- Haskell-Cafe - Haskell.org

unneeded strictness
by Serge D. Mechveliani 20 Oct '04

20 Oct '04

Dear Haskellers, I am still trying to find out how to organize naturally a `lazy' printing of an accumulated data field. People advised me no-buffering and also the tilde usage. But there is some more obstackle. The `trace' is accumulated in fl1 by repeating consD, where data D = D {fl1 :: String, fl2 :: String} consD :: Char -> D -> D consD a d = d {fl1 = a:(fl1 d)} (tilde is not needed here?) The whole process computes res :: D in a certain complex recursive manner. One of the ways to use res is to print out the `trace' field: let res = (complexProcess ...) :: D in show $ fl1 res I need fl1 res to be formed `lazily'. For example, take 2 $ fl1 res may be ready (and shown) immediately, and last $ fl1 res may be ready long after. If it was [Char] istead of D, then this would be easy. Because head $ (a:) xs = a For example, head $ ('a':) $ (error "") = 'a' But is this true for consD and headD, where headD :: D -> Char headD = head . fl1 ? That is headD $ consD a res =?= a If res is not a bottom, then, yes. Right? And if res occurs, say, (error ""), then consD cannot intrude into fl1 res, and the whole expression has a value _|_. Right? So, generally, headD $ consD a res is not equivalent to a; hence the head members of fl1 res have to wait at least until it becomes clear that res is not a bottom. Right? So far, I see the two way-outs. 1. To insert the usage of the `trace' tool -- which does not look nice. 2. To have a result res :: [D] instead of D. And as soon as a new step appears, form res ++[d {fl1 = [step]}] where d = last res, fl2 also can be modified. Now, map fl1 res is the needed trace, which, I believe, would be formed `lazily'. I wonder a) whether there is a nicer solution, b) what happens if the Language treats the constructors as the homomorphisms. For example, head $ (a:) xs = a ==> head $ fl1 $ D {fl1 = (a:) fl1} = a Here (D, fl1) is a constructor, and its insertion between the functions head and (a:) preserves the above relation between them (is this called `bottom chasing' ?). Probably, this will eliminate unneeded strictness -- ? Thank you in advance for the explanations. Copy, please, the answer to mechvel(a)botik.ru ----------------- Serge Mechveliani mechvel(a)botik.ru

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Re: [Haskell-cafe] Re: subsequence
by MR K P SCHUPKE 20 Oct '04

20 Oct '04

Well I don't just want to give you the answer, but this code finds a subsequence in a list... This is from the HList library, and it looks for an ordered subtype ... but it also does what you want (it uses a four valued modal logic, which returns AllFalse, if list a is not a subtype (in-order) of b, (if the types are different there is no point in comparing values) or it returns SomeTrue, which has a value of either True or False depending on whether the list is an ordered sublist of values. -------------------------------------------------------------------- -- test for list a containing list b (elements of b occur in-order, but may -- be interleaved with other elements. class (HList r,HList s) => HContains' r s where hContains' :: r -> s -> Bool instance HContains' HNil HNil where hContains' _ _ = True instance HList r => HContains' (HCons a r) HNil where hContains' _ _ = True instance HList r => HContains' HNil (HCons a r) where hContains' _ _ = False instance (HContains' r (HCons a s),HContains' r s,Eq a) => HContains' (HCons a r) (HCons a s) where hContains' (HCons a r) s@(HCons a' s') = if a == a' then hContains' r s' else hContains' r s instance HContains' r (HCons b s) => HContains' (HCons a r) (HCons b s) where hContains' (HCons _ r) s = hContains' r s class (HList r,HList s,Logic.MBool t) => HContains'' r s t | r s -> t where hContains'' :: r -> s -> t instance HContains'' HNil HNil Logic.AllTrue where hContains'' _ _ = Logic.AllTrue instance HList r => HContains'' (HCons a r) HNil Logic.AllTrue where hContains'' _ _ = Logic.AllTrue instance HList r => HContains'' HNil (HCons a r) Logic.AllFalse where hContains'' _ _ = Logic.AllFalse instance HContains'' r s t => HContains'' (HCons a r) (HCons a s) t where hContains'' (_ :: HCons a r) (_ :: HCons b s) = hContains'' (undefined :: r) (undefined :: s) instance HContains'' r s t => HContains'' (HCons a r) s t where hContains'' (_ :: HCons a r) s = hContains'' (undefined :: r) s class (HContains' r s,Logic.Modal t) => HContains r s t | r s -> t where hContains :: r -> s -> t instance HContains' r HNil => HContains r HNil Logic.AllTrue where hContains _ _ = Logic.AllTrue instance (HContains'' r (HCons b s) t',Logic.And t' Logic.SomeTrue t,HContains' r (HCons b s)) => HContains r (HCons b s) t where hContains r s = hContains'' r s `Logic.and` Logic.toSomeTrue (hContains' r s) ---------------------------------------------------------------------------- Keean.

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STG questions
by Andrew Harris 19 Oct '04

19 Oct '04

Hi - I am reading Simon Peyton Jones's "Implementing lazy functional languages on stock hardware: the Spineless Tagless G-Machine" (I have version 2.5) paper. I would like very much to understand the contents of Figure 5 which illustrate the mapping of the "map" function from STG code to C. However I have some questions. For example, * It looks like map_closure is the "closure" in that it has an array of StgWords and the first word is a pointer to an info table. This info table contains map_entry as the entry code. I am curious about the relationship between map_entry and map_direct. Why isn't the stuff in map_direct done in map_entry? * The ret_vec1 is a continuation that's placed on the B stack. I am assuming this is a "non-vectored" return that has contents similar to Figure 7 (which is for vectored returns), but is this true? Also, the "upd_nil" and "upd_cons" functions aren't defined in the example. Maybe they just don't do anything? * In the return_cons1() function, how did the head and tail get put into the RegData1 and RegData2 registers? * I may have missed it, but it looks like the fy and mfy use three contiguous words of heap each. Can I assume that the triple {Hp[-5], Hp[-4], Hp[-3]} are of a structure similar to figure 4, the "Layout of a closure", which in this case would have some pointer/non-pointer words? thanks for any help, -andrew

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DSL for business logic
by Bhinderwala, Shoeb 19 Oct '04

19 Oct '04

I understand that there will be issues integrating with Java, but I want to consider a Haskell solution nevertheless. Are monadic programming concepts essential to create a DSL. I am just learning Haskell and haven't grasped fully the concepts of monadic programming yet. Referring to Paul's paper "Modular Domain Specific Languages", it seems like a DSEL in Haskell is just a plain Haskell program that uses functions to model the business domain. I am referring to the DSEL for gemoetric region analysis and Fran both of which look like code in familiar Haskell syntax. Our business problem is in the financial space where we have rules that check compliance on portfolios. Some example rules are: - The portfolio may not invest more than 40% in equity securities. - The portfolio may not invest more than 5% in any sector relative to the S&P500 index. - The portfolio cannot buy any securities whose issuers are domiciled in Japan. Currently we have high level rule expressions that get mapped to low-level code that embodies the business logic of checking the rules. Essentially most of our rules do the following: 1. Group, sort, and filter the portfolio's holdings. 2. Examine individual groups (also called 'buckets') and compute aggregated values. Eg. percent market value of each bucket. 3. Compare the value for each group with threshold values to check compliance I am wondering if is is a good idea to express the above logic in a Haskell DSL by creating data types for Holding, Bucket, Portfolio, etc. Would I be going on the right track by modelling a concentration rule as: -- A concentration rule takes a list of Holdings and a list -- of filter rules and returns an aggregate number. conc :: [Holding] -> [holdingFilter] -> Float holdingFilter :: Holding -> Boolean -- Shoeb -----Original Message----- From: Paul Hudak [mailto:paul.hudak@yale.edu] Subject: Re: [Haskell-cafe] Integrating Haskell into a J2EE environment I wouldn't write off Haskell so quickly. All of what Shoeb describes concerning DSL issues might be much more easily solved in Haskell, and will certainly be more flexible than a hard-wired approach. The J2EE interface might be ugly, but if the functionality needed is not too great it might not be too bad. Generally speaking, these kinds of apps -- in this case "a DSL for high-level business rules" -- sounds like just the sort of thing that Haskell is good for. -Paul Doug Kirk wrote: > You're going to spend alot of time marshalling between Java and > Haskell values, and you'll either have to do it via JNI or by using > pipes [as in System.exec("haskellprogram param param param")], both of > ... > > Take care, > --doug > > > On Oct 5, 2004, at 4:33 PM, Bhinderwala, Shoeb wrote: > > Hi All, > > I am new to Haskell and this mailing list. > > We have a system that uses a custom high-level language to express > high-level business rules. Expressions in the high-level language get > compiled to Java bytecode. We express the grammar using BNF notation > as required by the javacc parser tool. This is then converted to an > AST using jjtree and from there we build the final Java code. Our > language could be considered a domain-specific language (DSL) and is > used by our business users to express very high-level business logic. > The language currently is very limited - we support boolean logic, > function invocations and if-then statements. We want to convert it > into a more powerful scripting language so that even lower level > business logic can be expressed in it. > > I came across a few papers that talk about writing a DSL with Haskell > as the underlying support language. How is this done. Is it possible > to create a sort of domain specific business scripting language > easily. How does that then compile to Haskell code. And how can the > Haskell code be invoked from Java. > > Essentially, I am thinking if I could use a Haskell like DSL language > to express our business rule logic and then be able to integrate into > and invoke the logic from a J2EE app server environment. Has anybody > done anything like this with Haskell. > > -- Shoeb

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One-way and two-way monads.
by Kwanghoon Choi 18 Oct '04

18 Oct '04

Dear All, List and IO are both monads when appropriate operations are defined. The IO monad, which is a one-way monad, does not have a function of type IO a -> a, in general, except an unsafe function "unsafePerformIO :: IO a -> a". A two-way monad, such as List, has such a function "head :: [a] -> a". My rough question is: Is there any intersting theory on the relationship between one-way monads and two-way monads? For example, a restriction on the IO monad would offer a *safe* "unsafePerformIO", and so the IO monad would become a two-way monad. Does this question make sense at all? Thanks in advance. Kwanghoon Choi

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Subsequence near solved hopefully
by Peter Stranney 17 Oct '04

17 Oct '04

Sorry for annoying you all, i'm just very eager to move on with this and solve it. so far i have; equalChar :: Char -> Char -> Bool equalChar x n | x == n = True | otherwise = False equalString :: String -> String -> Bool equalString [] [] = True equalString [] (c':s') = False equalString(c:s) [] = False equalString(c:s)(c':s') = equalChar c c'^ equalString s s' this function is to see if one string is equal to another. but when i compile this i get the error; - Instance of Integral Bool required for definition of equalString does anyone know why i am getting this error and what i will need to do to transform the function above so that it checks is a list a subsequence of another list Peter Stranney --------------------------------- Do you Yahoo!? Yahoo! Mail - You care about security. So do we.

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Re: subsequence
by Peter Stranney 17 Oct '04

17 Oct '04

>> I am stuck on an exercise in a haskell book (the craft of functional >> programming) chapter 7 exercise 7.18 I have to write a function to check >> does one list occur as a subsequence of another, i am having real >> difficulty trying to do this. I have a method which will tell me if one >> character occurs in the list but it won't work for a list of >>characters. >> the function should look like isSubsequence :: [chars] -> [chars] -> >> boolean >> >Try writing a list or string equality function first? That may help. Thanks for getting back to me, have written this so far find :: [Char] -> Char -> Bool find [] n = False find (x:xs) n | x == n = True | otherwise = find xs x this will work to find if one character appears in a list but i dont understand how to call this recursively to find if a list occurs within a list. I have tried writing another function to call this but it won't work. Does anyone have a solution? thanks again Peter Stranney __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com

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Problem
by Michael Sterling 17 Oct '04

17 Oct '04

I am trying to create a function in Haskell which accepts a list, and goes through each element in the list, appending the element to a separate string, and then I want to print out that string (currently, it takes a list of values, and prints out the same list of values).

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Subsequence
by Peter Stranney 17 Oct '04

17 Oct '04

I am stuck on an exercise in a haskell book (the craft of functional programming) chapter 7 exercise 7.18 I have to write a function to check does one list occur as a subsequence of another, i am having real difficulty trying to do this. I have a method which will tell me if one character occurs in the list but it won't work for a list of characters. the function should look like isSubsequence :: [chars] -> [chars] -> boolean any help on this, even a polymorphic type function would be greatly appreciated Thank you Peter Stranney Peter Stranney --------------------------------- Do you Yahoo!? vote.yahoo.com - Register online to vote today!

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OO idioms redux
by John Goerzen 17 Oct '04

17 Oct '04

OK, recently I posed a question about rethinking some OO idioms, and that spawned some useful discussion. I now have a followup question. One of the best features of OO programming is that of inheritance. It can be used to slightly alter the behavior of objects without requiring modification to existing code or breaking compatibility with existing APIs. As an example, say I have a ConfigParser class. This class can read in configuration files, provides various get/set methods to access them, and can write them back out. Now say I would like to make this a little more powerful. Maybe I want to support the use of environment variables in my config file, so if there's a reference to $FOO in the file, it will be replaced by the contents of $FOO in the environment. In OO, I would make a new EnvConfigParser class. I'd override the read() method. My new read() would probably start by calling the parent's read() method, to get parsing for free. Then it could iterate over the data, doing its environment variable substitution. Now, in Haskell, we obviously have no objects like this. We do have something that provides some of the same benefits -- typeclasses. However, from what I can determine, they don't support algorithm inheritance like objects do in an OOP. Specifically, it seems impossible to have two instances of a single typeclass that work on the same type, while having one share most of the code with the other. I'm wondering if there is a Haskell design pattern that I'm missing that would provid ethe kind of benefits that one gets from inheritance in the OO world. -- John -- John Goerzen Author, Foundations of Python Network Programming http://www.amazon.com/exec/obidos/tg/detail/-/1590593715

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