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Today's Topics:
1. self recursive lambda (sarfraz)
2. netwire and "dynamic" list of objects (Nathan H?sken)
3. Re: self recursive lambda (Antoine Latter)
4. Re: self recursive lambda (sarfraz)
----------------------------------------------------------------------
Message: 1
Date: Mon, 9 Apr 2012 17:57:22 +0200
From: sarfraz <dragoon.emp...@yahoo.fr>
Subject: [Haskell-beginners] self recursive lambda
To: beginners@haskell.org
Message-ID: <20120409155722.GA673@unvrs>
Content-Type: text/plain; charset="us-ascii"
Hi,
Can't seem to be able to debug the following:
import System.IO
nHead :: Int -> String -> String
nHead n str = if (length str) < n
then str
else if n ==0
then str
else (\x n str -> if n == 0
-- this lambda does not work
then str
else x (n - 1) (tail str))
main = interact (nHead 5)
I see where the problem is. But surely it is possible to make a
recursive lambda. (I want to use a lambda, i know how to do otherwise)
--
Sarfraz K.
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Message: 2
Date: Mon, 09 Apr 2012 18:40:52 +0200
From: Nathan H?sken <nathan.hues...@posteo.de>
Subject: [Haskell-beginners] netwire and "dynamic" list of objects
To: beginners@haskell.org
Message-ID: <4f831114.4090...@posteo.de>
Content-Type: text/plain; charset=ISO-8859-1
Hey,
I have a simple SDL based graphic demo, which I am trying (for
educational purposes) to rewrite utilizing netwire.
The demo basically consists of a State which contains a list of object
states.
data ObjectState = ...
data State = State [ObjectState]
Then there is a function updateState which updates all objects, removing
some and adding some new ones.
updateState :: State -> State
updateState (State oss) = State (catMaybes $ map updateObject $
newObjects ++ oss)
where
updateObject :: ObjectState -> Maybe ObjectState
updateObject = ...
newObjects :: [ObjectState]
newObjects = ...
OK, now I wrote a wire:
type MyWire = Wire.Wire () (Kleisli IO)
wire :: GameState -> MyWire () GameState
wire initGS = proc _ -> do
rec
gs' <- Wire.delay initGS -< gs
gs <- arr updateState -< gs'
returnA -< gs
Works wonderfully :).
But I avoided rewriting "updateState" in wire format. But I have no Idea
how to do that.
How do I handle a dynamic list of objects in a wire?
Thanks!
Nathan
------------------------------
Message: 3
Date: Mon, 9 Apr 2012 12:28:42 -0500
From: Antoine Latter <aslat...@gmail.com>
Subject: Re: [Haskell-beginners] self recursive lambda
To: beginners@haskell.org
Message-ID:
<CAKjSnQHdqvxjy8srRyC3q5BzWftVM2f=R1Y9FZAV4m0m6ZS=s...@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8
On Mon, Apr 9, 2012 at 10:57 AM, sarfraz <dragoon.emp...@yahoo.fr> wrote:
> Hi,
>
> Can't seem to be able to debug the following:
>
> import System.IO
>
> nHead :: Int -> String -> String
> nHead n str = if (length str) < n
> ? ? ? ? ? ? ?then str
> ? ? ? ? ? ? ?else if n ==0
> ? ? ? ? ? ? ? ? ? then str
> ? ? ? ? ? ? ? ? ? else (\x n str -> if n == 0
> -- this lambda does not work
> ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? then str
> ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? else x (n - 1) (tail str))
>
> main = interact (nHead 5)
>
> I see where the problem is. But surely it is possible to make a
> recursive lambda. (I want to use a lambda, i know how to do otherwise)
You can't make a recursive lambda directly - you need a helper function:
http://hackage.haskell.org/packages/archive/base/latest/doc/html/Data-Function.html#v:fix
Example usage:
> let factNonResursive = \fact n -> if n < 2 then 1 else n * fact (n - 1)
> fix factNonRecursive 5
Antoine
------------------------------
Message: 4
Date: Mon, 9 Apr 2012 20:13:48 +0200
From: sarfraz <dragoon.emp...@yahoo.fr>
Subject: Re: [Haskell-beginners] self recursive lambda
To: beginners@haskell.org
Message-ID: <20120409181347.GA619@unvrs>
Content-Type: text/plain; charset="iso-8859-1"
Ok thanks, got it working:
import System.IO
import Control.Monad.Fix
nHead :: Int -> String -> String
nHead n str = let rec = \x n str -> if n == 0 then str else x (n - 1) (tail str)
in if (length str) < n then str else fix rec n str
main = interact (nHead 5)
In the end I will be using:
import System.IO
nHead :: Int -> String -> String
nHead n str = let rec n str = if n == 0 then str else x (n - 1) (tail str)
in if (length str) < n then str else rec n str
main = interact (nHead 5)
But it's good to know that it was possible.
On Mon, Apr 09, 2012 at 12:28:42PM -0500, Antoine Latter wrote:
> On Mon, Apr 9, 2012 at 10:57 AM, sarfraz <dragoon.emp...@yahoo.fr> wrote:
> > Hi,
> >
> > Can't seem to be able to debug the following:
> >
> > import System.IO
> >
> > nHead :: Int -> String -> String
> > nHead n str = if (length str) < n
> > ? ? ? ? ? ? ?then str
> > ? ? ? ? ? ? ?else if n ==0
> > ? ? ? ? ? ? ? ? ? then str
> > ? ? ? ? ? ? ? ? ? else (\x n str -> if n == 0
> > -- this lambda does not work
> > ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? then str
> > ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? else x (n - 1) (tail str))
> >
> > main = interact (nHead 5)
> >
> > I see where the problem is. But surely it is possible to make a
> > recursive lambda. (I want to use a lambda, i know how to do otherwise)
>
> You can't make a recursive lambda directly - you need a helper function:
>
> http://hackage.haskell.org/packages/archive/base/latest/doc/html/Data-Function.html#v:fix
>
> Example usage:
>
> > let factNonResursive = \fact n -> if n < 2 then 1 else n * fact (n - 1)
> > fix factNonRecursive 5
>
> Antoine
>
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://www.haskell.org/mailman/listinfo/beginners
--
Sarfraz K.
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