- Beginners - Haskell.org

Module import problem
by Casey Rodarmor 28 Sep '08

28 Sep '08

Hi there! I have a problem with importing a module I'd like to use. My working directory, ~/proj, contains: ./Haskore -- a folder containing a version of haskore, this music thingy ./test.hs -- random stuff using haskore The main file in ~/proj/Haskore is ~/proj/Haskore/Haskore.hs, which contains the following module declaration: > module Haskore(module HaskoreLoader) where > import HaskoreLoader I've tried to put all the following in ~/proj/test.hs, with no luck: > import Haskore -- Could not find module `Haskore': > import Haskore.Haskore -- file name does not match module name `Haskore' Am I doing something wrong? Is there a way to place a module in an arbitrary directory, without having to modify it? Thanks so much for your help! Best, Casey Rodarmor

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Haskell Preprocessor?
by Mike Sullivan 25 Sep '08

25 Sep '08

Hi All, What is the easiest and/or best way to define global "constants" in Haskell (avoiding any side effects, of course). The functionality I'm thinking of is similar to the use of global "const" variables or #defines in C code. Judging from this article ( http://www.lochan.org/keith/publications/hspp-hw99.ps.gz) and the existence of the GHC -cpp command-line option, it seems that it is possible to use the C preprocessor on Haskell code. However, the article is fairly old ('99)... Is this still (or was it ever) the best way? The point of the article is that it's not... were any of the "Haskell preprocessor" suggestions in the paper (dubbed "HsPP") adopted by GHC, and can we use them? There also seems to be a (almost fully) native implementation of a Haskell preprocessor in Hackage ( http://hackage.haskell.org/cgi-bin/hackage-scripts/package/cpphs) Does anybody use it? What would you use if you wanted such functionality? Thanks! Mike

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Default values for Data Types?
by Mike Sullivan 25 Sep '08

25 Sep '08

Hi all, I was wondering if there is syntactic sugar in Haskell for defining a default value for fields in a data type. For instance, say I have a type that is defined in record syntax: type CustomerID = Int type Address = Maybe String data Customer = Customer { customerID :: CustomerID , customerName :: String , customerAddress :: Address } deriving (Show) Is there any way to define default values for some (or all) fields such that they may be omitted from a declaration, and still have it generate a valid object? e.g.) a = Customer{customerID = 12, customerName="Bill"} -- I would like a{customerAddress} to default to Nothing (for instance). It seems to me that this would be a nice feature to have, if it does not exist. Am I missing something? Mike

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Why is there no notion of a one-tuple (e.g., a '([])' as opposed to a '[]') in Haskell?
by Benjamin L.Russell 24 Sep '08

24 Sep '08

I'm having difficulty in understanding the following behavior: In GHCi: Prelude> :type [] [] :: [a] but: Prelude> :type ([]) ([]) :: [a] I.e., the types of both the empty-list '[]' and the one-tuple containing the empty-list '[]' are '[a]' (a list of a generic type variable). According to "Chapter 2. Types and Functions" (see http://book.realworldhaskell.org/read/types-and-functions.html) of Real World Haskell (beta) (see http://book.realworldhaskell.org/beta/) >Haskell doesn't have a notion of a one-element tuple. Why not? It seems that a tuple is similar to a list, except that the elements need not be all of the same type, and that a tuple, unlike a list, cannot be extended. Yet: Prelude> :type [] [] :: [a] and Prelude> :type [[]] [[]] :: [[a]] so the types of the empty-list '[]' and the one-element list containing the empty-list '[[]]' are different. Forgive me if I am missing something, but something about this asymmetry bothers me.... -- Benjamin L. Russell

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not able to compile Happy GLR files
by trailblayzr 22 Sep '08

22 Sep '08

I'm unable to compile the GLR examples (in the 'glr' directory) provided with Happy, version 1.16. I'm using ghc version 6.6.1. I've looked at the version of Happy currently in darcs and the pertinent part of the examples has not been updated there, either. This Happy seems out of date because it uses the flag "-package data"; my searching indicates that the 'data' package is old. Other errors remained after I removed the flag. Would someone please verify the error? Also, does anyone know of or have a GLR example that works? Thank you for reading or helping.

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fold over elem?
by Logesh Pillay 20 Sep '08

20 Sep '08

I want to check if all the numbers in a list A are elements of a bigger unordered list B. Is there some way to fold /A into elem /B?

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Request for Another State Monad Example
by Mike Sullivan 19 Sep '08

19 Sep '08

Hi All, As I'm sure all Haskell beginners do, I'm having a bit of a struggle wrapping my head around all of the uses for monads. My current frustration is trying to figure out how to use the state monad to attach some persistent state between different calls to a function. I have two questions that I would appreciate it if somebody could help me with. The ubiquitous state monad example for Haskell tutorials seems to be a random number generator, with a function like the following (from http://www.haskell.org/all_about_monads/html/statemonad.html#example) getAny :: (Random a) => State StdGen a getAny = do g <- get (x,g') <- return $ random g put g' return x My first question is very basic, but here it goes: I see it everywhere, but what does the "=>" signify? Specifically, in this example what does "(Random a) =>" do in the type signature? My second question is more of a request, I suppose. I think it would be useful to get another example that does not have the added complications of dealing with the Random package, and saves more than one piece of data as state. How would one go about (for example) creating a Fibonacci sequence generator that saves the last state, such that on each call it returns the next number in the Fibonacci sequence? Thank you, Mike

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Looking for a data structure
by Levi Stephen 17 Sep '08

17 Sep '08

Hi, I'm looking for how best to represent selections from a set of items. For example, consider a soccer/football type game. You would have a Player data type. The game would store a player pool of all players in that game. Each club will have a number of players from that player pool, each match will involve that club selecting a number of players from its list of players. What is the best way to structure this data and hopefully at the type level enforce this subset type relationship? Thanks, Levi

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Random nub questions!
by Casey Rodarmor 15 Sep '08

15 Sep '08

Hi everybody, Haskell is great! I have some questions: Why is this: (+ 1) a curried invocation of the the plus operator, but this: (+ 1 2) is an error? And in the same vein, why is this a negative number: (- 1) instead of a curried invocation of the subtraction operator? Why can't I do this: 1 `(+)` 2 even if it is a little pathological? And this might be a little implementation dependent, although I'll hazard it anyways: Why can't I assign to things when running GHCi? Given my experience with C like languages, the first thing I wanted to do with GHCi was to start assigning to variables. I know that Haskell is purely functional, but it seems like it should be able to allow assignment from the interpreter by raising errors on rebinding. (Just like in erl, the erlang interpreter.) All the best, Casey

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Named (custom) recursions vs clever comonadic recursions
by Federico Brubacher 14 Sep '08

14 Sep '08

Hey all, Today I had a small exchange with another reddit user about CT and why is important to Haskell programming... What I don't get yet (as the subject says) is a real-world example on where you might apply cathegory theory to a recursion. Here is the small thread : fbru02: Sorry for maybe the OT posting but I have a question of my own: While learning Haskell and being a curious procrastinator I started learning Category theory but although I get , I don't get it yet. What I want to ask is practical uses of cathegory theory ? I know that it is used for high availability systems for theoretical proof that a System has the correct side effects. Also I know there are in reality recursions that might need this stuff as they are not as simple as what one would write in a toy project. What are this recursion schemes ?? What other uses there are for CT? nbloomf: Disclaimer- I am only beginning to understand this stuff myself. I am also a mathematician, not a computer scientist or a professional programmer. Two papers that give a good overview of what categories can do are "Functional Programming with Bananas, Lenses, etc." and "Recursion Schemes from Comonads". My understanding of the role of categories in FP (at least partly) is as follows. Arbitrary recursion is very useful to writers of programs. However, like arbitrary GOTOs, it can make it hard to see what's "really" happening in a program. A long time ago people figured out that fold/reduce was a common recursive pattern, but it was generally only used on lists and couldn't express all the recursion people wanted to do. More recently it was discovered that if your language is strongly typed, you can think of the types as objects in a category. Then some type constructors become endofunctors, and the traditional fold can be expressed as a particular map, parameterized by the functor, having a nice universal property. Moreover, as the "Recursion schemes from comonads" paper discusses, by parameterizing on the comonad the fold operator can become several different recursion schemes. This means a much smaller amount of code can express a larger number of ideas- even ideas that haven't been had yet. Moreover, it makes optimizations (such as fold fusion) much more powerful. So the real power of category theory is as a metalanguage- these ideas might never have become "obvious" without the language of functors, algebras, initial objects, and such. fbru02 : thank you for great response. Have you yourself used this parametraizing of the comonad? In which context or solving which problem and why couldn't you solving easily making a normal named recursion :D Thanks a lot ! :D Thanks ! -- Federico Brubacher www.fbrubacher.com Colonial Duty Free Shop www.colonial.com.uy

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