June 2013 - Beginners

Haskell for Programmers Workshop: Denver, Colorado
by Peter Jones 15 Jun '13

15 Jun '13

Since this is shameless self-promotion I'll keep it short. I'm teaching a Haskell workshop for imperative programmers in Denver, Colorado, September 16-18. If you want more information please take a look at the workshop website: http://www.devalot.com/workshops/haskell/index.html -- Peter Jones --- Love to Develop Devalot: http://www.devalot.com

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HTTPS Get Request with unverifiable certificate
by Friedrich Wiemer 12 Jun '13

12 Jun '13

Hey, I'm trying to send a HTTPS-Get Request to a private server, which has a self-signed ssl-certificate. Currently I use Network.HTTP.Conduit and this code-snipped: > myGetRequest url = do > req <- parseUrl url > return $ req {secure = True} > > *Main Network.HTTP.Conduit> myGetRequest "https://my.private.server" >>= (\x -> withManager (httpLbs x)) which results in > *** Exception: TlsException (HandshakeFailed (Error_Protocol ("certificate rejected: FQDN do not match this certificate",True,CertificateUnknown))) I guess that's due to the unverifiable, self-signed certificate? Can I disable the test or accept my certificate? Thanks in advance! Friedrich

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Haskell GTK
by Giacomo Tesio 10 Jun '13

10 Jun '13

I have to write a "simple" GUI that plot some timeseries with a few parameterization (dates, times and so on...). The task should be simple, but I'm find using the GTK binding quite weird. I'm surprised that nothing better exists for GUI programming in haskell. I read a few about FRP but AFAIK it doesn't adress simple desktop UI full of input to validate and grid/tree to show. Any suggestion? I've already read the pages on Hackage without much success. At least, can someone point me to the right direction for the codebase organization? Giacomo

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Type for lists with one or more elements
by Frerich Raabe 10 Jun '13

10 Jun '13

Hi, is there an existing (or conventional) way to model lists of one or more items? I often deal with functions with don't make sense with empty lists, and the caller already verified that the lists are not empty. So to avoid getting warnings from ghc, I'd like to have a dedicated type for this case. I could do something like data List a = Singleton a | Cons a (List a) but before I go ahead I wonder: is there an existing Haskell package for this? Maybe even with prettier names than what I wrote above, and convenient functions for transforming from/to plain [] lists? It would probably be straightforward to write it myself, but because of that, I suspect that somebody else already did it. :-) -- Frerich Raabe - raabe(a)froglogic.com www.froglogic.com - Multi-Platform GUI Testing

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Enum Float and Double
by harry 10 Jun '13

10 Jun '13

Why are Float and Double Enums? It doesn't seem to achieve anything useful, other than creating confusion such as http://stackoverflow.com/questions/7290438/haskell-ranges-and-floats. I've seen some threads from several years ago pointing out numerous problems with any attempt at giving these instances a sensible definition, but no explanation as to why they were created in the first place, or why they haven't been removed since.

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Is this a GHC bug? Problem inferring type
by Peter Hall 09 Jun '13

09 Jun '13

This is confusing me, and I'm wondering if it's a compiler bug. I have this: foo = (++) `on` show I'm expecting the type to be: foo :: Show a => a -> a -> String But GHCI is inferring: foo :: () -> () -> String If I explicitly provide a type annotation, or if I use -XNoMonormorphismRestriction, then it all works fine. I would expect a compile error if the type couldn't be inferred due to the Monomorphism Restriction, but instead it has silently chosen the least useful type. Peter

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type inference: how ghc chooses e.g. between Float and Double?
by TP 07 Jun '13

07 Jun '13

Hi all, In ghci, let us type: > data Worker x y = Worker x y deriving Show > let a = Worker 2 3.4 > :t a a :: Worker Integer Double According to which procedure does ghci chooses Integer for 2, and Double for 3.4? I believed that in such a case I was compelled to indicate a type signature explicitly, for example: > let a = Worker (2::Int) (3.4::Float) > :t a a :: Worker Int Float I knew that numbers in Haskell are polymorphic constants instances of the Num typeclass: > :t 2 2 :: Num a => a But what is happening exactly when he chooses Integer and Double above? Thanks in advance, TP

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How to remove leading and trailing non-alpha characters, and multiple consecutive spaces?
by Costello, Roger L. 07 Jun '13

07 Jun '13

Hi Folks, I have a string that contains a person's name. Prior to the person's name there may be some non-alpha characters. After the person's name there may be some non-alpha characters. Between the person's first name and last name there should be only one space. I want to remove the leading and trailing non-alpha characters and remove the extra spaces. Here is an example string: s = " \" John Doe \" " After processing, I should have: John Doe Below is my implementation. Is there is a shorter and more efficient implementation? ---------------------------------- import Data.Char import Data.List s = " \" John Doe \" " -- remove leading non-alpha characters t1 = dropWhile (not . isAlpha) s -- returns "John Doe \" " -- break the string up into a list of words, -- delimited by white space t2 = words t1 -- returns ["John","Doe","\""] -- create a string consisting of the first -- name, space, last name t3 = t2!!0 ++ " " ++ t2!!1 -- returns "John Doe" -- Put it all together: t4 = ((words . dropWhile (not . isAlpha)) s)!!0 ++ " " ++ ((words . dropWhile (not . isAlpha)) s)!!1

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The Missing Arrow Function Strikes Back
by Adrian May 06 Jun '13

06 Jun '13

Hi All, I just banged up against this problem: http://haskell.1045720.n5.nabble.com/The-case-of-the-missing-Arrow-function… Was liftA2 (not the applicative one) a bad idea, or is there another way to do it, or what? TIA, Adrian.

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Concrete instance of abstract class
by Adrian May 06 Jun '13

06 Jun '13

Hi folks, I just wrote this: import Diagrams.Prelude import Diagrams.Backend.Cairo type Dia = Diagram Cairo R2 class Turtlish t where pic :: t s -> Dia state :: t s -> s move :: s -> t s -> t s (>>>),(+++) :: t s -> t s -> t s x >>> y = x +++ move (state x) y The idea is that s is a turtle state, t s contains such a state along with a growing diagram, and >>> superimposes the diagrams after shuffling the right hand t s around according to the s I extract from the left hand t s. But I have different turtles planned. I want a regular turtle, a Sankey turtle which also has a width, and we could imagine sub-turtles that only had the angle or only the position. But in all cases, I think the above class describes how I want to compose diagrams together. I already tried it with monads but I'm now thinking that I want something like the above * instead* of a monad. So how do I use it to make a regular turtle? Maybe something like: data TurtState = TurtState P2 CircleFrac data TurtWorld s = TurtWorld Dia s I know exactly what s is above but otherwise the kinds don't match below. instance Turtlish TurtWorld where pic (TurtWorld d _) = d state (TurtWorld _ s) = s (TurtWorld d1 _) +++ (TurtWorld d2 s2) = TurtWorld (d1 `atop` d2) s2 move (pp,aa) (TurtWorld d (p,a)) = TurtWorld (d # rotate aa # translate pp) ( (p # rotate aa + pp) , (a+aa) ) Naturally, it barfs over move saying: Couldn't match type `s' with `(R2, t0)' `s' is a rigid type variable bound by the type signature for move :: s -> TurtWorld s -> TurtWorld s at turtle.hs:20:3 In the pattern: (pp, aa) In an equation for `move': move (pp, aa) (TurtWorld d (p, a)) = TurtWorld (d # rotate aa # translate pp) ((p # rotate aa + pp), (a + aa)) In the instance declaration for `Turtlish TurtWorld' because it hasn't a clue what (pp,aa) is and wants s totally generic anyway. But what am I supposed to do instead? Isn't it an everyday thing to use a generic pattern with a specific type? TIA, Adrian.

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