September 2008 - Beginners

Exposing Ratio data constructor
by Casey Rodarmor 29 Sep '08

29 Sep '08

Hej hej! I was writing some code using Ratio, and even though I know it's tucked behind an abstraction barrier, I really wanted access to the Ratio data constructor ':%'. I wrote invertRatio like such: invertRatio r = denominator r % numerator But I really wanted to write it like this: invertRatio (n :% d) = d % n I understand that exposing ':%' causes problems, since it allows us not only to pick apart ratios, but to construct bad ones that would normally be caught when constructed with '%'. (Such as '1:%0'.) Is there any way to avoid this, while still letting the user benefit from the nice pattern matching syntax that exposing the data constructor allows? Kram, Casey Rodarmor

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Exposing Ratio data constructor in prelude
by Casey Rodarmor 29 Sep '08

29 Sep '08

Hej hej! I was writing some code using Ratio, and even though I know it's tucked behind an abstraction barrier, I really wanted access to the Ratio data constructor ':%'. I wrote invertRatio like such: invertRatio r = denominator r % numerator But I really wanted to write it like this: invertRatio (n :% d) = d % n I understand that exposing ':%' causes problems, since it allows us not only to pick apart ratios, but to construct bad ones that would normally be caught when constructed with '%'. (Such as '1:%0'.) Is there any way to avoid this, while still letting the user benefit from the nice pattern matching syntax that exposing the data constructor allows? Kram, Casey

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Re: [Haskell-beginners] Module import problem
by Casey Rodarmor 29 Sep '08

29 Sep '08

On Sun, Sep 28, 2008 at 9:09 PM, Chaddaï Fouché <chaddai.fouche(a)gmail.com> wrote: > 2008/9/28 Casey Rodarmor <caseyrodarmor(a)gmail.com>: >> The comparison I was thinking of was with python, where a module can >> be placed in an arbitrary directory, and then accessed using a >> relative path. If I have 'stuff.py' that contains class 'Foo', I can >> move it to 'hello/module/stuff.py', and then import it is >> hello.module.stuff. > > And if you have something in hello/ it must refer to the same module > by module.stuff instead. I'm not sure it's clearer... Yeah, I can see where you're coming from. > Anyway the hierarchical modules in Haskell means that every code must > import the same module with the same name which I find saner (though I > could wish the import/export syntax would be more advanced to allow > for shorter imports when I have a lot of imports in the same hierarchy > part). > >> A python module doesn't need to know where it >> lives. > > Well, it doesn't need to, but everyone else needs to, in Haskell, the > module need to know where it lives, but everyone else don't care. > >> >> I often find myself working on computers where I don't have >> administrative privileges, which means I might not be able to install >> libraries in the 'right' places. This approach also makes it simple to >> create self-contained projects that can easily be moved from machine >> to machine, where only the local directory structure is important. >> Would there be any downside to this in haskell? > > Note that if the modules are in the same project, they're all in the > same hierarchy (at least that's how I see a project) and so their > names are coherent with each other and with the relative directory > hierarchy, there is no problem there. > Now, I think your preoccupation has more to do with packaging several > libraries you're not sure will be available on the final machine with > your project, and reasonably, you don't want to put them all in the > same base directory (yeah, it would be a mess). I would argue that > Haskell being a compiled language, its distribution problematics > aren't exactly the same as Python... If you're just distributing an > application you presumably would just compile it on your development > setup and then distribute the executable. Very true, but I still really like source distributions, even for compiled languages. Even though it can mean headaches, a source distribution is sometimes more portable than a binary distribution. (Usually because a clever end user can fiddle a little bit and get it working, whereas an incompatible binary is a useless blob.) > But if you really want to distribute the code source in a self > sufficient package (assuming GHC on the final machine I guess ?), you > should use Cabal : it automates dependency checking, compilation and > installation with several source directory and configurable target > installation directory. It's pretty easy to use too (mainly I suggest > you do copy/paste of a good project file on Hackage). I'll definitely check out Cabal. I'm a student, and so I'm always developing on personal and school computers, as well coding with random project partners (who always seem to be running equally random OSs). And then, to top it off, I've got to turn in assignments without being exactly sure where the grader is going to run them. I'm *always* wanting to use some library in a project, and (apt-get|port|emerge) my_sweet_library is never an option :-( Thus my concern with portable source packages. :-) These days I usually write crazy makefiles that try to do their best to compile and link in all the libraries needed, wherever they might find themselves. > > -- > Jedaï >

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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 24 Sep '08

24 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 24 Sep '08

24 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 18 Sep '08

18 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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