June 2011 - Beginners

Creating beautiful code: can you make this divide-and-conquer implementation of the "tails" function beautiful?
by Costello, Roger L. 28 Jun '11

28 Jun '11

Hi Folks, Below is a divide-and-conquer implementation of the "tails" function. Notice the two patterns (x:y:xs) and (x:[]). And notice that (x:y:xs) is used by the "length" function and again by the "splitAt" function. That doesn't seem elegant. Can the function be simplified and made beautiful? /Roger tails' :: [a] -> [[a]] tails' (x:y:xs) = map (++zs) (tails' ys) ++ tails' zs where m = length (x:y:xs) n = m `div` 2 (ys,zs) = splitAt n (x:y:xs) tails' (x:[]) = [[x]]

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DSL for Android development
by Sean Charles 27 Jun '11

27 Jun '11

As a yet to be published iPhone hacker, now somewhat saddened by the AppStore bouncers, I've turned to Android. I have a lot of Java and J2ME experience but only eight months with Haskell. I hate java. The JVM is awesome. Clojure is wish-fulfillment for Android right now. Having used Chicken (scheme) in the past for generating applications for people I know that compiling language A to language B can be very successful, especially when the end-user doesn't know or care how he got his app, only that it works. So... where would be a good place to start with a DSL that could produce Java code as its output? This is new to me! I've written simple lexers and parsers and I know the ropes on that front but I don't know haskell well enough yet to know how to go about it. I am thinking that I'd need to create data types for the major classes, or maybe not, if i can produce my own abstraction that generated multiple classes in the output etc and so i descend into confusion about where to start! I suspect that state carrying monadic types will be prevalent, good job I understand monads. Yes that was sarcasm. I've done the Scheme-in-48 hours thing, so I am ok with parsec. I did consider writing a "true" language that will code-generate java instead of a DSL, that's still in my mind too. As I understand DSL, it's up to me to invent my language to work within the "domain" of Android and then make it create Java code for subsequent feeding to the android tools. As a first guess, I am going to study the "Hello Android" example and then see how I could create a DSL around that little program. As I see it, programming boils down to just a couple of things: function calls and decisions. I appreciate that there is more to it: XML files and images etc to be bound in but that's nothing a good bash script won't fix for me! Thanks, Sean Charles. PS: I know this is a *big* thing to take on but I love big and I love learning so let's rip this one to bits.

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Doing IO in terms of Arrow
by Haisheng Wu 27 Jun '11

27 Jun '11

Hello, I'm wondering if it is possible doing IO in terms of Arrow. I got a function search all files under directory recursively and I'd like to refartoring using Arrow terms. I tried to define a function like below but failed. isDirExist = Kleisli doesDirectoryExist Do I need to define a instance for IO to be Arrow? Or is there any existing solutions? I'm a newbie of Haskell and thanks your help. ~~~~~ getFilesInDir :: FilePath -> IO [FilePath] getFilesInDir inp = do isDir <- doesDirectoryExist inp files <- if isDir then (do names <- getDirectoryContents inp forM [ inp </> x | x <- names, isNotSpecialDir x ] getFilesInDir) else return [[inp]] return $ concat files ~~~~~

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Would you shed light on "undefined values" please?
by Costello, Roger L. 27 Jun '11

27 Jun '11

Hi Folks, In this book [1] the author defines the term "bottom": In order that we can say that, without exception, every syntactically well-formed expression denotes a value, it is convenient to introduce a special symbol (upside down T), pronounced 'bottom', to stand for the undefined value of a particular type. In particular, the value of infinity is the undefined value (bottom) of type Integer, and 1/0 is the undefined value (bottom) of type Float. Hence we can assert that 1/0 = bottom. He defines infinity as this: infinity :: Integer infinity = infinity + 1 The author says this when discussing the Bool datatype: It follows that there are not two but three Boolean values, namely False, True, and bottom. In fact, every datatype declaration introduces an extra anonymous value, the undefined value of the datatype. What is the undefined value (bottom) of type Bool? What is the undefined value (bottom) of type String? If I create my own datatype: data MyBool = F | T What is the undefined value (bottom) of type MyBool? I am not clear why "bottom" is an important concept. Would you explain please? /Roger [1] Introduction to Functional Programming using Haskell by Richard Bird

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Do I understand this well
by Roelof Wobben 27 Jun '11

27 Jun '11

Hello, Im stil reading chapter4 Now we have this : addVectors (x1, y1) (x2, y2) = (x1 + x2, y1 + y2) do I understand it that the outcome of x1+x2 will be saved in the last expression x1 + x2 But on which variable can the outcome be found. Roelof

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don't understand this
by Roelof Wobben 27 Jun '11

27 Jun '11

Hello, I must find the k st number in a list. So I have this list [1,2,3,4] and I have to find the second number. According to the solution the answer is : elementAt list i = list !! (i-1) But where do I fill in [1,2,3,4] and the 2 ? Roelof

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first number is a list
by Roelof Wobben 27 Jun '11

27 Jun '11

Hello, I have to find the first number is a list. So first I tried : let mylast = last [1,2,3,4]mylast This one works but you have to give 2 commands. So i was thinking about using list compreshion. So I did : [x | x <- [1,2,3,4] , last ] But then I see these error : Could not match expected type "Bool" with actual type [a0] -> a0 in the expression last in stdn of a list compreshion last in the expression : [x | x <- [1,2,3,4] , last ] What is my thinking error ? Roelof

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Re: [Haskell-beginners] [web-devel] yesod database select all rows?
by Michael Snoyman 26 Jun '11

26 Jun '11

On Mon, Jun 27, 2011 at 2:20 AM, Eric Schug <schugschug(a)gmail.com> wrote: > Hello, > It does not seem obvious to me how to perform the equivalent of > > SELECT * FROM table_name > SELECT field1,field2 FROM table_name > > examples show something of the form > rows <- runDB $ selectList [ Field1Eq value1, Field2Gt value2] [ ] 0 0 > But I don't see how the name of the table is determined nor how to select > additional fields to extract. > > What would this do? > rows <- runDB $ selectList [ ] [ ] 0 0 This all has to do with Haskell type trickery. If GHC can determine the expected type of rows based on how you use it, it will work. If it can't, it will complain about ambiguous types. If you want to select from the Person entity, you can do something like: rows <- runDB $ selectList ([] :: [Filter Person]) [] 0 0 Michael

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Re: [Haskell-beginners] Understanding some notation
by Jack Henahan 26 Jun '11

26 Jun '11

I know the type signature of (++), I'm just not seeing `n` as a list, I suppose. I'm reading it as being the Int that is passed to splitAt, though perhaps my thought process is just wonky. On Jun 26, 2011, at 6:41 PM, Mike Meyer wrote: > On Sun, 26 Jun 2011 18:14:44 -0400 > Jack Henahan <jhenahan(a)uvm.edu> wrote: > >> I have this code snippet: >> >> import Data.List >> >> aaa x (y:ys) = case splitAt x (y:ys) of >> (n, x:xs) -> x:n ++ xs >> (n, xs) -> n ++ xs >> >> I understand what it's meant to do (that is, split a list at index `x` and make a new list with that element at the head, or just return the list when given a singleton), but my brain is failing me when trying to read the notation `n ++ xs`. >> >> Is there some obvious explanation that I'm just forgetting? > > Yes. But more importantly, you're forgetting that you can ask the REPL: > > Prelude> :t (++) > (++) :: [a] -> [a] -> [a] > > So it takes two lists and produces a new one. Trying it: > > Prelude> [1, 2, 3] ++ [4, 5, 6] > [1,2,3,4,5,6] > > So ++ concatenates it's operands. > > <mike > -- > Mike Meyer <mwm(a)mired.org> http://www.mired.org/ > Independent Software developer/SCM consultant, email for more information. > > O< ascii ribbon campaign - stop html mail - www.asciiribbon.org ==== "Computer Science is no more about computers than astronomy is about telescopes." -- Edsger Dijkstra ====

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Understanding some notation
by Jack Henahan 26 Jun '11

26 Jun '11

I have this code snippet: import Data.List aaa x (y:ys) = case splitAt x (y:ys) of (n, x:xs) -> x:n ++ xs (n, xs) -> n ++ xs I understand what it's meant to do (that is, split a list at index `x` and make a new list with that element at the head, or just return the list when given a singleton), but my brain is failing me when trying to read the notation `n ++ xs`. Is there some obvious explanation that I'm just forgetting? ==== "Computer Science is no more about computers than astronomy is about telescopes." -- Edsger Dijkstra ====

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