September 2015 - Beginners

Config data
by Mike Houghton 19 Sep '15

19 Sep '15

Hi, I’ve haskell files parser.hs and email.hs email.hs is a module imported into parser and parser has the main. email has various passwords and server names needed to connect and send email. I want to have those details in a config file and read it in at start - using say config module. I’m ok with this part but the practicality of doing it eludes me... It seems to me that email module can either get the config details itself or be told them by parser. If email wants to get them how would it do this? It does not have a main that gets run so can’t load a config file.(can it ??) However parser can load the config in its main and then tell mail what the values are but how would mail save them? Of course I could chanage the signatures of the email send/receive functions in mail to take the connection details but that seems wrong. Thanks Mike

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Converting a data type to an abstract data type
by Ryan Warner 17 Sep '15

17 Sep '15

I had defined a data type similar to the following: data Record = Record { name :: String, age :: Int } Later, I realized I needed it be an ADT defined like: data Record a = Record { name :: String, age :: Int, resource :: a } The change turned out to be fairly well contained and probably only took me a half hour to propagate up. However, I see the potential for this to be a bigger job. Are there any editors that automate that kind of refactoring? -Ryan

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Re: [Haskell-beginners] [Haskell-cafe] Powerset of a set
by Francesco Ariis 14 Sep '15

14 Sep '15

On Mon, Sep 14, 2015 at 01:57:10PM -0500, JORGE MALDONADO wrote: > The powerset of set s is a set containing all subsets of s. > I need a clue on how to write Haskell code to get the superset of a set > using direct recursion and list comprehension. > > Best regads. This is good for haskell-beginners rather than haskell cafe. Clue: - say you have a list `l` [a,b,c,d,e] - you have the powerset of list `m` [b,c,d,e] - how can you use the `powerset of m` to calculate the `powerset of l`?

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How do ambigious types compile with Aeson
by Sevcsik András 13 Sep '15

13 Sep '15

Hi list, > ghci> decode $ fromStrict $ encodeUtf8 $ "{\"a\" : \"b\"}" > Nothing > ghci> decode $ fromStrict $ encodeUtf8 $ "{\"a\" : \"b\"}" :: Maybe Object > Just (fromList [("a",String "b")]) The question is: how can the first example compile? I would expect that since GHC cannot infer the type I want from Aeson to decode to, it would die with an error that "t0 type variable is ambigious". But instead, it compiles just fine, and fails runtime. How it's decided what type should Aeson try to decode to? What type is that gives me Nothing on whatever JSON input I give? Cheers, Andras Sevcsik -- Minden jót, Sevcsik András

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wierd quickcheck outcome
by Roelof Wobben 10 Sep '15

10 Sep '15

Hello, I have this function : fourDifferent:: Integer -> Integer -> Integer -> Integer -> Bool fourDifferent a b c d = ( a == b ) && ( a == c ) && (a == d) which I wants to test with this function : prop_fourDifferent :: Integer -> Integer -> Integer -> Integer -> Bool prop_fourDifferent a b c d = fourDifferent a b c d == ( a == b ) && ( a == c ) && (a == d) so I do quickCheck propFourDifferent and see this outcome : *Solution> quickCheck prop_fourDifferent *** Failed! Falsifiable (after 2 tests and 2 shrinks): 0 0 0 1 *Solution> fourDifferent 0 0 0 1 False *Solution> let a = 0 *Solution> let b = 0 *Solution> let c = 0 *Solution> let d = 1 *Solution> (a == b) && ( a == c) && ( a == d ) False *Solution> So why is it failing. both gives false , Roelof

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Applicative on Tree
by Mike Houghton 08 Sep '15

08 Sep '15

Hi, I’m looking at data Tree a = Node a [Tree a] deriving (Show) and trying to write the Applicative instance. I have I think a correct Functor : instance Functor (Tree) where fmap f (Node x tr) = Node (f x) ( fmap (fmap f) tr) but I’m well and truly stuck on Applicative :) I have instance Applicative (Tree) where pure x = Node x [] (Node f []) <*> tr = fmap f tr (Node f fs) <*> tr@(Node x xs) = ????? I cant get rid of the question marks! :) any pointers would be really appreciated! Thanks

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