August 2011 - Beginners

Re: [Haskell-beginners] main: <<loop>> ....?
by Sunil S Nandihalli 22 Aug '11

22 Aug '11

Is there a way to disable haskell automatic infinite loop detection .. that might make it simpler to figure out where it is happening by just looking at the trace-output in stdout.. Thanks On Mon, Aug 22, 2011 at 1:06 PM, David Virebayre <dav.vire+haskell(a)gmail.com> wrote: > 2011/8/22 Sunil S Nandihalli <sunil.nandihalli(a)gmail.com>: >> Hello everybody, >> main: <<loop>> -> where did this come from. I know I am not printing > > Your program has an endless loop that was detected by ghc. > > David. >

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How to convert a Data.Set to a Descending List?
by Sunil S Nandihalli 21 Aug '11

21 Aug '11

Hello everybody, Is there a more efficient way to convert a Set to a descending list? I was looking for a function similar to Set.toAscList something like Set.toDecList . I feel first converting to a ascending list and then reversing may be in-efficient given that I actually don't need all the elements only a last couple of elements.. Thanks, Sunil.

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Running a list of functions
by Manfred Lotz 20 Aug '11

20 Aug '11

Hi all, Maybe a stupid question. I have a list of functions [IO ()] and want to run those functions one after another. I did this runList :: [IO ()] -> IO () runList [] = return () runList (f:fs) = do f runList fs which works fine. However, I'm curious if there is a library function doing exactly this? -- Manfred

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Question: Data Type for user selection
by Hartmut 20 Aug '11

20 Aug '11

Haskellers, in the following testing code I want to model user selection criterias. Explanation of the requirement: - BasicSelect represents a single value (in the beginning: Number or String) - ExtendedSelect is a single value plus a sign if selection is meant inclusive or exclusive - MultipleSelections is a set of ExtendedSelect's I have some problems with that: - The BasicSelect shall only accept SelectionNum's or SelectionStr's. Currently it accepts anything? - The MultipleSelections shall only accept EmptySel or references to ExtendedSelect's. Can anybody give me a hint how do this? Does this work with plain data types? Hartmut module SelectionCriterias where data InclusiveOrExclusive = Inclusive | Exclusive data BasicSelect a = NumberSelect a | StringSelect a data Num a => NumberSelect a = SelectionNum a data Show a => StringSelect a = SelectionStr a data ExtendedSelect a = ExtendedSelect { basicSel :: BasicSelect a, inclOrExcl :: InclusiveOrExclusive } data MultipleSelections a = EmptySel | SingleSel a | MultipleSel [a] ----------------------------------------------------------------------------- -- Examples/Usage: -- e1 :: BasicSelect Integer e1 = NumberSelect 100 e2 = NumberSelect 110 -- e2 :: BasicSelect [Char] e3 = StringSelect "test3" e4 = StringSelect "test4" e5 = StringSelect "test5" -- f1 :: ExtendedSelect Integer f1 = ExtendedSelect { basicSel = e1, inclOrExcl = Inclusive } f2 = ExtendedSelect { basicSel = e2, inclOrExcl = Exclusive } -- f2 :: ExtendedSelect [Char] f3 = ExtendedSelect { basicSel = e3, inclOrExcl = Exclusive } -- multi1 :: MultipleSelections a multi1 = EmptySel -- multi2 :: MultipleSelections (ExtendedSelect Integer) multi2 = SingleSel f1 -- multi3 :: MultipleSelections (ExtendedSelect Integer) multi3 = MultipleSel [f1,f2] -- e.g. shall not be valid - because Bool not supported multi4 = MultipleSel [True, False]

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performance issues
by Sunil S Nandihalli 20 Aug '11

20 Aug '11

Hello everybody, I am a newbie to haskell. I was wondering if there are things I need to do to make the following code faster. I have tried changing the Integer to Int but did not help much.. https://gist.github.com/c1ee2d6397cd5b28ade5 Thanks in advance. Sunil.

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Trying to understand type families, continued...
by Michael Litchard 19 Aug '11

19 Aug '11

I'm playing around with type families, and using as a starting point, this example: class GMapKey k where data GMap k :: * -> * empty :: GMap k v lookup :: k -> GMap k v -> Maybe v insert :: k -> v -> GMap k v -> GMap k v Here's an instance that will lead to my question instance GMapKey Int where data GMap Int v = GMapInt (Data.IntMap.IntMap v) empty = GMapInt Data.IntMap.empty lookup k (GMapInt m) = Data.IntMap.lookup k m insert k v (GMapInt m) = GMapInt (Data.IntMap.insert k v m) Here is my experiment > class PM m where > data ServerModel m :: * -> * > movetoPreProcess :: m -> FilePath -> IO () I want to write an instance for this. As GMap can have an Int with some value v, I would like ServerModel to have a type I create (with another data declaration, call it Foo v) with values I define. Here's a little more detail for the sake of any explanation that might come my way. data ModelFoo a = ModelBar a | ModelBaz a ModelFoo is the type I want to use with ServerModel. What would the instance declaration look like? If ServerModel is defined within PM, should ModelFoo be as well?

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Could someone tell me where to find this example from the HaskellWiki?
by Michael Litchard 19 Aug '11

19 Aug '11

I was looking at this http://www.haskell.org/haskellwiki/GHC/Type_families And I found this http://darcs.haskell.org/testsuite/tests/ghc-regress/indexed-types/should_r… Which would be very useful. if it wasn't a dead link. Could someone mail me a copy, or a live url?

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Trying to understand type families.
by Michael Litchard 19 Aug '11

19 Aug '11

For this question, I'll refer to http://www.haskell.org/haskellwiki/GHC/Type_families#A_unit_instance In particular this example instance GMapKey () where data GMap () v = GMapUnit (Maybe v) empty = GMapUnit Nothing lookup () (GMapUnit v) = v insert () v (GMapUnit _) = GMapUnit $ Just v Could someone explain what the () is doing?

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Re: [Haskell-beginners] Where/Let clauses and subexpression repetition/memoization
by Brandon Allbery 19 Aug '11

19 Aug '11

On Aug 19, 2011 12:44 PM, "Tom Murphy" <amindfv(a)gmail.com> wrote: > On Fri, Aug 19, 2011 at 2:52 AM, Daniel Fischer < daniel.is.fischer(a)googlemail.com> wrote: >> [...] In most cases, you'll get recomputation, >> but GHC does a bit of CSE, so in some cases it will compute only once and >> share the result (which may be a bad thing - that's a further reason for >> not doing too much CSE, sometimes sharing has catastrophic results). > > This is a beginner's list, so I'll ask: what catastrophic result could it have, if it's computing pure functions? Catastrophic means nontermination, usually due to the stack or heap getting blown out. The classic example of catastrophic CSE is one of the simplest: computing the average of a list that won't fit in memory, such as [1..100000000000]. CSE, manual or automatic, would hold the entire list in memory, while manually fusing the sum and count operations over a fold runs in constant memory. Last I heard, GHC still didn't detect even that simple case, although it does handle some others. -- ...tabula non rasa. ++bsa

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Where/Let clauses and subexpression repetition/memoization
by Michael Serra 19 Aug '11

19 Aug '11

Dear Haskell Community, I have been bedeviled by a persistent confusion about one aspect of where/let clauses, which I think I can illustrate more easily with code than with words. Given code like this: averageR pxls = map (pcast . setR ravg) pxls' where (ravg,_,_) = averages pxls' pxls' = map pcast pxls What happens if I substitute pxls' for its right-hand side, averageR pxls = map (pcast . setR ravg) (map pcast pxls) where (ravg,_,_) = averages (map pcast pxls) My "superstition" here has been that the former only evaluates pxls' once, whereas the latter computes it twice. This seems like a basic issue which must have been confirmed or debunked somewhere in my readings, but it hasn't sunk in with me.

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