- Haskell-Cafe - Haskell.org

Silly question: accessing a slot in a data type
by Fernando Rodriguez 09 Jan '08

09 Jan '08

Hi, I'm writing a very simple address book. I defined the follwoing types for Contact and AddressBook: type Name= String type PhoneNumber= Integer data Contact = Contact {name :: Name, phone:: PhoneNumber} deriving Show -- The addressBook is a lista of Contacts data AddressBook= AddressBook [Contact] deriving Show -- Create a test one ag = Agenda [Contact "BillG" 618965332, Contact "Linus" 5897458] I wan't to write a function to search for a given phone by name and return a Maybe PhoneNumber. I know how to do this recursively on a "raw" list, but how do I access the list inside AddressBook? Also, how do I represent an AddressBook with an empty list and one with data on its list for my pattern matching? Thanks!

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Basic question concerning the category Hask (was: concerning data constructors)
by Yitzchak Gale 09 Jan '08

09 Jan '08

Hi Andrew, Andrew Bromage wrote: > I still say it "isn't a set" in the same way that a group "isn't a set". > Haskell data types have structure that is respected by Haskell > homomorphisms. Sets don't. Ah, that's certainly true. But what is that additional structure? In categories that have a forgetful functor to Set, the interesting part of their structure comes from the fact that their morphisms are only a proper subset of the morphisms in Set. So in what way are Set morphisms restricted from being Hask morphisms? In two ways, I think: 1. They must be computable. 2. They must satisfy certain strictness criteria involving undefined and seq. (1) can be rather boring, since computability is rarely is an issue in real-life programming problems. It seems that some people using concepts from categories in Haskell would prefer to ignore (2) as well. If so, we have been reduced to viewing Hask as more or less the same as a small subcategory of Set, with no additional structure that is of interest. In this view, there is nothing special about Haskell, except for the fact that category-theoretic concepts are a bit easier to express in Haskell than in many other languages. So perhaps a better name for this category would be Turing. I don't think that ignoring (2) is the right approach. I would like Hask to reflect the non-strict nature of Haskell, which is one of its most interesting features. Some people are worried that this version of Hask is missing certain nice properties that one would like to have. For example, it was recently claimed on this list that tuples are not products in that category. (Or some such. I would be interested to see a demonstration of that.) I am not impressed by those complaints. As usual in category theory, you define corresponding notions in Hask, and prove that they are preserved under the appropriate functors. That should always be easy. And if ever it is not, then you have discovered an interesting non-trivial consequence of laziness that deserves study. For example: I think that "monads" in Haskell should satisfy the monad laws in Hask, not just after applying the forgetful functor to Turing. That means that (>>=) must be strict in its second argument. That would very likely not break any existing programs, yet almost none of the existing monads satisfy this, not even IO. Regards, Yitz

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Trouble with indentation
by Fernando Rodriguez 09 Jan '08

09 Jan '08

Hi, I'm trying to write a function that finds out the week day using the Zeller congruence (http://en.wikipedia.org/wiki/Zeller's_congruence). However, ghc complains about: parse error on input `=' at the "if m <= 2 then " line. I believe this is some sort of layout error. Can someone point out what I am doing wrong? Here's the code: data DiaSemana = Lunes | Martes | Miercoles | Jueves | Viernes | Sabado | Domingo deriving (Show, Eq, Ord, Enum) diaDeSemana d m a = toEnum num :: DiaSemana where num = zeller x y z zeller x y z = (700 + (26 * x - 2) `div` 10 + d + y + y `div` 4 + z `div` 4 - 2 * z) `mod` 7 if m <= 2 then x = m + 10 y = (a - 1) `mod` 100 z = (a-1) 'div' 100 else x = m - 2 y = a `mod` 100 z = a `div` 100

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Currying and Partial Evaluation
by Fernando Rodriguez 09 Jan '08

09 Jan '08

Hi, Is currying in Haskell the same thing as Partial Evaluation (http://en.wikipedia.org/wiki/Partial_evaluation)? Am I getting partial evaluation for free just by using Haskell? Thanks!

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Re: PROPOSAL: Some more 'Applicative' combinators
by Sterling Clover 09 Jan '08

09 Jan '08

The more general question, which stems from a certain degree of ignorance on my part, is what uses folks have found for Alternative at all outside of parsing. --S On Jan 8, 2008, at 4:43 AM, apfelmus wrote: > Iavor Diatchki wrote: >> Bryan O'Sullivan wrote: >>> Why not make the task specificity clearer, with a separate >>> Control.Applicative.Parsing module? > > +1 > >> Now _this_ would definitely suggest that they are only good for >> parsing which would be misleading. > > Would it? I mean, I currently don't know a second use case for > them. Preferably, the names like skipMany , endBy , sepBy etc. > should still make sense in the new context. > > > Regards, > apfelmus > > _______________________________________________ > Libraries mailing list > Libraries(a)haskell.org > http://www.haskell.org/mailman/listinfo/libraries

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Please allow beginners to vocalize code. >>= :: -> &&& <- -<
by Richard Kelsall 09 Jan '08

09 Jan '08

A tip for people explaining Haskell to beginners: The acts of reading and speaking are closely integrated in the brain. When I am unable to 'say' something it is much more difficult to 'read' and absorb the text. There appears to be a misconception that it somehow helps beginners to understand things if they are not told how to 'say' these strange new symbols. Certainly for me, and I would guess for most people, this idea is completely wrong. When I 'read' a new operator such as >>= I want know how to 'say' it. I don't mean that posts on Haskell-Cafe should do this, but books and articles aimed at people who haven't used Haskell before should always vocalize the symbols as soon as they are introduced. On a related note, if there isn't already, it would be nice to have a page in the wiki that gives good ways of vocalizing the operators and vocalizing them in simple code snippets. I might get round to doing this sometime, maybe just a table something like this : Operator Formal Informal -------- ------ -------- :: has type -> maps to to example f :: Int -> Int f has type Int to Int Or, to descend into trivia, a subtle distinction might be usefully made between = and == f i = 54 * i f i is 54 times i x == 27 x equals 27 Richard.

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Hackage web page
by Neil Mitchell 09 Jan '08

09 Jan '08

Hi, The hackage web page confuses me: http://hackage.haskell.org/packages/hackage.html As a user, when I go to that page, I would be seeking one of four pieces of information: 1) How to install a cabal package 2) How to create and upload a cabal package 3) What packages are on hackage 4) What is hackage The first paragraph answers question 4, before pointing off at the trac pages, which contain similar and overlapping information, as well as completely other stuff. The second paragraph basically answers 3 and 1, but in rather roundabout ways, not including a link to the answer to 3, but including a link about CabalInstall which you can't use yet. The rest of the page is notes for the hackage developers. I just went to hackage looking for the answer to question 2. I can't find it. I have a rough idea where some of the information might be hiding, but we really need a walkthrough tutorial - and I'm sure I've seen one of these before. I would think something more like the style of this might work better: http://www-users.york.ac.uk/~socs235/welcome.htm Hackage has now graduated from being a nice idea to being a critial user-focused thingy, which is great. Perhaps the website needs a little bit of thought along those lines. This isn't intended to be a complaint at all - just a new perspective to the people who develop it and probably don't read the web page at all. Thanks Neil

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Cabal and DB packages
by manu 08 Jan '08

08 Jan '08

Hello, I'm trying to do something that should be fairly simple, installing some DB package so I can use MySQL or SQLite. However I've had troubles building HSQL, HaskellDB and Takusen before giving up (I am using ghc 6.8.1 and Cabal-1.2.2.0). ------------------ with HSQL : Database/HSQL.hsc:66:7: Could not find module `System.Time': it is a member of package old-time-1.0.0.0, which is hidden so I added 'old-time' to the 'build-depends' line in the .cabal file but then it fails building with Building hsql-1.7... [1 of 2] Compiling Database.HSQL.Types ( Database/HSQL/Types.hs, dist/ build/Database/HSQL/Types.o ) Database/HSQL/Types.hs:134:18: Not in scope: type variable `forall' Database/HSQL/Types.hs:134:25: Not in scope: type variable `a' Database/HSQL/Types.hs:134:27: Illegal operator `.' in type `forall a . (Int -> FieldDef -> (FieldDef -> CString -> Int -> IO a) -> IO a)' (Use -XTypeOperators to allow operators in types) etc... At this point, it goes over my head... --------------- What the hell, I'll use HaskellDB and FlatDB, but then I get this message while building HaskellDB : src/Database/HaskellDB/HDBRec.hs:106:11: Illegal signature in pattern: l f a Use -XPatternSignatures to permit it when I add 'PatternSignatures' to the 'Extensions:' line in .cabal, it fails configuring with : Setup.hs: haskelldb.cabal:8: Parse of field 'extensions' failed: Jeeeesus, nevermind, I'll try Takusen --------------- with Takusen : $ runhaskell Setup.hs configure Setup.hs:26:7: Could not find module `Distribution.Program': Use -v to see a list of the files searched for. I give up... ---------------- Is there any simple way to do it though ? Is there any problems with Cabal that I need to work around ? Help !! Manu

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Newbie question: the need for parentheses
by Fernando Rodriguez 08 Jan '08

08 Jan '08

Hi, I have this function that sums up a list of numbers: suma [] = 0 suma (h:t) = h + suma t However, why do I need the parenthes in the second clause? What does the compiler think I'm trying to do when I type suma [] = 0 suma h:t = h + suma t Thanks! :-)

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Compiler backend question
by Peter Verswyvelen 08 Jan '08

08 Jan '08

If I understand it correctly, the GHC compiler either directly generates machinecode, or it uses C as an intermediate language. I also read somewhere that C is not the most efficient intermediate representation for functional languages, and that one gets better performance when generating native machine code. However, from the discussions in this list, I could conclude that the low level machine code optimizer of GHC is far from state-of-the-art compared to industry standard C/C++ compilers. I was wondering, why doesn't GHC use the GCC (or any other standard compiler) backend intermediate code? The backend of GCC generates highly optimized code no? Or is the intermediate code format of GCC (or other compilers) not suitable for Haskell? Another question regarding the backend: a cool feature of the Microsoft Visual C++ (MVC) compiler is its ability to perform "LTCG" (link-time-code-generation), performing whole program optimization. It something like this possible with Haskell / (or the GCC backend?). Would it be possible to feed all the generated C code of the GHC compiler into the MVC compiler, to generate one big MVC / LTCG generated executable? It would be interesting to see how much the whole program optimization approach (which can do analysis on the program as if it was a single module) would improve performance... Cheers, Peter

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