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Today's Topics:
1. Re: Syntactic sugar to insert into a Map? (Markus L?ll)
2. Why does this list comprehension return an empty list?
(Costello, Roger L.)
3. Re: Why does this list comprehension return an empty list?
(Frerich Raabe)
4. Re: Why does this list comprehension return an empty list?
(Emanuel Koczwara)
5. Re: Why does this list comprehension return an empty list?
(Magnus Therning)
6. Shorten this code (Nadav Chernin)
----------------------------------------------------------------------
Message: 1
Date: Mon, 22 Jul 2013 23:49:51 -0400
From: Markus L?ll <markus.l...@gmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] Syntactic sugar to insert into a Map?
Message-ID:
<CALdaiuAm8yVAve-+pK9Ung6Wf9+0K3wNsvTKgb=1gc5hs0b...@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
Hi Martin,
I think given that integers are converted to fromInteger x and that
strings can be overloaded you could implement something like this with
a state or a writer monad:
mySet = do
1 2
3 4
5 6
To do this you'll need to implement the appropriate classes. But I
guess it's more a hack towards getting what you want than to being
true to what a monad is.
On Mon, Jul 22, 2013 at 3:06 AM, Ozgur Akgun <ozgurak...@gmail.com> wrote:
> Hi.
>
> On 21 July 2013 17:23, martin <martin.drautzb...@web.de> wrote:
>>
>> I just want to insert several key/value pairs, something like
>>
>> Map.empty
>> insert key1 val1
>> insert key2 val2
>> return theMap
>
>
> If you really want this syntax, what about using the writer monad:
>
> import Control.Monad.Writer
> import qualified Data.Map as M
>
> m = execWriter $ do
> insert 1 "foo"
> insert 2 "bar"
> where insert k v = tell (M.singleton k v)
> -- m = fromList [(1,"foo"),(2,"bar")]
>
>
> Hope this helps,
> Ozgur
>
>
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://www.haskell.org/mailman/listinfo/beginners
>
--
Markus L?ll
------------------------------
Message: 2
Date: Tue, 23 Jul 2013 08:53:28 +0000
From: "Costello, Roger L." <coste...@mitre.org>
To: "beginners@haskell.org" <beginners@haskell.org>
Subject: [Haskell-beginners] Why does this list comprehension return
an empty list?
Message-ID:
<b5fee00b53cf054aa8439027e8fe17751efb2...@imcmbx04.mitre.org>
Content-Type: text/plain; charset="us-ascii"
Hi Folks,
I have a list of singletons:
xs = [("a")]
f is a function that, given an argument x, it returns the argument:
f x = x
g is a function that, given an argument x, it returns the empty list:
g x = []
I have a list comprehension that extracts the singletons from xs using f and g,
and creates a pair from their output:
[(a,b) | a <- f xs, b <- g xs]
I executed this and the result is the empty list:
[]
That is odd. Why is the empty list the result?
Consider this list comprehension that extracts the singletons from xs using f
only, and creates a pair by matching up the singleton from xs with the empty
list:
[(a,[]) | a <- f xs]
The result is this:
[("a",[])]
That is the result that I expected to get with the first list comprehension.
Would you explain why this:
[(a,b) | a <- f xs, b <- g xs]
yields [], whereas this:
[(a,[]) | a <- f xs]
yields [("a",[])]?
Also, how would I modify this:
[(a,b) | a <- f xs, b <- g xs]
so that it produces this:
[("a",[])]
/Roger
------------------------------
Message: 3
Date: Tue, 23 Jul 2013 11:08:10 +0200
From: Frerich Raabe <ra...@froglogic.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] Why does this list comprehension
return an empty list?
Message-ID: <355f8941a5723081406da1acb5d4b...@roundcube.froglogic.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
On 2013-07-23 10:53, Costello, Roger L. wrote:
> I have a list of singletons:
>
> xs = [("a")]
>
> f is a function that, given an argument x, it returns the argument:
>
> f x = x
>
> g is a function that, given an argument x, it returns the empty list:
>
> g x = []
>
> I have a list comprehension that extracts the singletons from xs
> using f and g, and creates a pair from their output:
>
> [(a,b) | a <- f xs, b <- g xs]
>
> I executed this and the result is the empty list:
>
> []
>
> That is odd. Why is the empty list the result?
Consider how the list comprehension looks like when written as monadic
code:
f xs >>= \a ->
g xs >>= \b ->
return (a,b)
I.e. "for each element 'a' drawn from the result of "f sx", draw an
element 'b' from the result of "g xs" and yield a new element "(a,b)".
So the resulting list will always be 'length (f xs) * length (g xs)'
items in length. Consequently, if either of the two lists is empty, the
result will be empty.
--
Frerich Raabe - ra...@froglogic.com
www.froglogic.com - Multi-Platform GUI Testing
------------------------------
Message: 4
Date: Tue, 23 Jul 2013 11:12:23 +0200
From: Emanuel Koczwara <poc...@emanuelkoczwara.pl>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] Why does this list comprehension
return an empty list?
Message-ID: <1374570743.10100.5.camel@emanuel-laptop>
Content-Type: text/plain; charset="utf-8"
Hi,
Dnia 2013-07-23, wto o godzinie 08:53 +0000, Costello, Roger L. pisze:
> Hi Folks,
>
> I have a list of singletons:
>
> xs = [("a")]
>
> f is a function that, given an argument x, it returns the argument:
>
> f x = x
>
> g is a function that, given an argument x, it returns the empty list:
>
> g x = []
>
> I have a list comprehension that extracts the singletons from xs using f and
> g, and creates a pair from their output:
>
> [(a,b) | a <- f xs, b <- g xs]
>
> I executed this and the result is the empty list:
>
> []
>
> That is odd. Why is the empty list the result?
>
Becouse g xs has zero elements. Look at this example:
[(x,x) | x <- []] => []
Empty list has zero elements.
> Consider this list comprehension that extracts the singletons from xs using f
> only, and creates a pair by matching up the singleton from xs with the empty
> list:
>
> [(a,[]) | a <- f xs]
>
> The result is this:
>
> [("a",[])]
>
> That is the result that I expected to get with the first list comprehension.
>
> Would you explain why this:
>
> [(a,b) | a <- f xs, b <- g xs]
>
> yields [], whereas this:
>
> [(a,[]) | a <- f xs]
>
> yields [("a",[])]?
>
> Also, how would I modify this:
>
> [(a,b) | a <- f xs, b <- g xs]
>
> so that it produces this:
>
> [("a",[])]
Define g as:
g xs = [[]]
or maybe better:
g _ = [[]]
Now it has 1 element - empty list.
Best regards,
Emanuel
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Message: 5
Date: Tue, 23 Jul 2013 11:17:59 +0200
From: Magnus Therning <mag...@therning.org>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] Why does this list comprehension
return an empty list?
Message-ID:
<CAAExw5sUktB7BcNA_=an9ak8ci4ioajm34h12fbc1sl-_tp...@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8
On Tue, Jul 23, 2013 at 10:53 AM, Costello, Roger L. <coste...@mitre.org> wrote:
> Hi Folks,
>
> I have a list of singletons:
>
> xs = [("a")]
The type of that, as written, is [String].
> f is a function that, given an argument x, it returns the argument:
>
> f x = x
>
> g is a function that, given an argument x, it returns the empty list:
>
> g x = []
>
> I have a list comprehension that extracts the singletons from xs using f and
> g, and creates a pair from their output:
>
> [(a,b) | a <- f xs, b <- g xs]
>
> I executed this and the result is the empty list:
>
> []
>
> That is odd. Why is the empty list the result?
If you say it out loud what you are looking for it makes sense:
construct a list tuples containing all combinations of 'a' and 'b'
where 'a' is taken from the list 'f xs' and 'b' from the list 'g xs'.
Now 'g xs' is empty, which means there is no possible value for 'b' to
take, hence there is no way to create even a single such tuple.
/M
--
Magnus Therning OpenPGP: 0xAB4DFBA4
email: mag...@therning.org jabber: mag...@therning.org
twitter: magthe http://therning.org/magnus
------------------------------
Message: 6
Date: Tue, 23 Jul 2013 13:41:32 +0300
From: Nadav Chernin <nadavcher...@gmail.com>
To: beginners@haskell.org
Subject: [Haskell-beginners] Shorten this code
Message-ID:
<cae80g0kqqw7rrw+fmq6jdetsbqg_6jfz6y_ayyowv4kyvfj...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
Hi, all
Below is my solution to SPOJ->Polybius
square<http://www.spoj.com/problems/POLYBIUS>
Please try to shorten it:
*import Data.Maybe*
*d=[1..5]*
*f s=unwords$map(\c->fromJust$lookup c(('
',""):('J',"24"):zip(['A'..'I']++['K'..'Z'])[show(x+10*y)|y<-d,x<-d]))s*
*main=getLine>>(interact$unlines.map f.lines)*
Thanks, Nadav
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