Re: [Haskell-cafe] Data creation pattern?
Hi Eugene You don't need to supply all the arguments to a constructor at once: makeWithOne :: String - (String - String - Object) makeWithOne s1 = \s2 s3 - Object s1 s2 s3 -- or even: -- makeWithOne s1 = Object s1 This builds a higher-order function that can be applied later to two Strings to finally make the Object. Best wishes Stephen ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Data creation pattern?
On Thu, May 13, 2010 at 04:43:26PM +0100, Stephen Tetley wrote:
Hi Eugene
You don't need to supply all the arguments to a constructor at once:
makeWithOne :: String - (String - String - Object)
makeWithOne s1 = \s2 s3 - Object s1 s2 s3
-- or even:
-- makeWithOne s1 = Object s1
This builds a higher-order function that can be applied later to two
Strings to finally make the Object.
Hello, Stephen!
Well, that's true, but I don't really know the order of incoming data. So for
constructor MyObject { prop1, prop2, prop3 :: String } there can be the cases
of order
prop1
prop2
prop3
prop3
prop2
prop1
prop1
prop3
prop2
...
so I may need to initialize prop1, then prop3, and finally prop2 to make
instance of MyObject. And similarly for another cases.
I don't see any easy solution for now except sort name-value pairs in some
pre-defined order, and then use something like
props :: [(String,String)]
result = MyObject (props' !! 0) (props' !! 1) (props' !! 2)
where
props' = map snd props
which looks ugly and involves some additional calculations for sorting.
--
Eugene Dzhurinsky
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Re: [Haskell-cafe] Data creation pattern?
Hi Eugene Is something like this close to what you want: For example this builds an object with ordered strings... makeOrdered :: String - String - String - Object makeOrdered a b c = let (s,t,u) = sort3 (a,b,c) in Object s t u Alternatively you could build the with the Strings as they appear in the input then use an Object 'transformer' - a function from Object to Object (fun :: Object - Object), e.g orderObject :: Object - Object orderObject (Object a b c) = Object s t u where s = min3 a b c t = med3 a b c u = max3 a b c demo1 = orderObject (Object 4 6 1) Support code -- inefficient, but I had max3, etc to hand... sort3 :: Ord a = (a,a,a) - (a,a,a) sort3 (a,b,c) = (s,t,u) where s = min3 a b c t = med3 a b c u = max3 a b c -- Or without the intermediate triple: -- -- sort3 :: Ord a = a - a - a - (a,a,a) -- sort3 a b c = (s,t,u) -- where --s = min3 a b c --t = med3 a b c --u = max3 a b c -- | max of 3 max3 :: Ord a = a - a - a - a max3 a b c = max (max a b) c -- | min of 3 min3 :: Ord a = a - a - a - a min3 a b c = min (min a b) c -- | median of 3 med3 :: Ord a = a - a - a - a med3 a b c = if c = x then x else if c y then y else c where (x,y) = order a b order p q | p = q= (p,q) | otherwise = (q,p) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Data creation pattern?
On 13 May 2010 19:14, Stephen Tetley stephen.tet...@gmail.com wrote: Hi Eugene Hi Eugeny Whoops - apologies for the the name change... ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Data creation pattern?
Hi. Stephen Tetley wrote: Hi Eugene Is something like this close to what you want: For example this builds an object with ordered strings... makeOrdered :: String - String - String - Object makeOrdered a b c = let (s,t,u) = sort3 (a,b,c) in Object s t u Or just: makeOrdered a b c = let (s:t:u:_) = sort [a, b, c] in Object s t u (no support code required) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Data creation pattern?
On 13 May 2010 19:24, Steffen Schuldenzucker sschuldenzuc...@uni-bonn.de wrote: Or just: makeOrdered a b c = let (s:t:u:_) = sort [a, b, c] in Object s t u Hi Steffen True - but it does include a partial pattern (that will always match in this case, of course). ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Data creation pattern?
On Thu, May 13, 2010 at 07:15:22PM +0100, Stephen Tetley wrote: On 13 May 2010 19:14, Stephen Tetley stephen.tet...@gmail.com wrote: Hi Eugene Hi Eugeny Whoops - apologies for the the name change... In fact it doesn't make any difference, so both these names are equal :) -- Eugene Dzhurinsky pgpHrKKK4J1fm.pgp Description: PGP signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Data creation pattern?
On Thu, May 13, 2010 at 07:14:25PM +0100, Stephen Tetley wrote: Hi Eugene Is something like this close to what you want: Not really. First of all, there're many properties, not 3. So it may end up with plenty of support (boilerplate) code. Also, names of these parameters are not sortable. Of course I could make these names as another data type with deriving from Eq/Ord - but that increase complexity as well. Original idea was 1) create Array 2) if line param_N_name=value appear, then 2.1) try to take object N from array 2.2) if no object exists - then create one 2.3) set the property name to value 2.4) put resulting object back into array 3) take next line and go to 2 so if it is possible to have partially initialized objects in Haskell, then this approach should work. If now - well, then replace creation of object with adding name/value pair to an array. And then create objects from those arrays. -- Eugene Dzhurinsky pgpLJ5Y81wAVT.pgp Description: PGP signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Data creation pattern?
On Thursday 13 May 2010 20:43:44, Eugeny N Dzhurinsky wrote:
On Thu, May 13, 2010 at 07:14:25PM +0100, Stephen Tetley wrote:
Hi Eugene
Is something like this close to what you want:
Not really. First of all, there're many properties, not 3. So it may end
up with plenty of support (boilerplate) code.
Also, names of these parameters are not sortable. Of course I could make
these names as another data type with deriving from Eq/Ord - but that
increase complexity as well.
Original idea was
1) create Array
2) if line param_N_name=value appear, then
2.1) try to take object N from array
2.2) if no object exists - then create one
2.3) set the property name to value
2.4) put resulting object back into array
3) take next line and go to 2
so if it is possible to have partially initialized objects in Haskell,
If the fields aren't strict, there's no problem having
data MyObject = MyObject { param1, param2, param3 :: String }
emptyObject = MyObject undefined undefined undefined
-- Boilerplate code, but you can let that write the computer for you
update :: String - MyObject - String - MyObject
update param1 obj val = obj{param1 = val}
update param2 obj val = obj{param2 = val}
update param3 obj val = obj{param3 = val}
update _ obj _ = obj
and start from emptyObject, updateing the fields as they come (if the
fields aren't all Strings, read or parse the value).
import qualified Data.Map as Map
import Data.List (foldl')
-- break all lines into:
-- object ID (N)
-- field to be set (name)
-- value
breakUpLine :: String - (Int, String, String)
-- construct a map ID - object from the lines of the file
construct :: [String] - Map Int MyObject
construct = foldl' updateMap (Map.empty)
where
updateMap mp (i, fld, val) =
case Map.findWithDefault emptyObject i mp of
obj - Map.insert i (update fld obj val) mp
then this approach should work. If now - well, then replace creation of
object with adding name/value pair to an array. And then create objects
from those arrays.
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Re: [Haskell-cafe] Data creation pattern?
On Thu, May 13, 2010 at 09:03:48PM +0200, Daniel Fischer wrote: so if it is possible to have partially initialized objects in Haskell, If the fields aren't strict, there's no problem having ... Wow! Thank you, that's it :) -- Eugene Dzhurinsky pgpPXFjii1ixC.pgp Description: PGP signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
