Real World Haskell Book Club Google Group

2011-06-13T07:32:31Z

MonadTransformer exercise question

2009-06-17T17:10:28Z

Hi guys

I have problems wrapping my head around the 3rd exercise (on page 436)
in chapter 18. The task is to "rewrite the constrainedCount function
to record results using the WriterT transformer in your new App
stack." I do have a working solution, as well as an alternative
solution which doesn't work. But I don't understand why. Perhaps

What's with tails?

2009-06-02T03:31:41Z

First off, to the authors of RWH, thanks for the wonderful book! This
is the first book I've read where I felt like I was back in college
and had an actual professor teaching the subject matter to me! I
think because of the user contributed comments, this book reads much
better than just about any other technical book I have ever read.

RWH Book Club 5/14/2009

2009-05-13T14:56:41Z

This Thursday, 5/14/2009 will continue the Real World Haskell book
club. We will continue on the Collective Intelligence book
walkthrough as like before.

Details:
5/14/2009 from 8PM-10PM EDT (-400 GMT)

In person location:
12012 Sunset Hills Rd
Reston, VA 20190
Google Maps -
[link]

RWH Book Club 5/7/2009

2009-05-06T20:32:51Z

This Thursday, 5/7/2009 will continue the Real World Haskell book
club. We will continue on the Collective Intelligence book walkthrough as
like before.
Details:
5/7/2009 from 8PM-10PM EDT (-400 GMT)
In person location:
12012 Sunset Hills Rd
Reston, VA 20190
Google Maps -
[link]

RWH Book Club 4/30

2009-04-30T04:54:13Z

This Thursday, 4/30/2009 will continue the Real World Haskell book
club. We have started a project to port some of the Programming
Collective Intelligence book to Haskell and have been making some good
process

Details:
4/30 from 8PM-10PM EDT (-400 GMT)

In person location:
12012 Sunset Hills Rd

RWH Book Club 4/23/2009

2009-04-21T18:03:39Z

This Thursday, 4/23/2009 will continue the Real World Haskell book
club. We
have moved it from Mondays to Thursdays to accommodate more people.
We'll have started a project to port some of the Programming
Collective
Intelligence book to Haskell since the other Google Group doesn't seem
to have much activity.

RWH Meeting 4/16/2009

2009-04-15T17:02:59Z

Today, April 9th will continue the Real World Haskell book club. We
have moved it from Mondays to Thursdays to accommodate more people.
We'll be starting a project to port some of the Programming Collective
Intelligence book to Haskell since the other Google Group doesn't seem
to have much activity.

RWH Book Club 4/9

2009-04-09T18:03:22Z

Today, April 9th will continue the Real World Haskell book club.
We have moved it from Mondays to Thursdays to accommodate more
people.

Details:
4/9 from 8PM-10PM EDT (-400 GMT)

In person location:
12012 Sunset Hills Rd
Reston, VA 20190
Google Maps -
[link]

RWH Book Club 4/2

2009-04-01T13:11:30Z

Tomorrow, April 2nd will continue the Real World Haskell book club.
We have moved it from Mondays to Thursdays to accommodate more
people.

Details:
4/2 from 8PM-10PM EDT (-400 GMT)

In person location:
12012 Sunset Hills Rd
Reston, VA 20190
Google Maps -
[link]

Beginning with RWH?

2009-03-24T23:31:25Z

Hi All,

Did anyone here pick up RWH without any prior knowledge, and then blow
through the entire book while understanding everything? This was going
to be my approach, until I think I didn't exactly `grok' some stuff
around Chapter 9-10. Now I feel that I am learning very little in
subsequent chapters because I didn't lay the foundation. (Note that

Can't follow foldTree example in Chapter 9

2009-03-24T01:54:59Z

I have been struggling quite a bit with Chapter 9.
Unfortunately I don't really know how to debug or trace so I can get
stopped dead pretty easily.

I am just trying to understand the code given in the book.

On page 230 foldTree is defined, on page 231 the countDirectories
iterator function is defined.

RWH Book Club 3/23/2009

2009-03-23T23:42:04Z

Tonight will once again be another night of the Real World Haskell
book club. Tonight we'll be covering monads, so hope to get a decent
turnout. Lately, attendance has been falling and I hope to change
that as we get a bit more advanced.

Details:
3/23 from 8PM-10PM EDT (-400 GMT)

In person location:

Chapter 8 - Exercise with Exceptions

2009-03-20T18:37:25Z

Has anyone written a solution to #1 in the third group of exercises?

The one that asks you to re-write `globToRegex' so that it has the
following type signature:
globToRegex :: String -> Either GlobError String

I am quite stumped on this one.

In case you don't want to look it up in the book, here is the source

Chapter 25 - a little test adding some I/O

2009-03-18T17:53:24Z

Hi,
After reading chapter 25, I made a very simple script to see what I
can get from Haskell on simple I/O and calculations, compared to e.g
Python. It is very similar to the function of chapter 25: it reads a
bunch of columns of numbers in a file, and writes the mean of each
column. So the main difference is that there is I/O.

answers to all exercises?

2009-03-17T11:35:18Z

Hello
I wanted to ask - is any place, where answers for all exercises are
collected? Sometimes, it's good idea to compare your solution with other
solutions, and find more elegant ones...

Chapter 15 Exercise 1

2009-03-17T03:12:44Z

I'm scratching my head over this one. Is there a way to use
QuickCheck in the MonadHandle monad? I feel like I'm missing
something obvious, but there doesn't seem to be enough pure code to
actually create a test property.

Broken rss links?

2009-03-16T16:48:58Z

Hi everyone. So I've been following the RWH comments in Google Reader
through the nice RSS feed. The feed is still up and there's a number
of comments every day, but strangely the links are broken for me!
An example, the latest item in it links to
[link]

RWH Book Club 3/16/2009

2009-03-16T16:01:11Z

Details:
3/16 from 8PM-10PM EDT (-400 GMT)

In person location:

RWH Book Club Continues 3/9/2009

2009-03-06T20:57:05Z

Sorry I've been out of pocket lately, but we're continuing the RWH
book club this Monday 3/9 from 8PM-10PM EST (-500 GMT).

If you're in the DC area, you can join us in person at the Microsoft
Technology Center:
12012 Sunset Hills Rd, Reston, VA 20190

If you plan on attending in person, please little-r me so that I can

Chapter 7: hazards of lazy IO

2009-02-22T19:49:35Z

There are two "bear trap" warnings one right after the other on page
179 and 180 about the hazards on not understanding lazy IO. I want to
make sure that I specifically understand the hazards of referring to
variables bound by lazy IO operations (actions?) on page 180.

The warning says "...you must not close the Handle until you have

withstanding slings and arrows of chapter 6

2009-02-18T15:24:13Z

I thought I would go through the exercises at the end of Chapter 6.
Although they seem to be somewhat of an afterthought, not really
relevant to all of the material just introduced.

But I took a swing at it anyway. I'm not sure why, after a somewhat
exhausting chapter, arrows are snuck in at the last moment and are the

RWH Book Club Canceled 2/16/2009

2009-02-16T21:26:32Z

All:

Due to a US public holiday, we will not be meeting tonight in Reston.
Feel free to use the #novalang IRC channel if you wish to still meet.

Regards,
Matt

Error when using Control.Exception.handle

2009-02-14T20:14:08Z

Hello,
I'm getting the following error when trying to use handle function
(Ch8, p. 209):

<interactive>:1:0:
Ambiguous type variable `e' in the constraint:
`Exception e'
arising from a use of `handle' at <interactive>:1:0-25
Probable fix: add a type signature that fixes these type variable

Re: [RWHBC] are u in tension friend just visit my site and u will cool

2009-02-13T07:27:34Z

<snip moronic spam/>
Can the administrator of this group please remove this idiot from
the subscriber list?
Thanks,
Erik

Meeting tonight 2/9/2009 - 2000-2200 EST

2009-02-09T21:05:23Z

Tonight we're meeting once again on Real World Haskell. This time
we'll be covering Chapter 4 starting around page 97. If you're in the
Northern Virginia area, you can join us in person as part of the NoVA
Languages group. Here are the details:

Date/Time:
2/9/2009 - 2000 - 2200 EST (0100 GMT)

Question to chapter nine

2009-02-08T13:25:31Z

Hi everybody. I have some trouble wrapping my head around some of the
examples in that chapter. If sessions are still being held, I'd wait
till chapter nine is discussed.

Overlapping Instances (chapter 6)

2009-01-28T21:42:18Z

Dear all,
I am currently working through chapter 6 and struggling with the
problem of overlapping instances (page 153).
If I type in
> toJValue [("foo","bar")]
I get the error message as mentioned in the book:
Overlapping instances for JSON [([Char], [Char])]
arising from a use of `toJValue' at <interactive>:1:0-24

Meeting tonight 1/26/2009 - 2000-2200 EST

2009-01-26T18:56:10Z

Tonight we're meeting once again on Real World Haskell. This time, we
hope to finish up the last of Chapter 3 and mostly cover Chapter 4
tonight. If you're in the Northern Virginia area, you can join us in
person as part of the NoVA Languages group. Here are the details:

Date/Time:
1/26/2009 - 2000 - 2200 EST (0100 GMT)

Meeting Tomorrow?

2009-01-18T22:40:41Z

Are we still planning on meeting tomorrow in Reston as normal even with the
holiday?

Answers to RWH questions?

2009-01-18T08:37:09Z

Is there an official list of answers to the RWH exercises?

I'm blogging my own answers (I'm currently on chapter 4).
[link]
onwards...

It's easy to check that an answer is right, but it's difficult to know
if an answer is the most elegant way to do it in Haskell.

Bootstrap Detroit Area Study Group for Real World Haskell

2009-01-15T19:55:52Z

Heya - looking for other interested geeks in metro-Detroit area that
are interested in Haskell and starting a local group.
Email me if you exist :)
Regards,
Davis

Understanding an algebraic data type

2009-01-13T21:35:05Z

I'm reading chapter 5 of the book, and try to understand the algebraic
data type Doc. It's defined like this:

Reminder - Chapter 3/4? 1/12/2009 - 2000 EST (0100 GMT)

2009-01-12T18:53:00Z

All:

Just as a reminder that tonight is another edition of the Real World
Haskell Book Club. As before, we're going to have DimDim and IRC
available for us to use for this event. We're also holding the in-
person event in Reston, VA as part of the novalang group. At a bare
minimum, we will be doing Chapter 3, and if time allows, Chapter 4.

gtk2hs on Mac 10.5

2009-01-12T05:30:54Z

I'm trying to follow Chapter 23, "GUI Programming with gtk2hs", and
I'm having problems even getting started. I'm working under Mac OS X
10.5. I installed Mac Ports, did "sudo port selfupdate" (no update
needed), then "sudo port install gtk2hs". Apparently gtk2hs needs
just about everything, because for the next couple of hours it was

Suggested changes to example code; monad "or" and "and"?

2009-01-08T06:20:16Z

If there's ever a second edition of RWH, here are some suggested
changes to some of the example code:
p. 106
dlts = foldr step [] . lines where
step l ds
| "#define DLT_" `isPrefixOf` l = secondWord l : ds
| otherwise = ds
The following is a more concise and, I think, clearer definition:

Exception Section Out of Date

2009-01-08T01:38:57Z

Sections about Exception in Real World Haskell are out of date.
At least in GHC6.10, Exception becomes a typeclass, no longer just a
type. Any user defined exception (as long as it is an instance of
Typable and Show) can be an instance of Exception.
Also, functions like throw, catch, handle can deal with both

First Session Wrapup

2009-01-07T00:38:56Z

All:

I want to thank everyone who attended last night's first session of
the Real World Haskell Book Club. We had a good 37 or so participants
actively following along and around 14 or so joining in at the
novalang group in Reston, VA.

Because of the time differences, feel free to continue using the

First Session Reminder - 1/5/2009 20:00 EST

2009-01-04T19:46:10Z

All:

We'll be kicking off the first installment of the Real World Haskell
Book
Club with a joint venture with the Northern Virginia Language Group
(NoVALang) on January 5th,
2009 from 20:00 until whenever Eastern Standard Time (GMT 0100 hours).

In this session, we will be covering chapters 1 and 2 of the Real

Incomplete discussion of GeneralizedNewtypeDeriving in "How to Give a Type a New Identity"

2009-01-04T00:32:12Z

There's some nasty handwaving in chapter 6 "Using Typeclasses" in the
section on "newtype". There are references made to using "deriving" to
expose underlying typeclass instances, but no discussion at all of
GeneralizedNewtypeDeriving, or the limitations present if you don't
enable it.

Errata: p.149: "deriving (Show)" line should be missing from "CannotShow

2009-01-03T23:33:43Z

I believe there is an error on page 149 of chapter 6, in the
"Automatic Derivation" section.

There is some example code which reads in part:
data CannotShow = CannotShow
deriving (Show)

-- will not compile, since CannotShow is not an instance of Show
data CannotDeriveShow = CannotDeriveShow CannotShow

What happened to [String] return values for regex matches? (Chapter 8)

2008-12-31T04:26:50Z

When I try to return a [String] from a regex match, I get something
like:

ghci> "ii uu" =~ "(uu|ii)" :: [String]

<interactive>:1:0:
No instance for (RegexContext Regex [Char] [String])
arising from a use of `=~' at <interactive>:1:0-19
Possible fix:
add an instance declaration for

JSON package on hackage?

2008-12-30T05:34:06Z

I wanted to follow the example of the JSON package in chapter 5, but I, for
one, really struggled with it. I found it quite confusing to figure out when
to include what files where to make everything typecheck. Anyway, I did
succeed, I believe. Is anyone interested in a hackage version of the module?

Colorado Area Study Group for Real World Haskell

2008-12-28T18:30:00Z

I deliberately started a new discussion topic with Colorado in the
topic to attract local attention.

I am hoping to also have a physical meeting in Colorado
contemporaneous with the joint session.
I am impressed with the power of virtual meetings, but still like the
idea of gathering with like minded individuals in person from time to

A question about space leaks

2008-12-28T17:33:04Z

Hi,
I'm working through the book, which arrived a few days before. Thereby
I found the term "space leak" a bit confusing. Am I right if I assume
that these space leaks are not comparable to memory leaks in C? Would
the term "space/memory consumption" fit more? I guess that the memory
needed for the stake (of thunks) is freed after the evaluation.

"Writing lazy functions"

2008-12-28T14:06:42Z

I am confused by what I am suppose to take away from this section:

[link]

I see that globToRegex' is not tail recursive, and it is regardless of
evaluation strategy, I think. (The comments seem to say the same.)

Type classes

2008-12-27T06:48:36Z

From Chapter 6. Using Typeclasses > Typeclasses at work: making JSON
easier to use:

type JSONError = String

class JSON a where
toJValue :: a -> JValue
fromJValue :: JValue -> Either JSONError a

instance JSON Bool where
toJValue = JBool
fromJValue (JBool b) = Right b
fromJValue _ = Left "not a JSON boolean"

Why the short variable names and function-returning methods?

2008-12-25T23:56:08Z

Just as an overall style question I was wondering why Haskell code has
the following characteristics:

Single letter variable names:
This might be more mathematical but even the authors agree that this
is bad for readibility (top of pg 348). In contrast the Lisp people
(possibly because the compiler can't infer type info like Haskell use

Simpler version of asInt_either (chapter 4)?

2008-12-23T20:32:32Z

My answer to this exercise seems overly complicated. Is there a way
to simplify the following?

Cheers,
Ben
______________________________ ____________________
import Data.Char (digitToInt, isDigit)

type ErrorMessage = String
asInt_either :: String -> Either Int ErrorMessage
asInt_either ('-':xs) = either (\x-> Left $ negate x) (Right)

A wiki to note down ideas while reading RWH

2008-12-23T03:08:29Z

Hello friends,
I have just begun my exploration of Haskell with RWH; I have set up a
Dokuwiki to collect interesting thoughts/ideas which readers might have
when they go through RWH.
[link]
With Regards,
Pramode C.E
--------------