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[Haskell-begin] Re: [Haskell-cafe] Trouble with non-exhaustive patterns
by C.M.Brown 22 Jul '08

22 Jul '08

Hi Fernando, I hope you don't mind, but I've moved this over to the Haskell-beginners mailing list, where I think this kind of question will be more appropriate. In Haskell, it helps to think of functions in terms of an input and an output, that is, what is the thing that is going into the function; and what is the thing that is coming out of the function? In your function, final, the input is clearly a list of something (we can denote this by the Haskell type [a] which means a list of some type, a). Its return type is clearly an element of the list, so that must be the something type (the 'a' from the [a]). This gives the Haskell type: final :: [a] -> a (final takes a list of 'a' and gives back a single 'a'. The 'a' is a type variable, and it is used to denote that anything can be put in its place, so we can give final a list of integers, characters, whatever). Now, let's take a look at your definition of final. If we take a closer look, in fact only two equations satisfy this type: final [a] = a final (_:t) = final t The other takes a list and returns a list. The equation, final [] = [] takes an empty list and returns an empty list (its type is therefore [a] -> [a]). This is why you got an error, as Haskell doesn't know how what to do with the conflicting equation. What we need is the final element of the list. How do we do that? Let's think of the simple cases first. The final element of a list containing a single element is just that element, final [a] = a But what about if the list contains more elements? Or is an empty list? The empty list may be confusing, as an empty list contains no elements, so in effect, we can't return anything. We can, however, return an error message. fun [] = error "empty List" And the final element of any list, must be the final element of its tail: final (_:t) = final t this gives us: final :: [a] -> a final [] = error "Empty List" final [a] = a final (_:t) = final t I hope that gives some insight. Kind regards, Chris. On Mon, 21 Jul 2008, Fernando Rodriguez wrote: > > Hi, > > I defiend the following function to get the last element of a list: > > final [a] = a > final (_:t) = final t > > and it works as expected. Since I didn't want to have a non exhaustive pattern, > I added the following case: > > final [] = [] - I consider that the end of an empty list is the empty list > final [a] = a > final (_:t) = final t > > Suddenly, the function stoped working with a rather cryptic (for a newbie > at least) error message: > > *Temp> final [4,5] > > <interactive>:1:9: > No instance for (Num [a]) > arising from the literal `5' at <interactive>:1:9 > Possible fix: add an instance declaration for (Num [a]) > In the expr*Temp> ession: 5 > In the first argument of `final', namely `[4, 5]' > In the expression: final [4, 5] > > What have I done so wrong? > > Thanks in advance, > Fernando > > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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[Haskell-begin] Alternating sequence
by Dirk Markert 22 Jul '08

22 Jul '08

I am trying to generate the following list: 2, 3, 5 -- and then alternating (+2) resp (+4) -- 7, 11, 13, 17, 19, 23 I came up with the following solution 2:3:unfoldr (\(a,b) -> Just (a,(a+b, if b == 2 then 4 else 2))) (5,2) Are there easier ways to generate the desired list? Dirk

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[Haskell-begin] Book Suggestion
by Steve Klabnik 21 Jul '08

21 Jul '08

Chad- I recently purchased "The Haskell School of Expression" by Paul Hudak. http://books.google.com/books?id=lQbth9j5j9oC&dq=the+haskell+school+of+expr… I felt that it was a pretty decent introduction to the language. However, I was disappointed by its use of an (apparently) Win32 only library. While I haven't found the time to work through the exercises, that's what the book said. This may have changed. I plan on working through the book again in a few weeks after my summer classes are over.

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[Haskell-begin] invoking GHCi programtically ?
by Krassimir Krustev 20 Jul '08

20 Jul '08

Is it possible to invoke GHCi from Haskell code? I want to achieve the following effect: have a shell script invoke runhaskell myprog.hs < input.dat and at some point (on conditions) have the prog bring me the ghci prompt GHCi Main> so I can examine my program state interctively, for example place breakpoints, run my functions piecewise? and then do GHCi Main>:continue and then have the prompt again (if conditions) GHCi Main> I did try ghci myprog.hs < input.dat Command-line arguments ghci myprog.hs a b c are similarly by ghci not acceptable In the overall, I want to have the stdin redirected from file once in the GHCi session, place breakpoints and do :step(s) with that stream in this way and cannot also do GHCi Main>:main < input.dat but in my case (I think) this not also an option since its not me but the wrapper script doing the setup. krassi

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[Haskell-begin] .hi files
by B 19 Jul '08

19 Jul '08

So it looks like the ghc libraries come installed in *.hi files. This looks to be a binary library. Is that the case? Is there a tool to see what Haskell functions and etc are in a .hi file? Ben

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For starters...
by Rafael Gustavo da Cunha Pereira Pinto 19 Jul '08

19 Jul '08

I am trying to write a toy function to sum all members of a list but the biggest. This list is entirely composed by integers bigger than 0. I have: f::[Integer]->Integer f a = x a - y a where x=sum y=foldr max 0 Is there any way to implement that using composition, so I have a point-free implementation? -- Rafael Gustavo da Cunha Pereira Pinto Electronic Engineer, MSc.

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Re: [Haskell-begin] Tangential Studies?
by Steve Klabnik 19 Jul '08

19 Jul '08

> > I'd also recommend > > Richard Bird. Introduction to Functional Programming in Haskell. > > Added to my list, thank you. > While not directly related, taking math courses improves your Haskell skills > automatically :) Discrete mathematics is the umbrella term for the math you > want, which includes things like graph theory, combinatorics and logic. > Intuitionistic logic and type theory are directly related to Haskell. > > Looking at the way the classes are structured, apparently most of these classes have a single, identical pre-requisite. Maybe I should bump one of my fall classes back to spring so that I can take the pre-req in the fall, and take a few of those before I graduate. Hm... Try some problems at > > http://projecteuler.net/ > > I've done one or two. I remember getting stuck on a problem involving prime numbers, due to not generating them efficiently enough to have it run in any reasonable amount of time. Maybe I'll dust that off and ask for help here.

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Re: [Haskell-begin] Book Suggestion
by Steve Klabnik 19 Jul '08

19 Jul '08

> http://haskell.org/soe/software1.htm > > I have the graphics library running fine under linux, I haven't got to > music yet. I found that the SOEGraphics module was actually called > SOE, and comes with the SOE source. > Excellent. When it comes time to start, that'll help a lot, thank you. -Steve

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[Haskell-begin] Tangential Studies?
by Steve Klabnik 19 Jul '08

19 Jul '08

Hello everyone- Unfortunately, at my university, we don't do much in the way of functional programming. At all. So I've been trying to work on it outside of class as much as possible. I'm going through the exercises in SICP. I've picked up "The Haskell School of Expression," as well as " Basic Category Theory for Computer Scientists." (which I'm finding is not very basic, at least for someone as new to the topic as I am) I've subscribed to this mailing list. But what else can I do? While my fall classes are scheduled, my spring classes must consist of 15 credits of anything I want to take. Should I take some math courses? Which ones? Anything related that I should study? The best way to improve coding ability is to actually code. But I'm attempting to do everything I can, and so suggestions as to how you all improve yourselves and your abilities would be welcomed.

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Book Suggestion
by Chad Wilson 18 Jul '08

18 Jul '08

Have any of you fellow n00bs ordered a book on Haskell? Or are you just using the haskell.org web site? I think I need to get me a book to keep as a reference. -- Chad Wilson

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