Re: [Haskell-cafe] Search monad

2007-03-20 Thread Tomasz Zielonka 
On Mon, Mar 19, 2007 at 05:11:19PM +, Edsko de Vries wrote:
 What I'm wondering about is if this monad is an instance of a more
 general monad(if so, which one?), and generally if people have any
 comments about these definitions.

It's like the Writer monad used with different monoids.

Pozdrawiam
Tomek
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe

[Haskell-cafe] Search monad

2007-03-19 Thread Edsko de Vries 
Hey,

I have a structure containing Xs in various places, like so

data X
data Structure = Structure .. [X] .. [X] ..

And I defined mapStructure 

mapStructure :: (X - X) - (Structure - Structure)

I then wanted to use mapStructure to define queries as well as
transformations on structures. I generalized mapStructure to
mapStructureM:

mapStructure :: Monad m = (X - m X) - (Structure - m Structure)

and then defined the following search monad:

 data Search f a b = Found (f a) b
 
 class Monad (m a) = SearchMonad m a where
 found :: a - m a a
 
 fromSearch :: Search f a b - f a
 fromSearch (Found a _) = a
 
 search :: (SearchMonad m a) = (a - Bool) - a - m a a
 search f a
 | f a = found a
 | otherwise = return a

Instances of the monad for finding one and for finding all elements:

 instance SearchMonad (Search Maybe) a where
 found a = Found (Just a) a
 
 instance SearchMonad (Search []) a where
 found a = Found [a] a
 
 instance Monad (Search Maybe a) where
 return b = Found Nothing b
 Found (Just a) a' = f = case f a' of
 Found _ b - Found (Just a) b
 Found Nothing a' = f = f a'
 
 instance Monad (Search [] a) where
 return b = Found [] b
 Found as a' = f = case f a' of
 Found as' b - Found (as ++ as') b

Here is a simple sample session with ghci

*Util fromSearch $ mapM (search even) [1,3,5] :: Maybe Int
Nothing
*Util fromSearch $ mapM (search even) [1,2,3,4,5] :: Maybe Int
Just 2
*Util fromSearch $ mapM (search even) [1,2,3,4,5] :: [Int]
[2,4]

What I'm wondering about is if this monad is an instance of a more general 
monad(if so, which one?), and generally if people have any comments about these 
definitions.

Thanks!

Edsko 
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe

Re: [Haskell-cafe] Search monad

2007-03-19 Thread Jeff Polakow 
Hello,

  You might want to look at the scrap your boilerplate papers and/or their 
implementation in GHC in Data.Generics.

-Jeff

[EMAIL PROTECTED] wrote on 03/19/2007 01:11:19 PM:

 Hey,
 
 I have a structure containing Xs in various places, like so
 
 data X
 data Structure = Structure .. [X] .. [X] ..
 
 And I defined mapStructure 
 
 mapStructure :: (X - X) - (Structure - Structure)
 
 I then wanted to use mapStructure to define queries as well as
 transformations on structures. I generalized mapStructure to
 mapStructureM:
 
 mapStructure :: Monad m = (X - m X) - (Structure - m Structure)
 
 and then defined the following search monad:
 
  data Search f a b = Found (f a) b
  
  class Monad (m a) = SearchMonad m a where
  found :: a - m a a
  
  fromSearch :: Search f a b - f a
  fromSearch (Found a _) = a
  
  search :: (SearchMonad m a) = (a - Bool) - a - m a a
  search f a
  | f a = found a
  | otherwise = return a
 
 Instances of the monad for finding one and for finding all elements:
 
  instance SearchMonad (Search Maybe) a where
  found a = Found (Just a) a
  
  instance SearchMonad (Search []) a where
  found a = Found [a] a
  
  instance Monad (Search Maybe a) where
  return b = Found Nothing b
  Found (Just a) a' = f = case f a' of
  Found _ b - Found (Just a) b
  Found Nothing a' = f = f a'
  
  instance Monad (Search [] a) where
  return b = Found [] b
  Found as a' = f = case f a' of
  Found as' b - Found (as ++ as') b
 
 Here is a simple sample session with ghci
 
 *Util fromSearch $ mapM (search even) [1,3,5] :: Maybe Int
 Nothing
 *Util fromSearch $ mapM (search even) [1,2,3,4,5] :: Maybe Int
 Just 2
 *Util fromSearch $ mapM (search even) [1,2,3,4,5] :: [Int]
 [2,4]
 
 What I'm wondering about is if this monad is an instance of a more 
 general monad(if so, which one?), and generally if people have any 
 comments about these definitions.
 
 Thanks!
 
 Edsko 
 ___
 Haskell-Cafe mailing list
 Haskell-Cafe@haskell.org
 http://www.haskell.org/mailman/listinfo/haskell-cafe


---

This e-mail may contain confidential and/or privileged information. If you 
are not the intended recipient (or have received this e-mail in error) 
please notify the sender immediately and destroy this e-mail. Any 
unauthorized copying, disclosure or distribution of the material in this 
e-mail is strictly forbidden.___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe