD) and you
can control what is lift more clear.

I have no experience although.



2012/3/2 Alexander V Vershilov alexander.vershi...@gmail.com

 I've found a solution, I should not use lift for runWriterT, and should
 explicilty lift all computation of level I need, i.e. (lift.lift) for ask
 and lift for register.

 Thu, Mar 01, 2012 at 02:19:29PM +0400, Dmitry Olshansky wrote
   If I'm running register outside runWriterT everything will work.
 
  Maybe just
 
   lift $ register $ print freed2
 
  or I didn't catch something?
 
 
 
  2012/3/1 Alexander V Vershilov alexander.vershi...@gmail.com
 
  Hello.
 
  I'm trying to add monad stack into network-conduit, and everything
  works except some details [1].
 
  I've run runReaderT $ runTCPServer (wrapper around runResourceT) and
  inside conduit I want to run writer to gather results of inner
 computation.
  In inner computation I want to use IO, data from outter stack
 (ReaderT)
  so I'm running {-1-}:
 
   (k,t) - lift $ runWriterT $ ask = \x - tell [x] {- 1 -}
 
  and {-2-}
 
   (k,t) - lift $ runWriterT $ do {- 2 -}
 x - ask
 liftIO $ print $x+1
 tell [x]
 
  and that will work (except I've thought I should not lift
 runWriterT, but
  calling functions inside.
 
  And finally in computation that will run once I want to register
 cleaning
  function (for example register $ putStrLn cleaned) ({-3-})
 
   (k,t) - lift $ runWriterT $ do {- 3 -}
 x - ask
 liftIO $ print $x+1
 register $ print freed2
 tell [x]
 
  but I've got type error. If I'm running register outside runWriterT
  everything
  will work.
 
  I would apperated if there will be any suggestions how to make this
 code
  better or use register in internal computation (runWriterT)
 
  [1] https://gist.github.com/1941151
  --
  Best regards,
   Alexander V Vershilov
 
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  Haskell-Cafe@haskell.org
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Re: [Haskell-cafe] Conduits and large ConduitResult chunks

[Michael Snoyman]

On Thu, Mar 1, 2012 at 11:50 PM, Nathan Howell
nathan.d.how...@gmail.com wrote:
 I'm porting lzma-enumerator over to conduits and I've run into a snag.
 The output chunks from lzma can be quite large, so I'd like to stream
 the results out in smaller chunks instead of tens (or hundreds) of
 megabytes at a time. It seems as though this should be possible, as it
 is with enumeratees, but I can't see how. Is there a way to get this
 working without unsafeInterleaveIO?

 https://github.com/alphaHeavy/lzma-conduit/blob/2b1451f5596b2db5a23df9a0fb128c7444e112b2/src/Data/Conduit/Lzma.hs#L165

 thanks,
 -n

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I just wrote a blog post on this topic:
http://www.yesodweb.com/blog/2012/03/more-powerful-conduit

Feedback requested!

Michael

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