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Today's Topics:
1. Re: Cabal error (Ivan Hugo Guevara)
2. "Couldn't match expected type" and simple "Couldn't match
type" (Ken Kawamoto)
3. cleanest way to unwrap a list? (Christopher Howard)
4. Re: cleanest way to unwrap a list? (Shakthi Kannan)
5. Re: cleanest way to unwrap a list? (Ben Kolera)
6. Re: cleanest way to unwrap a list? (Jay Sulzberger)
7. Re: cleanest way to unwrap a list? (Jay Sulzberger)
8. Re: cleanest way to unwrap a list? (Henk-Jan van Tuyl)
9. Re: cleanest way to unwrap a list? (Graham Gill)
----------------------------------------------------------------------
Message: 1
Date: Sun, 12 Aug 2012 10:36:55 -0300
From: Ivan Hugo Guevara <[email protected]>
Subject: Re: [Haskell-beginners] Cabal error
To: Gregory Guthrie <[email protected]>
Cc: "[email protected]" <[email protected]>
Message-ID:
<cacu-qgk7ck8mq9ynycf8delt-j-jathn2kkgibdx_ptkxop...@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
I think so.... According to this:
http://hackage.haskell.org/package/regex-tdfa-1.1.8, you need 7/8
packages to make it work.
--
Atte. Iv?n Guevara
------------------------------
Message: 2
Date: Mon, 13 Aug 2012 09:01:45 +0900
From: Ken Kawamoto <[email protected]>
Subject: [Haskell-beginners] "Couldn't match expected type" and simple
"Couldn't match type"
To: [email protected]
Message-ID:
<cagbyekouytnkjyx9wykg5gruqvwoy9e-fpq3vtkqaa16czu...@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
Hi Haskellers,
I found GHC sometimes complains
"Couldn't match expected type X with actual type Y"
, and simply
"Couldn't match type X with Y"
in other cases.
not a big deal, but was wondering what makes this difference.
-- Ken
------------------------------
Message: 3
Date: Sun, 12 Aug 2012 21:21:59 -0800
From: Christopher Howard <[email protected]>
Subject: [Haskell-beginners] cleanest way to unwrap a list?
To: Haskell Beginners <[email protected]>
Message-ID: <[email protected]>
Content-Type: text/plain; charset="iso-8859-1"
Hi. Is the some generic, clean syntax to unwrap a nested list, modify
the value, and put it back together? Say, for example, I have the list
[[1,2],[3,4]] and want to add 1 to each inner element, resulting in
[[2,3],[4,5]].
After reading about the list monad, I was rather excited, because I
(mistakenly) thought something like this would work:
code:
--------
a = do b <- [[1,2],[3,4]]
c <- b
return (c + 1)
--------
That would be awesome, because I would be able to modify the list at
each level of unwrapping, while leaving the code very neat and readable.
However, what the above example actually does is produce a /single/ list
from the values:
code:
--------
*Main> a
[2,3,4,5]
--------
Obviously wishing won't change how the list monad works, but I thought
it might be worth asking if there is some other monad or syntactic trick
that does something along the lines of what I am looking for.
--
frigidcode.com
indicium.us
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Message: 4
Date: Mon, 13 Aug 2012 11:07:48 +0530
From: Shakthi Kannan <[email protected]>
Subject: Re: [Haskell-beginners] cleanest way to unwrap a list?
To: Christopher Howard <[email protected]>
Cc: Haskell Beginners <[email protected]>
Message-ID:
<cabg-yt3cm9uee7q6r9oazdnxldzkm-ba52cddnjzbiazax3...@mail.gmail.com>
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Hi,
--- On Mon, Aug 13, 2012 at 10:51 AM, Christopher Howard
<[email protected]> wrote:
| Say, for example, I have the list
| [[1,2],[3,4]] and want to add 1 to each inner element, resulting in
| [[2,3],[4,5]].
\--
Like this?
ghci> let xxs = [[1,2], [3,4]]
ghci> [ [ x+1 | x <- xs] | xs <- xxs ]
[[2,3],[4,5]]
SK
--
Shakthi Kannan
http://www.shakthimaan.com
------------------------------
Message: 5
Date: Mon, 13 Aug 2012 15:48:30 +1000
From: Ben Kolera <[email protected]>
Subject: Re: [Haskell-beginners] cleanest way to unwrap a list?
To: Christopher Howard <[email protected]>
Cc: Haskell Beginners <[email protected]>
Message-ID: <[email protected]>
Content-Type: text/plain; charset="utf-8"
Maybe this is overkill, but if you want a more generic way for accessing and
changing nested immutable data in general, you could check this package out as
it is pretty exciting (to me at least):
http://hackage.haskell.org/package/lens
For more information on exactly what a lens is, read here:
http://dl.dropbox.com/u/7810909/media/doc/lenses.pdf
Cheers,
Ben
On Monday, 13 August 2012 at 3:21 PM, Christopher Howard wrote:
> Hi. Is the some generic, clean syntax to unwrap a nested list, modify
> the value, and put it back together? Say, for example, I have the list
> [[1,2],[3,4]] and want to add 1 to each inner element, resulting in
> [[2,3],[4,5]].
>
> After reading about the list monad, I was rather excited, because I
> (mistakenly) thought something like this would work:
>
> code:
> --------
> a = do b <- [[1,2],[3,4]]
> c <- b
> return (c + 1)
> --------
>
> That would be awesome, because I would be able to modify the list at
> each level of unwrapping, while leaving the code very neat and readable.
> However, what the above example actually does is produce a /single/ list
> from the values:
>
> code:
> --------
> *Main> a
> [2,3,4,5]
> --------
>
> Obviously wishing won't change how the list monad works, but I thought
> it might be worth asking if there is some other monad or syntactic trick
> that does something along the lines of what I am looking for.
>
> --
> frigidcode.com (http://frigidcode.com)
> indicium.us (http://indicium.us)
>
> _______________________________________________
> Beginners mailing list
> [email protected] (mailto:[email protected])
> http://www.haskell.org/mailman/listinfo/beginners
------------------------------
Message: 6
Date: Mon, 13 Aug 2012 01:51:07 -0400 (EDT)
From: Jay Sulzberger <[email protected]>
Subject: Re: [Haskell-beginners] cleanest way to unwrap a list?
To: Haskell Beginners <[email protected]>
Message-ID: <[email protected]>
Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed
On Sun, 12 Aug 2012, Christopher Howard <[email protected]>
wrote:
> Hi. Is the some generic, clean syntax to unwrap a nested list, modify
> the value, and put it back together? Say, for example, I have the list
> [[1,2],[3,4]] and want to add 1 to each inner element, resulting in
> [[2,3],[4,5]].
I think there is a forgetful functor from a part of Haskell to
Scheme, and this code lies in the image of this functor:
> (define map-1+
(lambda (l)
(map (lambda (n) (+ 1 n)) l)))
#<unspecified>
> (map-1+ (list 0 1 2 3 4))
(1 2 3 4 5)
> (define map-map-1+
(lambda (l-of-l-of-numbers)
(map map-1+ l-of-l-of-numbers)))
#<unspecified>
> (map-map-1+ (list (list 1 2) (list 3 4)))
((2 3) (4 5))
> (quit)
Process scheme finished
Further we have for this forgetful functor, that the original
Haskell code and the transformed-to-Scheme code have the "same"
running behavior, and the "same" input output behavior. And
under this functor the image of the object called "map" in
Haskell is the object called "map" in Scheme.
Haskell has map in the prelude:
map :: (a -> b) -> [a] -> [b]
map f xs is the list obtained by applying f to each element of xs, i.e.,
map f [x1, x2, ..., xn] == [f x1, f x2, ..., f xn]
map f [x1, x2, ...] == [f x1, f x2, ...]
Above quote taken from
http://www.haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html
So, likely, I do not know Haskell, something like:
m1 = map 1+ l
m2 = map m1 l
should work. Likely a bit more source code might be required for
various Haskell systems. And the procedure 1+ might have to be
defined, up to getting the right type of number specified, as
1+ n = (n + 1)
oo--JS.
>
> After reading about the list monad, I was rather excited, because I
> (mistakenly) thought something like this would work:
>
> code:
> --------
> a = do b <- [[1,2],[3,4]]
> c <- b
> return (c + 1)
> --------
>
> That would be awesome, because I would be able to modify the list at
> each level of unwrapping, while leaving the code very neat and readable.
> However, what the above example actually does is produce a /single/ list
> from the values:
>
> code:
> --------
> *Main> a
> [2,3,4,5]
> --------
>
> Obviously wishing won't change how the list monad works, but I thought
> it might be worth asking if there is some other monad or syntactic trick
> that does something along the lines of what I am looking for.
>
> --
> frigidcode.com
> indicium.us
>
>
------------------------------
Message: 7
Date: Mon, 13 Aug 2012 01:55:28 -0400 (EDT)
From: Jay Sulzberger <[email protected]>
Subject: Re: [Haskell-beginners] cleanest way to unwrap a list?
To: Haskell Beginners <[email protected]>
Message-ID: <[email protected]>
Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed
On Mon, 13 Aug 2012, Jay Sulzberger wrote:
>
>
> On Sun, 12 Aug 2012, Christopher Howard <[email protected]>
> wrote:
>
>> Hi. Is the some generic, clean syntax to unwrap a nested list, modify
>> the value, and put it back together? Say, for example, I have the list
>> [[1,2],[3,4]] and want to add 1 to each inner element, resulting in
>> [[2,3],[4,5]].
>
> I think there is a forgetful functor from a part of Haskell to
> Scheme, and this code lies in the image of this functor:
>
> > (define map-1+
> (lambda (l)
> (map (lambda (n) (+ 1 n)) l)))
> #<unspecified>
> > (map-1+ (list 0 1 2 3 4))
> (1 2 3 4 5)
> > (define map-map-1+
> (lambda (l-of-l-of-numbers)
> (map map-1+ l-of-l-of-numbers)))
> #<unspecified>
> > (map-map-1+ (list (list 1 2) (list 3 4)))
> ((2 3) (4 5))
> > (quit)
>
> Process scheme finished
>
> Further we have for this forgetful functor, that the original
> Haskell code and the transformed-to-Scheme code have the "same"
> running behavior, and the "same" input output behavior. And
> under this functor the image of the object called "map" in
> Haskell is the object called "map" in Scheme.
>
> Haskell has map in the prelude:
>
> map :: (a -> b) -> [a] -> [b]
>
> map f xs is the list obtained by applying f to each element of xs, i.e.,
>
> map f [x1, x2, ..., xn] == [f x1, f x2, ..., f xn]
> map f [x1, x2, ...] == [f x1, f x2, ...]
>
> Above quote taken from
>
> http://www.haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html
>
> So, likely, I do not know Haskell, something like:
>
> m1 = map 1+ l
> m2 = map m1 l
Ah, above likely wrong. OK, what about
m1 l = map 1+ l
m2 l = map m1 l
or perhaps
m1 = map 1+
m2 = map m1
oo--JS.
>
> should work. Likely a bit more source code might be required for
> various Haskell systems. And the procedure 1+ might have to be
> defined, up to getting the right type of number specified, as
>
> 1+ n = (n + 1)
>
> oo--JS.
>
>
>>
>> After reading about the list monad, I was rather excited, because I
>> (mistakenly) thought something like this would work:
>>
>> code:
>> --------
>> a = do b <- [[1,2],[3,4]]
>> c <- b
>> return (c + 1)
>> --------
>>
>> That would be awesome, because I would be able to modify the list at
>> each level of unwrapping, while leaving the code very neat and readable.
>> However, what the above example actually does is produce a /single/ list
>> from the values:
>>
>> code:
>> --------
>> *Main> a
>> [2,3,4,5]
>> --------
>>
>> Obviously wishing won't change how the list monad works, but I thought
>> it might be worth asking if there is some other monad or syntactic trick
>> that does something along the lines of what I am looking for.
>>
>> --
>> frigidcode.com
>> indicium.us
>>
>>
>
>
------------------------------
Message: 8
Date: Mon, 13 Aug 2012 07:58:40 +0200
From: "Henk-Jan van Tuyl" <[email protected]>
Subject: Re: [Haskell-beginners] cleanest way to unwrap a list?
To: "Christopher Howard" <[email protected]>
Cc: Haskell Beginners <[email protected]>
Message-ID: <[email protected]>
Content-Type: text/plain; charset=iso-8859-15; format=flowed;
delsp=yes
On Mon, 13 Aug 2012 07:37:48 +0200, Shakthi Kannan <[email protected]>
wrote:
> Hi,
>
> --- On Mon, Aug 13, 2012 at 10:51 AM, Christopher Howard
> <[email protected]> wrote:
> | Say, for example, I have the list
> | [[1,2],[3,4]] and want to add 1 to each inner element, resulting in
> | [[2,3],[4,5]].
> \--
>
> Like this?
>
> ghci> let xxs = [[1,2], [3,4]]
>
> ghci> [ [ x+1 | x <- xs] | xs <- xxs ]
> [[2,3],[4,5]]
Or this?
> map (map (+ 1)) [[1,2], [3,4]]
[[2,3],[4,5]]
Regards,
Henk-Jan van Tuyl
--
http://Van.Tuyl.eu/
http://members.chello.nl/hjgtuyl/tourdemonad.html
Haskell programming
--
------------------------------
Message: 9
Date: Mon, 13 Aug 2012 01:59:38 -0400
From: Graham Gill <[email protected]>
Subject: Re: [Haskell-beginners] cleanest way to unwrap a list?
To: Christopher Howard <[email protected]>
Cc: Haskell Beginners <[email protected]>
Message-ID: <[email protected]>
Content-Type: text/plain; charset="iso-8859-1"; Format="flowed"
What about just
map (map f) xxs
where for your example
xxs == [[1,2], [3,4]]
f == (+1)
Graham
On 13/08/2012 1:21 AM, Christopher Howard wrote:
> Hi. Is the some generic, clean syntax to unwrap a nested list, modify
> the value, and put it back together? Say, for example, I have the list
> [[1,2],[3,4]] and want to add 1 to each inner element, resulting in
> [[2,3],[4,5]].
>
> After reading about the list monad, I was rather excited, because I
> (mistakenly) thought something like this would work:
>
> code:
> --------
> a = do b <- [[1,2],[3,4]]
> c <- b
> return (c + 1)
> --------
>
> That would be awesome, because I would be able to modify the list at
> each level of unwrapping, while leaving the code very neat and readable.
> However, what the above example actually does is produce a /single/ list
> from the values:
>
> code:
> --------
> *Main> a
> [2,3,4,5]
> --------
>
> Obviously wishing won't change how the list monad works, but I thought
> it might be worth asking if there is some other monad or syntactic trick
> that does something along the lines of what I am looking for.
>
>
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
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