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Today's Topics:
1. Re: Fwd: My first functioning haskell project - a
steganography utility (Brandon S Allbery KF8NH)
2. why is something different within a function when it comes
out? (prad)
3. regular expressions (prad)
4. Re: why is something different within a function when it
comes out? (Michael Mossey)
5. Re: why is something different within a function when it
comes out? (Chadda? Fouch?)
6. newtype record syntax (Tom Doris)
7. Re: newtype record syntax (Brent Yorgey)
8. Re: newtype record syntax (Tom Doris)
----------------------------------------------------------------------
Message: 1
Date: Tue, 13 Jul 2010 14:11:28 -0400
From: Brandon S Allbery KF8NH <[email protected]>
Subject: Re: [Haskell-beginners] Fwd: My first functioning haskell
project - a steganography utility
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=UTF-8
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On 7/13/10 11:20 , Tim Cowlishaw wrote:
> actually, looking at it again, it'd be something like
>
> maybeMonoid :: (Monoid a) => Maybe a -> Maybe a -> a
> maybeMonoid :: x y = (fromMaybe mempty x) `mappend` (fromMaybe mempty y)
Which in turn can be written as:
> maybeMonoid = mappend `on` (fromMaybe mempty)
("on" is in Data.Function: "g `on` f" expresses the "\x y -> (f x) `g` (f
y)" idiom.)
- --
brandon s. allbery [linux,solaris,freebsd,perl] [email protected]
system administrator [openafs,heimdal,too many hats] [email protected]
electrical and computer engineering, carnegie mellon university KF8NH
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Message: 2
Date: Tue, 13 Jul 2010 22:33:10 -0700
From: prad <[email protected]>
Subject: [Haskell-beginners] why is something different within a
function when it comes out?
To: haskellbeginners <[email protected]>
Message-ID: <20100713223310.1cf90...@gom>
Content-Type: text/plain; charset=US-ASCII
i have this:
main = do
c <- readFile "test.txt"
let tleP = "<title>\n(.*)\n</title>"
let tle = c=~tleP::[[String]]
putStrLn $ tle!!0!!1
let g = xtract tleP c
putStrLn $ show g
xtract p c = do
let r = c=~p::[[String]]
return r!!0!!0!!1
for the first putStrLn i get:
League of Humane Voters Home Page
for the second i get
"League of Humane Voters Home Page"
but only after i have done r !!0 !!0 !!1
whereas i only needed tle !!0 !!1
it seems to me that both tle and r serve the same purpose though they
come out as different types r being a string and tle being i don't know
what because i get a "Not in scope: 'tle'
so even though both r and tle are set as [[String]], they don't seem to
be the same creature.
--
In friendship,
prad
... with you on your journey
Towards Freedom
http://www.towardsfreedom.com (website)
Information, Inspiration, Imagination - truly a site for soaring I's
------------------------------
Message: 3
Date: Tue, 13 Jul 2010 23:37:45 -0700
From: prad <[email protected]>
Subject: [Haskell-beginners] regular expressions
To: haskellbeginners <[email protected]>
Message-ID: <20100713233745.5463c...@gom>
Content-Type: text/plain; charset=US-ASCII
there seem to be a lot of regular expression modules:
Text.Regex.TDFA
Text.Regex.PCRE
Text.Regex.PCRE.Light
there is a bit info on how to use it in real world haskell but only for
the POSIX stuff (eg TDFA)
any suggestions or preferences?
--
In friendship,
prad
... with you on your journey
Towards Freedom
http://www.towardsfreedom.com (website)
Information, Inspiration, Imagination - truly a site for soaring I's
------------------------------
Message: 4
Date: Wed, 14 Jul 2010 01:10:04 -0700
From: Michael Mossey <[email protected]>
Subject: Re: [Haskell-beginners] why is something different within a
function when it comes out?
To: prad <[email protected]>
Cc: haskellbeginners <[email protected]>
Message-ID: <[email protected]>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
I'm fairly new, so I can't fully explain the behavior you got, but I
know at least one thing you did wrong:
prad wrote:
> i have this:
>
>
> main = do
>
> c <- readFile "test.txt"
> let tleP = "<title>\n(.*)\n</title>"
> let tle = c=~tleP::[[String]]
> putStrLn $ tle!!0!!1
>
> let g = xtract tleP c
> putStrLn $ show g
>
>
> xtract p c = do
> let r = c=~p::[[String]]
> return r!!0!!0!!1
>
You probably mean to write
> xtract p c = r !! 0 !! 1
> where r = c=~p::[[String]]
You used do-notation when you meant to write a simple function.
The use of "return" in do-notation is one point of confusion with
beginners. It does not operate like "return" in an imperative language.
I'm not sure what your 'xtract' did, but the compiler probably accepted
the do-notation because it specified a list monad of some sort.
I recommend you pay close attention to introductory examples, noticing
in particular when they are unlike imperative languages. Find some
tutorials on monads and the do-notation.
Best,
Mike
------------------------------
Message: 5
Date: Wed, 14 Jul 2010 10:54:22 +0200
From: Chadda? Fouch? <[email protected]>
Subject: Re: [Haskell-beginners] why is something different within a
function when it comes out?
To: prad <[email protected]>
Cc: haskellbeginners <[email protected]>
Message-ID:
<[email protected]>
Content-Type: text/plain; charset=UTF-8
On Wed, Jul 14, 2010 at 7:33 AM, prad <[email protected]> wrote:
> xtract p c = do
> Â Â let r = c=~p::[[String]]
> Â Â return r!!0!!0!!1
return is just a function in Haskell, and given that function
application has priority over any operator, this mean your last line
is :
> (return r) !! 0 !! 0 !! 1
return type is "(Monad m) => a -> m a" where m is a type constructor
variable (a type constructor is a parametrized type, think array
parametrized on the element type in most language, or template in C++)
constrained to be an instance of the Monad typeclass. Monad is an
important typeclass in Haskell but here it's enough to look at (!!)
type "[a] -> Int -> a" to see that (return r) should be of a list
type, list is a monad so this code typecheck, but return is perfectly
redundant :
For the list monad :
> return x = [x]
so in this case your last line put r in a singleton list :
> [r] !! 0 !! 0 !! 1
then (!! 0) extract r
> r !! 0 !! 1
And you get what you wanted in the first place...
So you could write just "r !! 0 !! 1" instead of "return r!!0!!!0!!1"
for the same result.
In fact xtract could be written :
> xtract p c = (c =~ p :: [[String]]) !! 0 !! 1
do-notation (which is just syntactic sugar to write monadic code
easily) and return (which is just a function, not a keyword) are only
useful when you're working with monads, if you're not you shouldn't
use them.
--
Jedaï
------------------------------
Message: 6
Date: Wed, 14 Jul 2010 10:39:23 +0100
From: Tom Doris <[email protected]>
Subject: [Haskell-beginners] newtype record syntax
To: [email protected]
Message-ID:
<[email protected]>
Content-Type: text/plain; charset="iso-8859-1"
Can someone give an explanation of the use of record syntax in newtype
declarations that helps one understand how it's meant to be interpreted? I
understand the "facts" according to the language definition, but I just
don't see why this notation was used; does it have any relationship with
records?
The only docs I can find that cover this are in the language report itself:
>|A newtype declaration may use field-naming syntax, though of course there
may only be one field. Thus:
>| newtype Age = Age { unAge :: Int }
>|brings into scope both a constructor and a de-constructor:
>| Age :: Int -> Age
>| unAge :: Age -> Int
I understand what this says, I just don't understand why Haskell does it
this way - unAge ends up defining a function, using the notation used to
declare a field in a record, and the type of the function is inferred from
the type of the field and the constructor - it just seems a bit
barmy. I'd appreciate anyone who can help, or a reference to some docs that
make sense of this.
Thanks
t
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Message: 7
Date: Wed, 14 Jul 2010 11:00:12 +0100
From: Brent Yorgey <[email protected]>
Subject: Re: [Haskell-beginners] newtype record syntax
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
On Wed, Jul 14, 2010 at 10:39:23AM +0100, Tom Doris wrote:
> Can someone give an explanation of the use of record syntax in newtype
> declarations that helps one understand how it's meant to be interpreted? I
> understand the "facts" according to the language definition, but I just
> don't see why this notation was used; does it have any relationship with
> records?
Yes, the reason this notation is used is that it corresponds exactly
to defining a record containing one field. Declaring field names in a
record really just gives you some projection functions to get the
fields out (and also some pattern-matching magic); declaring a field
name in a newtype gives you a projection function to unwrap the
newtype.
Does that help?
-Brent
------------------------------
Message: 8
Date: Wed, 14 Jul 2010 12:41:52 +0100
From: Tom Doris <[email protected]>
Subject: Re: [Haskell-beginners] newtype record syntax
To: Brent Yorgey <[email protected]>
Cc: [email protected]
Message-ID:
<[email protected]>
Content-Type: text/plain; charset="iso-8859-1"
Yes, makes sense, I always thought of the projection functions as being
secondary. Thanks
On 14 July 2010 11:00, Brent Yorgey <[email protected]> wrote:
> On Wed, Jul 14, 2010 at 10:39:23AM +0100, Tom Doris wrote:
> > Can someone give an explanation of the use of record syntax in newtype
> > declarations that helps one understand how it's meant to be interpreted?
> I
> > understand the "facts" according to the language definition, but I just
> > don't see why this notation was used; does it have any relationship with
> > records?
>
> Yes, the reason this notation is used is that it corresponds exactly
> to defining a record containing one field. Declaring field names in a
> record really just gives you some projection functions to get the
> fields out (and also some pattern-matching magic); declaring a field
> name in a newtype gives you a projection function to unwrap the
> newtype.
>
> Does that help?
>
> -Brent
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
>
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