- Beginners - Haskell.org

find an element in a list
by Fabien R 26 Dec '15

26 Dec '15

As a newbie, I'm studying the pdf 'the Haskell road to logic, math and programming' and I'm stuck with one exercise. I want to extract x if x is at the beginning of a list. I thought to use something like this: extractIfBegins x [xs] | [xs] == (x:ys) = [ys] | otherwise = [xs] But ghci complains that ys is not defined. Without giving the answer, can someone give a hint about the approach to follow ? -- Fabien

3 3

warning in making instances of Functor class!
by Abhijit Patel 25 Dec '15

25 Dec '15

why am i getting warning in the following code?? though it works perfectly!!! Prelude> newtype Pair1 c b a=Pair1 {getPair1 :: (a,b,c)} Prelude> :{ Prelude| instance Functor (Pair1 m n) where Prelude| fmap f (Pair1 (x,y,z))=Pair1 (f x,y,z) Prelude| :} <interactive>:55:10: Warning: No explicit implementation for ‘Prelude.fmap’ In the instance declaration for ‘Functor (Pair1 m n)’ -- The below shows it is working fine! Prelude> getPair1 $ fmap (*100) (Pair1 (2,3,1)) (200,3,1)

2 1

FW: Initial startup of GHCi (Windows) v. 7.10.3
by pmcilroy＠gmail.com 25 Dec '15

25 Dec '15

Running on windows 10, GHC interactive fails to load any standard packages. The startup message does not include the prompt about initializing packages do, as shown commonly on Haskell getting started tutorials. Instead I see: GHCi, version 7.10.3: http://www.haskell.org/ghc/ :? for help Prelude> :l Data.Text <no location info>: module ‘Data.Text’ is a package module Failed, modules loaded: none. Prelude> Any pointers on what to do next?

2 1

Re: [Haskell-beginners] Doubts about functional programming paradigm
by Doug McIlroy 22 Dec '15

22 Dec '15

Though this post was prompted by beginners(a)haskell.org, it seemed pertinent enough for cross-posting to haskell-prime. > I just wanted to make a point that learning haskell is *much* > harder than learning most other programming languages I would not put it that way. One gets so much farther in capability almost instantly with a functional language. I know. At the dawn of functional programming, John McCarthy could teach in one hour everything from cons up to symbol manipulation that would have been a month's project in Fortran. (My account of that hour is at http://www.paulgraham.com/mcilroy.html.) What's hard about Haskell is that its landscape extends into terrains not imagined elsewhere. As long as you stay in the flat Floridian landscape of Fortran et al, you don't need to explore the Himalayas of Haskell. But of course the very exhilaration and inspiration of the high peaks draws one into the "*much* harder" training necessary to surmount them. > the learning aids that are available are not yet cohesive > enough to present a clear path ahead for the average programmer. I agree with this. Alas, there does not exist a definition of the Haskell one reads about on the haskell-cafe mailing list. I treasure my hardcover Haskell 98 report--so rare you can't even find it in the used-book marketplace. Haskell 2010 was obsolete as soon as it was promulgated. Contemporary Haskell as practiced by cognoscenti flaps in a gale of language pragmas. There is no authoritative source about these pragmas. They are listed and described in the GHC User Guide, but that source all too often defines solely by example, not even bolstered by a formal syntax specification. I earnestly hope the newly revived Haskell-prime committee can rectify this state of affairs, and that GHC will provide a compliant implementation. Doug McIlroy

5 6

Pattern matching is actually about matching constructors??
by Olumide 22 Dec '15

22 Dec '15

Hello, On chapter 7 of LYH there's an example of a user-defined operator .++ infixr 5 .++ (.++) :: List a -> List a -> List a Empty .++ ys = ys (x :-: xs) .++ ys = x :-: (xs .++ ys) which is used as follows let a = 3 :-: 4 :-: 5 :-: Empty let b = 6 :-: 7 :-: Empty a .++ b (:-:) 3 ((:-:) 4 ((:-:) 5 ((:-:) 6 ((:-:) 7 Empty)))) Following this the text reads: "Notice how we pattern matched on (x :-: xs). That works because pattern matching is actually about matching constructors. We can match on :-: because it is a constructor for our own list type ..." Source: http://learnyouahaskell.com/making-our-own-types-and-typeclasses#recursive-… Is the operator :-: a constructor? I'm confused because the definition of :-: is not prefixed by the data keyword? - Olumide

3 3

Mixing pattern matching and guards
by martin 22 Dec '15

22 Dec '15

Hello all, I recently wrote this piece of code import qualified Data.Set as S [...] purge :: (Eq cp, Eq ip) => Product cp ip -> Product cp ip purge (Punion ps) | ps' == S.empty = Pempty where ps' = S.filter (/= Pempty) ps purge x = x and I thought I had missed the "otherwise" case in the guard and I was prepared to see a "non exhaustive ..." error, but to my amazement it works *Main> purge $ Punion $ S.fromList [Packed 1 Pempty] Punion (fromList [Packed 1 Pempty]) As this is a Punion, it should match the first pattern, but not the guard. It seems to fall right through to the second pattern. Is this the way it works?

2 1

Re: [Haskell-beginners] How to nest arbitrary things
by Imants Cekusins 21 Dec '15

21 Dec '15

> how to write an "unpack" function is this what Lenses are all about? https://www.fpcomplete.com/school/to-infinity-and-beyond/pick-of-the-week/a… tutorial I stumbled across Lenses only recently so can not write a working example yet. if Lenses are relevant to this topic, could someone kindly post an appropriate (Truck Parcel Can) datatype and an "unpack" with Lenses example?

2 6

How to nest arbitrary things
by martin 21 Dec '15

21 Dec '15

Hello all, I was trying to model things which can contain other things. This is easy as long as containers and the contained items all can be described in the same fashion. However when I want to model - say - trucks containing boxes containing parcels containing cans and trucks, boxes, parcels and cans are not of the same type, then this nested thing will become a type in its own right and it will be of a different type than trucks containing cans (which are not wrappen in parcels ...) As long as I can squeeze trucks, boxes ... into one type, possibly by using a "container_type" attribute, there is no problem. Is this the only way to do this? Is there an idiom?

7 11

Problem reading sound data from mic with TChan
by Martin Vlk 21 Dec '15

21 Dec '15

Hi, I am working on a little toy project related to controlling graphics display based on data from the computer microphone. I am basing the whole thing on concepts from game programming, so I have a main loop which reads inputs, updates the world state and generates outputs. As part of inputs there is microphone. I read data from it using the pulse-simple library. The "simpleRead" function blocks if there is not enough data available so I can't use it directly in the main loop or I risk delays. So I figured I'll use a separate thread to read from the mic and write data into a TChan. The main loop in separate thread then can read from the TChan as needed and test for availability of data to avoid delaying the main loop. Here is my current code: http://lpaste.net/147522 The data is written into TChan in the "handleMic" function and read from the TChan on line 85. The problem I have is that the TChan never seems to contain any data when I read from it and that confuses me. Why? Does anyone see where is my problem? Many Thanks Martin

3 3

Questions About Rose Trees Analysis
by Lorenzo Isella 21 Dec '15

21 Dec '15

Dear All, I am trying to learn Haskell (for the second time), but I have to admit I am still struggling with relative simple stuff, so I ask if anybody can give me a hand. I am dealing with the issues of extracting info out of some rose trees and probably I am also having some troubles with the notation. The rose tree is defined with the following notation data Rose a = a :> [Rose a] deriving (Eq, Show) and the root can be detected simply as root (a :> rs) = a I would like to have the expression (with the ":>" notation for the Node) of the function to find the children of the root. Example of expected behavior of this function children children (1 :> [2 :> [], 3 :> []]) = [2 :> [], 3 :> []] On top of that, I am trying to get the functions to have the functions size :: Rose a -> Int leaves :: Rose a -> Int that count the number of nodes in a rose tree, respectively the number of leaves (nodes without any children). For the children function, I have tried stuff like children (a :> rs) = rs quite unsuccessfully. Any suggestion is appreciated. Cheers Lorenzo

5 6