Anyone ever needed this? Me and John Wiegley were discussing a decent
name for it, John suggested inv as in involution. E.g.
inv reverse (take 10)
inv reverse (dropWhile isDigit)
trim = inv reverse (dropWhile isSpace) . dropWhile isSpace
That seems to be the only use-case I've ever come across.
On 17 August 2013 19:11, Christopher Done wrote:
> Anyone ever needed this? Me and John Wiegley were discussing a decent
> name for it, John suggested inv as in involution. E.g.
In terms of a decent name: as soon as I saw the subject, I thought you
were somehow inverting a function :/
In terms o
On 17/08/13 10:11, Christopher Done wrote:
> Anyone ever needed this? Me and John Wiegley were discussing a decent
> name for it, John suggested inv as in involution. E.g.
First thing I thought was ‘inverse’…
>
> inv reverse (take 10)
> inv reverse (dropWhile isDigit)
> trim = inv reverse (dropWhi
In J (a sort of dialect of APL), there's a thing called "under", written
"&.". The expression "(f &. g) x" is equivalent to "(g^:_1) (f (g x))"
where "g^:_1" is J's "obverse" of g, which in cases where it exists is
usually the inverse of g (
http://www.jsoftware.com/help/dictionary/intro26.htm).
damodar kulkarni writes:
> Thanks for this nice analogy and explanation. This brings "monad
> transformers" to my mind.
> "without" monad transformers, the monads are bit crippled in their
> applicability (please correct me if I am wrong)
> and
> "with" monad transformers the code becomes to some
On Sat, Aug 17, 2013 at 11:11:07AM +0200, Christopher Done wrote:
> Anyone ever needed this? Me and John Wiegley were discussing a decent
> name for it, John suggested inv as in involution. E.g.
>
> inv reverse (take 10)
> inv reverse (dropWhile isDigit)
> trim = inv reverse (dropWhile isSpace) .
Note that at least for the dropWhile example, there is a specialized
function, dropWhileEnd, which is most likely more efficient than reversing
the list twice.
On Aug 17, 2013 3:35 PM, "Tom Ellis" <
tom-lists-haskell-cafe-2...@jaguarpaw.co.uk> wrote:
> On Sat, Aug 17, 2013 at 11:11:07AM +0200, Chr
Hi,
Am Samstag, den 17.08.2013, 11:11 +0200 schrieb Christopher Done:
> inv reverse (take 10)
if you want that fast and lazy, check out
http://www.joachim-breitner.de/blog/archives/600-On-taking-the-last-n-elements-of-a-list.html
Greetings,
Joachim
--
Joachim “nomeata” Breitner
m...@joachim-
Christopher Done writes:
> Anyone ever needed this? Me and John Wiegley were discussing a decent
> name for it, John suggested inv as in involution. E.g.
>
> inv reverse (take 10)
> inv reverse (dropWhile isDigit)
> trim = inv reverse (dropWhile isSpace) . dropWhile isSpace
>
> That seems to be t
Hello,
I've a problem connecting to my postgresql database.
Can You help me fix the ambigious type signature?
(The example is identical to the first 5-liner-example in the package
documentation)
http://hackage.haskell.org/packages/archive/postgresql-simple/0.3.5.0/doc/html/Database-PostgreSQL
On Sat, Aug 17, 2013 at 8:23 AM, Mathijs Kwik wrote:
> damodar kulkarni writes:
>
> > Thanks for this nice analogy and explanation. This brings "monad
> > transformers" to my mind.
> > "without" monad transformers, the monads are bit crippled in their
> > applicability (please correct me if I am
On Sat, Aug 17, 2013 at 1:35 PM, Hartmut Pfarr
wrote:
> (The example is identical to the first 5-liner-example in the package
> documentation)
>
As I read it, the example has a typo: it should be using `query_` instead
of `query`. See
http://hackage.haskell.org/packages/archive/postgresql-simple/
This is indeed a job for lens, particularly, the Iso type, and the "under"
function. Lens conveniently comes with a typeclassed isomorphism called
"reversed", which of course has a list instance.
>>> under reversed (take 10) ['a'.. 'z']
"qrstuvwxyz"
-- Dan Burton
On Aug 17, 2013 10:23 AM, "Anton
The lens docs even have an example of another helper function, "involuted"
for functions which are their own inverse.
>>> "live" & involuted reverse %~ ('d':)
"lived"
inv f g = involuted f %~ g
http://hackage.haskell.org/packages/archive/lens/3.9.0.2/doc/html/Control-Lens-Iso.html#v:involuted
-
Thx, I changed now from query to query_
Now the coding is like that:
{-# LANGUAGE OverloadedStrings #-}
import Database.PostgreSQL.Simple
import Database.PostgreSQL.Simple.FromRow
hello :: (FromRow a) => IO [a]
hello = do
On Sat, Aug 17, 2013 at 11:59:24PM +0200, Hartmut Pfarr wrote:
> {-# LANGUAGE OverloadedStrings #-}
>
> import Database.PostgreSQL.Simple
> import Database.PostgreSQL.Simple.FromRow
>
> hello :: (FromRow a) => IO [a]
> hello = do
> conn <- connect defaultConnectInfo
> query_ conn "select 2 +
On Sat, Aug 17, 2013 at 5:59 PM, Hartmut Pfarr
wrote:
> query_ conn "select 2 + 2"
>
> I've no errors any more.
> But: I don't see any result (for sure, it is not coeded yet)
>
Yes, because you're not capturing it; it's the return value from `query_`,
which you are throwing away above instead o
> Dan Burton writes:
under reversed (take 10) ['a'.. 'z']
> "qrstuvwxyz"
Excellent, thanks!
--
John Wiegley
FP Complete Haskell tools, training and consulting
http://fpcomplete.com johnw on #haskell/irc.freenode.net
__
Q: Are the "continuations" in Scheme related to the "monads" from
Haskell? If so, could someone elaborate on that?
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Yes they are. Purely intuitively, you can see how writing code in a monadic
style (using >>= a lot) is very similar to writing in continuation-passing
style.
You can express this the most directly with the continuation monad. Then,
from this monad, you can express other monads. In some sense, the
... thx all for helping. Now the coding works: it puts the following out.
Kind regards
Hartmut
*Main> main
Only {fromOnly = 4}
--
Only {fromOnly = 101}
Only {fromOnly = 102}
Only {fromOnly = 103}
--
blub 101 51
blub 102 52
blub 103 53
The
Thanks again for the detailed and explanatory answer.
That's the reason I'm writing these huge responses, because I hope I can
> shorten this journey for others.
>
This has certainly helped me grasp some aspects in this regard.
While Monad Transformers are awesome and can solve many problems qui
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