Re: [Haskell-cafe] introspection | meta data
Can you name these fields? If so, haskell has (sorta clumsy) named
records,
and you can select and update fields by name, and you can replace
'setSFField 3 sf x' with 'sf {somefield=x}'
I did think of this, but unfortunatly my algorithm cant use names (without
hard coding all possible combinations )
> So what is the general haskell approach to this type of
introspection/meta
> data problem... ?
A C array of pointers maps closest to a MutableArray, which is mostly a
list
with different performance. Unless you're casting pointers, in which case
Dynamic types or the Generic stuff is maybe what you want. Or a redesign
;)
I looked at haskell arrays, but since I cant point to an element in my tuple
it wont work out. The ANSI C use of arrays is a really simple (but nasty)
way to provide a means for me to loop over a fixed struct record. The
Generic library looks super, only had time to browse some slides thus far,
but will defiantly try understand that.
As for a redesign and
Others have given good answers for this, but I suspect you may have chosen
the
wrong data structure...
I would be keen on any ideas you have on how to design this in haskell.
Learning to think in haskell is after all the goal :-) The example below is
trivial and real world, both reasons why I chose to use it.
Any comments welcome, none expected :-)
Thanks,
--
I idea is to perform a search over a number fields in a hierachical fashion
where each field can have a wild card. The real world example is printer
selection. In a multi-national company all users tend to be on one (or a
few) central servers, but require there printouts to come to them locally
whereever they are. Users typically range in 1000s and so "by user"
defintions are out.
Simplified Search fields:
Environemnt, Users, Report, Version, Host Printer
-- --
The "most" speicific field is host on the right with it becomming more
general moving to towards the left.
Setup data could choose to override all of report "RPT1" version "Ver1" to
Printer "Bobs Printer"
*ALL, *ALL, "RPT1", "Ver1", "*ALL", "Bobs Printer"
but Simon may be an exception, then a record could be added like so:
*ALL, "SIMON", "RPT1", "Ver1", *ALL, "Simons Printer"
This record would be found first for simon, but former found found for
everyone else.
A search starts from fully specific data i.e no wild cards.
The basic algortihm I worked out is:
1. Search setup data
2. If no record found
2a: Set current field to most specific field (host in this case)
2b: Toggle current field ( if Wildcard then make it value, if value make
it wildcard )
2c: if current field is *ALL goto 1 above (we stop here to perform a
search on the current permutation)
2d: Loop to 2b until no more fields
And my haskell working proto type is this:
module Main where
-- Env User Report Version HostPrinter
egdata1 = [(("PD7334EU", "*ALL", "*ALL","*ALL", "*ALL"), "Default
Printer"),
(("PD7334EU", "USER1", "*ALL","*ALL", "*ALL"),
"User1Printer"),
(("PD7334EU", "USER2", "Report1", "Version1", "*ALL"),
"User1Report1Printer"),
(("PD7334EU", "*ALL", "Report2", "*ALL", "*ALL"),
"Report2Printer")]
type SearchFilter = (String, String, String, String, String)
type Record = (SearchFilter, String)
findPrinter :: String -> String -> String -> String -> String -> [Record]
-> String
findPrinter env user report version host printerdata =
findPrinter' sf sf printerdata
where
sf = (env, user, report, version, host)
findPrinter' :: SearchFilter -> SearchFilter -> [Record] -> String
findPrinter' ("*ALL", "*ALL", "*ALL", "*ALL", "*ALL") _ _ = ""
findPrinter' sf origsf printerdata
| printer == "" = findPrinter' (toggle sf origsf 5) origsf
printerdata
| otherwise = printer
where
printer = searchPrinter sf printerdata
searchPrinter :: SearchFilter -> [Record] -> String
searchPrinter _ [] = ""
searchPrinter sf ((x,p):xa)
| sf == x= p
| otherwise = searchPrinter sf xa
toggle :: SearchFilter -> SearchFilter -> Int -> SearchFilter
toggle sf origsf 0 = sf
toggle sf origsf n
| newValue == "*ALL" = newSF
| otherwise= toggle newSF origsf (n-1)
where
newValue = toggleField (getSFField n sf) (getSFField n origsf)
newSF= setSFField n sf newValue
toggleField :: String -> String -> String
toggleField "*ALL" x = x
toggleField _ _ = "*ALL"
getSFField :: Int -> SearchFilter -> String
getSFField 1 (x,_,_,_,_) = x
getSFField 2 (_,x,_,_,_) = x
getSFField 3 (_,_,x,_,_) = x
getSFField 4 (_,_,_,x,_) = x
getSFField 5 (_,_,_,_,x) = x
setSFField :: Int -> SearchFilter -> String -> SearchFilter
setSFField 1 (a,b,c,d,e) f = (f,b,c,d,e)
setSFField 2 (a,b,c,d,e) f = (a,f,c,d,e)
setSFField 3 (a,b,c,d,e) f = (a,b,f,d,e)
setSFField 4 (a,b,c,d,e) f = (a,b,c,f,e)
setSFField 5 (a,b,c,d,e) f = (a,b,c,d
[Haskell-cafe] introspection | meta data
Hi I recently had to implement an algorthm in C, and found the time to give it a go in Haskell to aid in learning haskell. I found myself "needing" runtime meta information on data types (well tuples which are data (a,b) = (a,b)). Does haskell allow this ? Basically I need to loop over the fields in my record. I came up with functions like this: -- Toggle fields to/from generic value toggle sf origsf 0 = sf toggle sf origsf n | newValue == "*ALL" = newSF | otherwise= toggle newSF origsf (n-1) -- <- Loop through fields by No where newValue = toggleField (getSFField n sf) (getSFField n origsf)-- < accessor functions newSF= setSFField n sf newValue getSFField :: Int -> SearchFilter -> String getSFField 1 (x,_,_,_,_) = x getSFField 2 (_,x,_,_,_) = x setSFField :: Int -> SearchFilter -> String -> SearchFilter setSFField 1 (a,b,c,d,e) f = (f,b,c,d,e) setSFField 2 (a,b,c,d,e) f = (a,f,c,d,e) The only problem with this is that if I want to add or remove a field, I need to change a lot of code. In most OO lanaguages I could use "reflection" to loop through my fields. In C (which also doesnt have meta data) I got round it by using pointers in an array. To add or remove a field just requires adding or removing a pointer to my array which is one line of code. The only thought I had was of using lists, but the this would mean I loose pattern matching against all values at once which is appealing for calrify of code. So what is the general haskell approach to this type of introspection/meta data problem... ? Thanks, _ Protect your PC - get McAfee.com VirusScan Online http://clinic.mcafee.com/clinic/ibuy/campaign.asp?cid=3963 ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How do I include polymorphic function type signatures in a data element ?
*grin* its so obvious once you know the answer :-)
Thx
Original Message Follows
On Tue, 20 Jul 2004, Crypt Master wrote:
> Exmaple:
>
>
> type Fitness = Integer
> data Population a = Population [(Fitness, a)]
> deriving (Show)
>
> data GAParams = GAParams { randomNums :: [Integer] ,
>someFunc :: (Int->Int->(Population a))}
>
Like for 'Population a' you have to specify a type parameter for
'GAParams', i.e.
|
v
data GAParams a = GAParams { randomNums :: [Integer] ,
someFunc :: (Int->Int->(Population a))}
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[Haskell-cafe] How do I include polymorphic function type signatures in a data element ?
Hi
How do I include polymorphic function type signatures in a data element ?
Exmaple:
type Fitness = Integer
data Population a = Population [(Fitness, a)]
deriving (Show)
data GAParams = GAParams { randomNums :: [Integer] ,
someFunc :: (Int->Int->(Population a))}
This errors with parse input error on '}'. However something like this works
fine:
data GAParams = GAParams { randomNums :: [Integer] ,
someFunc :: (Int->Int->GAParams) }
/
I am just having issues with polymorphic types.
Any ideas ?
The context is included below in case it helps:
import Random
type Fitness = Integer
data Population a = Population [(Fitness, a)]
deriving (Show)
data GAParams = GAParams { randomNums :: [Integer] ,
someFunc :: (Int->Int->GAParams) }
gaSolutionSpaceFrom :: Population a -> GAParams -> [Population a]
gaSolutionSpaceFrom p gaParams = (evolvePopulation p gaParams) :
gaSolutionSpaceFrom p newGAParams
where
(r,rs) = splitAt (length pl) (randomNums
gaParams)
Population pl = p
newGAParams = (gaParams{randomNums=rs})
evolvePopulation :: Population a -> GAParams -> Population a
evolvePopulation p gaParams = (mutate (cross (select p)))
selectMatingPoolByRouletteWheel :: Population a -> GAParams -> Population a
selectMatingPoolByRouletteWheel (Population popList) gaParams =
(Population [ (rwSelect rw rnd) | rnd <- rndNums ])
where
rw = createRW (Population popList)
rndNums = (randomNums gaParams)
rwSelect :: [(Fitness, a)] -> Fitness -> (Fitness, a)
rwSelect [] _ = error "rwSelect random number outside roullete wheel range
or list empty"
rwSelect ((x,a):xs) z = if x <= z then
rwSelect xs z
else
(x,a)
createRW :: Population a -> [(Fitness, a)]
createRW (Population xs) = (scanl1 f xs)
where f (n,a) (m,b) = (n + m, a)
select p = p
cross p = p
mutate p = p
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[Haskell-cafe] so how does one convert an IO a into an a ?
Hi Thanks for all the usefull responses, from theory to practical :-). I am busy digesting it all. I have more questions, but more and more I can answer them myself :-) Till the next misunderstanding ;-) C _ Add photos to your e-mail with MSN 8. Get 2 months FREE*. http://join.msn.com/?page=features/featuredemail ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] so how does one convert an IO a into an a ?
Hi Thanks for your help so far, but I am still not getting this IO stuff. After reading your previous help and reading several articles on it I still cant phathom how you convert the IO Int into an Int. One person mentioned how random just returns an interative program which when eveluated returns the Int. Also from the school of expression book he says " The right way to think of (>>=) above is simply this: It "Executes" e1 ..." in relation to "do pat <- e1 ...". so I have this: rollDice :: IO Int rollDice = getStdRandom (randomR (1,6)) rl :: [Int] rl = [ (getRndNum x) | x <- [1..] ] getRndNum :: Int -> Int getRndNum x = do n <- rollDice return n *PS Pretend return is correctly aligned under n. dont what ahppens in copy and paste* now my understanding therefore is that "do n <- rollDice" should execute the the itererative program returned by rollDice. So now n should be my Int since IO Int was a program which when evaluted returns an Int ? However this is what haskell thinks of my thoery: *** Term : getRndNum *** Type : Int -> IO (Maybe Int) *** Does not match : Int -> Int So I am still in IO Int land despite having used the >>= in the do syntax. Worse Still I am in IO (Maybe Int) land. Monads within Monads. In yours, and many other examples I found online, the results are always passed to print which seems to know how to deal with an IO Int. Is this specially coded or overloaded or something ? There are plenty of examples which use return like so: do k <- getKey w return k which is what I tried above to no avail. It seems awefully complicated just to get hold a simple Int, but naturally complicity is directly related to ones understanding. Mine is sumewhat lacking ... any help would be appreciated. Thanks, S _ Tired of spam? Get advanced junk mail protection with MSN 8. http://join.msn.com/?page=features/junkmail ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Random Numbers for the beginner ?
Hi I have tried I swear, even googled for 45 minutes, but I cant seem to get random numbers working. In the documentation is has: rollDice :: IO Int rollDice = getStdRandom (randomR (1,6)) But if I type "getStdRandom (randomR (1,6))" into hugs in the context of module which imports Random, I get get errors. ERROR - Unresolved overloading *** Type : (Random a, Num a) => IO a *** Expression : getStdRandom (randomR (1,6)) Roll dice takes no parameters and returns an IO Int. So in thoery (mine at least ;-) ) running this as an expresion should work. I should get an IO Int back from the interpreter ? So I added RollDice to my module. This doesnt error, but it doesnt return anything except blank spaces: HasGal> rollDice HasGal> Integers or nums should automatically have show correct? So this should show me something ? Ultimatly I want to get randomRs infinite list working so I can build randNums = (take (length popList) [1..]) where the length of pop list is how many random numbers I want. My code works as it, just need to replace [1..] with some random numbers. Thanks, C _ Add photos to your e-mail with MSN 8. Get 2 months FREE*. http://join.msn.com/?page=features/featuredemail ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Using Product Algebraic types
- I always get a bad feeling when people start to think about efficiency right from the beginning: First get your program correct and readable, then measure, and only then optimize (if at all). Programmers are notoriously bad when guessing about efficiency, which even more true for lazy functional programs. Sorry to have produced the *sigh* factor. I asked the question only to start learning, which your answer has certainly helped me with. Maybe I can present a differnt persepctive on performance though. I have to say up front though that if I was interested in squeezing the last drop of performance out of everything I wouldnt be learning haskell as all the web sites warn you about this :-) I do beleive in clarity, readbaility, correctness, time to develop etc.., it just that performance is one of those factors. My day job is all mostly about performance (from iterative programming perspective), I make complex business applications useable from a perfromance perspective. While most people say "just buy more hardware" or make it "correct or readable" first, my experince shows this leads to a lot of wasted effort which could have been avoided with a few seconds thought. Have you ever tried telling an IT Director that his 500 000 - 1 000 000 pound equipment purchase isnt cutting it for a "simple" business application :-) Let me tell you, they can have really selective hearing when it suits them :-) In my world, getting a program "correct" includes effieincy within reason. For exmaple, a 5 minute response time for interactive process can be pushed on employees with a relativilty minial cost of a small loss in productivity. However 5 minutes as a response for customer sitting on your web site is going to cost you millions and probably put you out of business. There are dozens of senarios like this where a "correct" program is as good as no program if it doesnt perform fast enough. What you say about guessing at performance is correct. I have given lectures on performance which start off with slides which say the trick to performance tuning is: "Measure, Measure, Measure" But I have found through experience (in the iterative world) that certian patterns of bad performance are easily avoided at development time because we know from experince they are bad at runtime. Performance has 3 flavours in my mind: 1) Hard Core 2) Design (Algortihms and data representation) 3) Common sense coding The fiirst, Hard code, invovles assemlber optimizations, branch reductions using boolean algebra etc.. No one argues these are mostly not worth the effort and so these are mostly ingnored expect for very special cercumstances. The phrase "buy more hardware" applies in this case 99% of the time. At the other end of the spectrum you have design. Few people will argue that getting your design right up front is very important. Changing Algorithms or Data Representation usually invovles changes of a significance that you have wasted your time on the "incorrect" version as you are starting mostly from scratch. Admitatly this looks a lot easier in haskell than in other impertive lanaguages. The 3rd area is where I differ from most people. Common sense coding is simply not calculating or looking something up over and over in an inner loop etc... While the programs can be changed afterwards to incorporate these, a few seconds thought by the developer avoids the bottle neck in the first place. An Example of the impact of such ineffiencies is this: Recenetly I "optimized" an interactive job which was taking over 10 minutes to run. Without understanding much about it (its highly complex) I was able to bring it down to < 1 minute applying simple common sense things. I never changed or looked at the "logic" behind it. Happy user base = productive company :-) That said haskell compiler seem pretty smart from what you have shown me so far ... which is a welcome change :-) Thanks for your reponse. This is the most helpfull community I have encountered so far .. :-) _ Help STOP SPAM with the new MSN 8 and get 2 months FREE* http://join.msn.com/?page=features/junkmail ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie questions
Hi Thanks for responding and taking the time to respond so fully. Its much appreciated! >From what I can see, a key difficulty you're having is with the "evolvepopulation" function. You've given it the type a -> a, which pretty much requires it to be an identity function, because it's not allowed to make any assumptions about the values it takes. I was going trying for generic as possible, I am keen to expore how much more expressive one can be in functional lanaguages. To make things easier to read, try defining types for Population and Individual (they can always be generalized later). > type Individual = Int > type Population = [Individual] A use fill tip, thanks. [Snip] It looks like you're defining evolution as the composition of mutate, cross, and select, but got tripped up by the order of evaluation. Specifically, "mutate cross select p" is the same as "((mutate cross) select) p", which probably isn't what you want. It only recently dawned on me that functions are left asscoaitive and your absolutly right its not what i had in mind. If you have: > mutate :: Population -> Population > cross :: Population -> Population > select :: Population -> Population Then you could define > evolve p = mutate (cross (select p)) > > -- alternative: > -- evolve = mutate . cross . select I now get what the f.g is as well :-) Thanks, the alternate format with the original has just clicked something else into place. Starting from an initial population, you want to generate a sequence by iterating evolve, starting from an initial population. Haskell provides a function "iterate" for this purpose, or you can define your own. Yeah other posters suggested this as well. I now have the the same function as you mentioned which is very nice: gaSolutionSpaceFrom :: a -> [a] gaSolutionSpaceFrom = iterate evolvePopulation [snip] I found it helpful to remember that you can think of a function f :: a -> b -> c as taking two arguments of types a and b and returning a result c, or as taking a single argument of type a and returning a result of type b -> c. Once you become comfortable with partial application, a lot of Haskell code starts to make more sense. I understood the partial application when explained in simple terms, but I find I have a hard time when it used in more coimplicated examples. The currying in the "The Haskell School of Expression" left with a big "Huh", even though a simple application makes sense to me. "The craft of Functional Programming" it seesm thicker more methdoical, so I may get some more of the basics down in the next few weeks. Thanks again. -- Message: 8 Date: Sat, 03 Jul 2004 08:38:55 + From: "Crypt Master" <[EMAIL PROTECTED]> Subject: Re: [Haskell-cafe] Newbie questions To: [EMAIL PROTECTED] Cc: [EMAIL PROTECTED] Message-ID: <[EMAIL PROTECTED]> Content-Type: text/plain; format=flowed [Snip] People say function application in Haskell is written without brackets but this can be misleading, here you do need brackets to indicate that 'gaSolutionSpace [1,2,3,4,5]' is one argument and not two. So you should write: take 5 (gaSolutionSpace [1,2,3,4,5]) [Snip] Thanks alot, this was very helpfull. It also makes more sense now that I looked up the associativity of functions and found it to be left assoc. For some reason I assumed it would automaically bracket form the right as such (take (5 (gaSolutionSpace [1,2,3,4,5]))) but its actually this (((take 5) gaSolutionSpace) [1,2,3,4,5]) Thanks again. _ Tired of spam? Get advanced junk mail protection with MSN 8. http://join.msn.com/?page=features/junkmail -- Message: 9 Date: Sat, 03 Jul 2004 08:46:24 + From: "Crypt Master" <[EMAIL PROTECTED]> Subject: [Haskell-cafe] Re: Newbie questions To: [EMAIL PROTECTED] Cc: [EMAIL PROTECTED] Message-ID: <[EMAIL PROTECTED]> Content-Type: text/plain; format=flowed >-- gaSolutionSpace :: [a] -> [a] >>gaSolutionSpace x = x : gaSolutionSpace (evolvepopulation x) -Stop deceiving yourself until it's too late. -Why did you comment out the type annotation? *Sheepish Grin* its historical, my original thought and attempt was that you would recieve a list of populations and evolve it to a bigger list of populations. Hence the [a] -> [a]. It didnt work out too well as this is what I came up with: gaSolSpace [x:xs] = gaSolutionSpace [x : evolePopulation x] Eventually i realised that I needed to evolve a single population, not a list, which let to a -> [a] and thanks to Keith I now have this: gaSolutionSpaceFrom :: a -> [a] gaSolutionSpaceFrom = iterate evolvePopulation Thanks _ Add photos to your e-mai
RE: [Haskell-cafe] Using Product Algebraic types
Hi Man, your a genius :-) Thanks for the help , still dijesting it. Interestingly enough I was playing with how to use sacnl1 just before I got this message from you, but as you mentioned I was battling with "kind" errors so I never got to test my idea besides on paper. Am I correct in assuming that your definition of Popoulation is now using tuple and not product types ? If so it it better to use tuples ? In the book the craft of func programing, it shows product type examples like this: data People = Person Name Age type Name = String type Age = Int Later it shows polymoric definitions like this: data Pairs a = Pr a a You mentioned that I had applied the polymorphic type "a" to Fitness, but but in the above example of person and people they have done the exactly what I did ? Used a space to seperate elements. So I am a little confused as to why mine didnt work. Regarding the use of newtype and data I saw another thread on this and I will use that get some insights on the differences. Your "rw" took some following until I realised currying was invovled *grin* > rw (Population xs) = Population (scanl1 f xs) > where f (n, a) = (+ n) `pair` id Can I ask one question, I am not concerned with performance at this point, but what sort of overhead does a function like id have. It seems unneccesary to me ( I am not critising your solution, I am vert thankfull for your help ) in a large populations you will land up doing a fair amount of extra but simple "reductions" ( I hope thats the right word. ) just to "copy" the unkown "a". Or does it have to be a function for some reason and so you had to use "id" ? Thanks, S Original Message Follows Crypt Master, CM> I need to be able to work with a list of items whos CM> structure is onyl partially know. That is at the level CM> of this module I dont care about what rest of it is. CM> So I have this: < type Fitness = Integer < data Population a = Population [Fitness a] Well, first of all: this will not compile. You've declared Fitness to be an synonym of Integer and Integer is not a parametric data type, i.e. it has kind *. In your definition for Population, however, you apply Fitness to a type argument. This will give you a kind error. CM> Hopefully this reads Population is constructed using CM> the Population constructor and is a list who elements CM> each conists a fitness value and some other value. So, no, it does not. I guess this is what you want: > type Fitness = Integer > data Population a = Population [(Fitness, a)] > deriving (Show) Now Population constructs a Population value from a list of which the elements are pairs of a Fitness value and a value of a specified type a. CM> Since I cant do poloymorphioc types with synonyms I CM> went with the data type. Well, actually, you can: > type Population' a = [(Fitness, a)] but type synonyms have the restriction that they cannot be partially applied. Another option might be > newtype Population'' a = Population'' [(Fitness, a)] which is only slightly different from the definition above involving data. CM> My current task is to build a roulette wheel CM> distribution of the fitness value. Basically I want to CM> build and incremental summing of the fitness value so CM> that each individual is paired with its upper range CM> like so CM> CM> Population [10 x, 20 y, 30 z] CM> New Population = [10 x, 20+10 y, 30+30 z] This can be accomplished by > rw :: Population a -> Population a > rw (Population xs) = Population (scanl1 f xs) > where f (n, a) = (+ n) `pair` id where pair is the maps a pair of functions to a function on pairs: > pair :: (a -> c) -> (b -> d) -> (a, b) -> (c, d) > f `pair` g = h > where h (a, b) = (f a, g b) A little test: > main :: IO () > main = print > $ rw (Population [(10, 2), (20, 3), (30, 5)]) This prints: "Population [(10,2),(30,3),(60,5)]". HTH, Stefan _ MSN 8 helps eliminate e-mail viruses. Get 2 months FREE*. http://join.msn.com/?page=features/virus ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Question on Exercise from SOE
Hi Not looking to get flamed, just offering my opinion. I got the SOE first, and while I like the higher level of it, the way he thinks etc... I found it hard to learn haskell from it. I just recieved "Haskell: The Craft of cuntional Programming" this weekend, and have made huge leaps forward (for me) based on the information within. I would have prefered to start with this book in place of SOE. So my advice is SOE is great but back it up with The craft .. S _ Add photos to your e-mail with MSN 8. Get 2 months FREE*. http://join.msn.com/?page=features/featuredemail ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Using Product Algebraic types
Hi My question is how do I select or work with product Alhebraic types ? Let you give you some context: I need to be able to work with a list of items whos structure is onyl partially know. That is at the level of this module I dont care about what rest of it is. So I have this: type Fitness = Integer data Population a = Population [Fitness a] Hopefully this reads Population is constructed using the Population constructor and is a list who elements each conists a fitness value and some other value. Since I cant do poloymorphioc types with synonyms I went with the data type. My current task is to build a roulette wheel distribution of the fitness value. Basically I want to build and incremental summing of the fitness value so that each individual is paired with its upper range like so Population [10 x, 20 y, 30 z] New Population = [10 x, 20+10 y, 30+30 z] My first thought was a list comprehension: rwList :: Population -> Population rwList (Population popList) = Population [ (rwListSum i) | i <- popList] rwListSum :: [a] -> [a] rwListSum (fitness individual) = (fitness*2, individual) ingoring for a minute that this wont achieve the above goal, I immediatly ran into the issue of how do I represent in the inner portion of the Population List, i.e "Fitness a". "(fitness individual)" doesnt work ? I tried treating it as 2 normal arguments, but of course how can I return it then ... ? A function can only return one thing. If its not a tuple then it wont work. Hence my question of how you represent product types ? I did come up with one idea, that is of making the internal part a type on its own so now I have this: type Fitness = Integer data InternalIndividual a = InternalIndividual Fitness a data Population a = Population [InternalIndividual a] ... rwList :: Population -> Population rwList (Population popList) = Population [ (rwListSum i) | i <- popList] rwListSum :: InternalIndividual -> InternalIndividual rwListSum (InternalIndividual fitness individual) = (InternalIndividual fitness*2 individual) To my surpirse this compiles. I have been able to fully test it yet though. So is there a way to do this without a extra type ? Is there a better way to represent this ? Thanks for your time, PS Sorry for all the dumb questions, I always find asking better than just wondering for ages. _ MSN 8 with e-mail virus protection service: 2 months FREE* http://join.msn.com/?page=features/virus ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] List syntax -- [] vs ()
Hi Thanks. A light bulb just went on :). I am still in awe of how much "special syntax" is made of such simple base. Haskell certainly is an interesting thing to learn, its like fractals, complexity and beuty from simplicity. S Original Message Follows---- Crypt Master, CM> I have noticed that lists seem to swtich between CM> using [] and using (). for example: CM> CM> listSum [] = 0 CM> listSum (x:xs) = x + listsum xs The parentheses are just 'normal' parentheses that are needed because application binds stronger than (:). Without the parentheses, you would get listSum x : xs which is the same as (listSum x) : xs . HTH, Stefan _ MSN 8 helps eliminate e-mail viruses. Get 2 months FREE*. http://join.msn.com/?page=features/virus ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Running Total or Incremental Sum as a higher order type ?
Hi I was attempting to get a running a total of a list i.e for [1,2,3] the result should be [1,3,6] -- [1, 1+2, 2+3] I build this function: incrementalSum [] x = [] incrementalSum (x:xs) runningTotal = currentSum : incrementalSum xs currentSum where currentSum = runningTotal + x I was trying to see how I could higher order functions for this but came up empty. Surely this is a pattern which has been abstracted ? I feel I have missed the obvious here. Thanks, S _ MSN 8 helps eliminate e-mail viruses. Get 2 months FREE*. http://join.msn.com/?page=features/virus ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] List syntax -- [] vs ()
Hi I have noticed that lists seem to swtich between using [] and using (). for example: listSum [] = 0 listSum (x:xs) = x + listsum xs but when specificy lists you use [] as in [1,2,3]. or type signatures are [a] -> [a] It also seems when they mentioned on the right hand side it also always []. Is it just for pattern matching that you use the "tuple" syntax ? How does haskell know we dont mean a tuple ? Or do we mean a tuple when we say (x:xs) ? Thanks _ MSN 8 with e-mail virus protection service: 2 months FREE* http://join.msn.com/?page=features/virus ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie questions
Hi Very very helpful, thanks. I wasnt sure about the "From" on the end, but I get the subtle change in way you see it. Much better than mine. Using you other advice I got: gaSolutionSpaceFrom :: a -> [a] gaSolutionSpaceFrom p = iterate evolvePopulation p and even managed to curry it :-) : gaSolutionSpaceFrom :: a -> [a] gaSolutionSpaceFrom = iterate evolvePopulation Thanks again. -- Kieth: [Snip] I think you mean: gaSolutionSpaceFrom :: a -> [a] gaSolutionSpaceFrom x = x : gaSolutionSpaceFrom (evolvepopulation x) gaSolutionSpace = gaSolutionSpaceFrom createRandomPopulation Note that "a" above should be replaced with your population type. Also note the "iterate" function in the standard library does just this. _ Tired of spam? Get advanced junk mail protection with MSN 8. http://join.msn.com/?page=features/junkmail ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Newbie questions
-- gaSolutionSpace :: [a] -> [a] gaSolutionSpace x = x : gaSolutionSpace (evolvepopulation x) -Stop deceiving yourself until it's too late. -Why did you comment out the type annotation? *Sheepish Grin* its historical, my original thought and attempt was that you would recieve a list of populations and evolve it to a bigger list of populations. Hence the [a] -> [a]. It didnt work out too well as this is what I came up with: gaSolSpace [x:xs] = gaSolutionSpace [x : evolePopulation x] Eventually i realised that I needed to evolve a single population, not a list, which let to a -> [a] and thanks to Keith I now have this: gaSolutionSpaceFrom :: a -> [a] gaSolutionSpaceFrom = iterate evolvePopulation Thanks _ Add photos to your e-mail with MSN 8. Get 2 months FREE*. http://join.msn.com/?page=features/featuredemail ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie questions
[Snip] People say function application in Haskell is written without brackets but this can be misleading, here you do need brackets to indicate that 'gaSolutionSpace [1,2,3,4,5]' is one argument and not two. So you should write: take 5 (gaSolutionSpace [1,2,3,4,5]) [Snip] Thanks alot, this was very helpfull. It also makes more sense now that I looked up the associativity of functions and found it to be left assoc. For some reason I assumed it would automaically bracket form the right as such (take (5 (gaSolutionSpace [1,2,3,4,5]))) but its actually this (((take 5) gaSolutionSpace) [1,2,3,4,5]) Thanks again. _ Tired of spam? Get advanced junk mail protection with MSN 8. http://join.msn.com/?page=features/junkmail ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Newbie questions
Hi I consider myself a pretty good imperative programmer, been at it for decades, and am tryibng my hand at Haskell now. Unfrituantly I am not "getting" it all that quickly despite having the multimedia haskell book. To start ghetting some hands on, I thought iI owuld do something which i have done many times before in imperatives, a basic Genetic Algorithm Library. Unfortunatly I am comming unstuck straight away and was hoping some of you kind folk could point in the direction of dry land :-) iIf your not familiar with GAs its not too important, simply a GA searches over the whole solution space randomly. Well not quiet, its a guided search, its guided by evolution ie by the fitness of each indicidual of a random initial population. Evolution is simulated by 3 basic operators, selection, cross over and mutation. selection is which indiviuals get to breed, cross over is how they breed, and mutation is just things intetresting. So after reading the multimedia book, Iimmediately thought of a defining the solution space as a infinite list. I was hoping then I could do things like take 5 gaSolutionSpace to get 5 iterations or generations. My first attempt tied to use lis syntax [], but it wouldnt compile and after seeing "numsFrom" ina tutorial I redefined it as such. Here is what I have so far: -- gaSolutionSpace :: [a] -> [a] -- gaSolutionSpace [] = gaSolutionSpace createRandomPopulation -- recursive base case gaSolutionSpace x = x : gaSolutionSpace (evolvepopulation x) evolvepopulation :: a -> a evolvepopulation p = mutate cross select p -- createRandomPopulation :: [Num a] createRandomPopulation = [1,23,4,5,6] cross p = p mutate p = p select p = p The take operator doesnt work on this. from hugs: HAGA> take 5 gaSolutionSpace [1,2,3,4,5] ERROR - Type error in application *** Expression : take 5 gaSolutionSpace [1,2,3,4,5] *** Term : take *** Type : Int -> [e] -> [e] *** Does not match : a -> b -> c -> d I am not sure I follow this. I assume its cause I didnt use the list notation in definaing gaSolutionSpace. Any ideas on how to do that. ? What about using map. It occurs to me that you can define gaSolutionspace as a map of the evolvolepopuloation function acrossan infinite solution space, but I dont know how to makethis infinte list to map over ...? There ar 2 papaers on GAs in haskell, but they use monads. I realise for performance I will probably have to use them too, but for now I would liek to do it without mondas even if performance isnt optimal. Sorry again for the newbie questions, but any help is appreciated. Stephen _ Add photos to your e-mail with MSN 8. Get 2 months FREE*. http://join.msn.com/?page=features/featuredemail ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Newbie questions
Hi I consider myself a pretty good imperative programmer, been at it for decades, and am tryibng my hand at Haskell now. Unfrituantly I am not "getting" it all that quickly despite having the multimedia haskell book. To start ghetting some hands on, I thought iI owuld do something which i have done many times before in imperatives, a basic Genetic Algorithm Library. Unfortunatly I am comming unstuck straight away and was hoping some of you kind folk could point in the direction of dry land :-) iIf your not familiar with GAs its not too important, simply a GA searches over the whole solution space randomly. Well not quiet, its a guided search, its guided by evolution ie by the fitness of each indicidual of a random initial population. Evolution is simulated by 3 basic operators, selection, cross over and mutation. selection is which indiviuals get to breed, cross over is how they breed, and mutation is just things intetresting. So after reading the multimedia book, Iimmediately thought of a defining the solution space as a infinite list. I was hoping then I could do things like take 5 gaSolutionSpace to get 5 iterations or generations. My first attempt tied to use lis syntax [], but it wouldnt compile and after seeing "numsFrom" ina tutorial I redefined it as such. Here is what I have so far: -- gaSolutionSpace :: [a] -> [a] -- gaSolutionSpace [] = gaSolutionSpace createRandomPopulation -- recursive base case gaSolutionSpace x = x : gaSolutionSpace (evolvepopulation x) evolvepopulation :: a -> a evolvepopulation p = mutate cross select p -- createRandomPopulation :: [Num a] createRandomPopulation = [1,23,4,5,6] cross p = p mutate p = p select p = p The take operator doesnt work on this. from hugs: HAGA> take 5 gaSolutionSpace [1,2,3,4,5] ERROR - Type error in application *** Expression : take 5 gaSolutionSpace [1,2,3,4,5] *** Term : take *** Type : Int -> [e] -> [e] *** Does not match : a -> b -> c -> d I am not sure I follow this. I assume its cause I didnt use the list notation in definaing gaSolutionSpace. Any ideas on how to do that. ? What about using map. It occurs to me that you can define gaSolutionspace as a map of the evolvolepopuloation function acrossan infinite solution space, but I dont know how to makethis infinte list to map over ...? There ar 2 papaers on GAs in haskell, but they use monads. I realise for performance I will probably have to use them too, but for now I would liek to do it without mondas even if performance isnt optimal. Sorry again for the newbie questions, but any help is appreciated. Stephen _ Help STOP SPAM with the new MSN 8 and get 2 months FREE* http://join.msn.com/?page=features/junkmail ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
