- Beginners - Haskell.org

Get rid of Maybes in complex types
by Baa 06 Jul '17

06 Jul '17

Hello Dear List! Consider, I retrieve from external source some data. Internally it's represented as some complex type with `Maybe` fields, even more, some of fields are record types and have `Maybe` fields too. They are Maybe's because some information in this data can be missing (user error or it not very valuable and can be skipped): data A = A { a1 :: Maybe B ... } data B = B { b1 :: Maybe C ... } I retrieve it from network, files, i.e. external world, then I validate it, report errors of some missing fields, fix another one (which can be fixed, for example, replace Nothing with `Just default_value` or even I can fix `Just wrong` to `Just right`, etc, etc). After all of this, I know that I have "clean" data, so all my complex types now have `Just right_value` fields. But I need to process them as optional, with possible Nothing case! To avoid it I must create copies of `A`, `B`, etc, where `a1`, `b1` will be `B`, `C`, not `Maybe B`, `Maybe C`. Sure, it's not a case. After processing and filtering, I create, for example, some resulting objects: data Result { a :: A -- not Maybe! ... } And even more: `a::A` in `Result` (I know it, after filtering) will not contain Nothings, only `Just right_values`s. But each function which consumes `A` must do something with possible Nothing values even after filtering and fixing of `A`s. I have, for example, function: createResults :: [A] -> [Result] createResults alst = ... case of (a1 theA) -> Just right_value -> ... Nothing -> logError undefined -- can not happen Fun here is: that it happens (I found bug in my filtering code with this `undefined`). But now I thought about it: what is the idiomatic way to solve such situation? When you need to have: - COMPLEX type WITH Maybes - the same type WITHOUT Maybes Alternative is to keep this Maybes to the very end of processing, what I don't like. Or to have types copies, which is more terrible, sure. PS. I threw IOs away to show only the crux of the problem. --- Cheers, Paul

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Incomplete patterns
by Jona Ekenberg 06 Jul '17

06 Jul '17

Hello, I am currently going through the CIS194 spring 13 course, since it has been recommended by several haskellions. I find the exercises extremely valuable, and the reading material is succinct (in a good way). I just finished the exercises for lesson two, and while everything works, I get this warning: Lektion02.hs:(38,1)-(44,21): warning: [-Wincomplete-patterns] … Pattern match(es) are non-exhaustive In an equation for ‘insert’: Patterns not matched: (LogMessage _ _ _) (Node _ (LogMessage _ _ _) _) I understand that there is a pattern I'm not covering, but I can't really figure out which one. Here is my function: insert :: LogMessage -> MessageTree -> MessageTree insert (Unknown _) mt = mt insert m Leaf = Node Leaf m Leaf insert m@(LogMessage _ t _) (Node left value@(LogMessage _ t' _) right) | t <= t' = Node (insert m left) value right | t > t' = Node left value (insert m right) insert m@(LogMessage _ _ _) (Node left (Unknown _) right) = Node left m right (here's a pastebin with intact indentation: http://lpaste.net/356716) I am not sure on how I should progress to find out which pattern I'm missing. Any help is appreciated. Kind regards, Jona

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Reversing a list
by Jona Ekenberg 04 Jul '17

04 Jul '17

Hello, I'm currently going through the exercises in chapter 3 of Real World Haskell. One of the exercises challenges me to create a function which takes a list and makes a palindrome of it. I tried to solve it this way: palindrome :: [t] -> [t] palindrome xs = xs ++ (reverse xs) where reverse [] = [] reverse (x:xs) = (reverse xs) ++ x But when I try to compile this I get this error: Kapitel3.hs:14:32-33: error: … • Couldn't match type ‘t’ with ‘[t]’ ‘t’ is a rigid type variable bound by the type signature for: palindrome :: forall t. [t] -> [t] at /Users/jona/programmering/haskell/boken/Kapitel3.hs:13:15 Expected type: [[t]] Actual type: [t] • In the first argument of ‘reverse’, namely ‘xs’ In the second argument of ‘(++)’, namely ‘(reverse xs)’ In the expression: xs ++ (reverse xs) • Relevant bindings include xs :: [t] (bound at /Users/jona/programmering/haskell/boken/Kapitel3.hs:14:12) palindrome :: [t] -> [t] (bound at /Users/jona/programmering/haskell/boken/Kapitel3.hs:14:1) Compilation failed. It looks like I have used a function that want a list of lists, but I don't understand where. Also, is there some way to declare the type of my reverse-function in this case? Kind regards, Jona Ekenberg

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pure instead of return?
by Silent Leaf 02 Jul '17

02 Jul '17

hi, i just wanted to know, if i use pure instead of return, if there's any single risk of getting a different implementation of the function, or anything of that kind. also, am i right to assume that GHC will do what is needed so there's no actual redirection in the compiled code, in the case pure is defined in terms of return? thanks

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lost a typeclass maybe?
by Silent Leaf 30 Jun '17

30 Jun '17

hi, i keep trying to find something that feels terribly obvious but i can't make any link. say i have a function of the following type: foo :: (a, b) -> ([a], [b]) -> ([a], [b]) or perhaps more generally: foo :: SomeClass f => f a b -> f [a] [b] -> f [a] [b] is SomeClass supposed to be BiFunctor or something else? clearly, what i want to do is to combine the elements of the first pair into the elements of the second, preferrably without pattern matching, that is, merely in function of (:). i think the problem with bifunctor is that it seems to only allow the application of both arguments in a separate fashion. but here the first argument is in one block, that is (a,b). i know, ofc we could do something like: foo pair pairList = bimap (fst pair :) (snd pair:) pairList or maybe use curry or whatever. but i'd like my pair to not need to be unboxed! is there not a way to not have to manually call fst and snd? are both of these functions typeclass methods by any chance? then we could write a generalized function that could work for any f = (:) or any kind of pair-like thingy. mind you i'm not sure to which extent it would keep the opacity of the type constructor (,). especially, it's a bit like unboxing the Maybe type constructor: you can do it manually by pattern matching, but when you have the exact same issue but with IO, it's not possible anymore to unbox the underlying type equally, i bet one could invent IO a b, in a way that you could not just get a and b, but you could somehow implement opaqueBimap :: (i -> k i) -> f a b -> f (k a) (k b) -> f (k a) (k b) with here of course f = (,), k = [] or List, and (i -> k i) = (:)

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special polymorphic default of typeclass depending on other typeclasses?
by Silent Leaf 29 Jun '17

29 Jun '17

hi, say i have the following typeclass: class Foo a where bar :: a -> String looks a lot like the Read typeclass, right? (at least i think it should?) well say it's a different meaning (in other terms i can't or do not want to use Read, but i'd like to implement a default version of bar for those instances that also implement Read. is there a way to do so?

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Re: [Haskell-beginners] special polymorphic default of typeclass depending on other typeclasses?
by Jean Lopes 29 Jun '17

29 Jun '17

It appear more like Show, you can add a constraint to the generic instance instance Show a => Foo a where bar x = show x Em 29 de jun de 2017 2:16 PM, "Silent Leaf" <silent.leaf0(a)gmail.com> escreveu: hi, say i have the following typeclass: class Foo a where bar :: a -> String looks a lot like the Read typeclass, right? (at least i think it should?) well say it's a different meaning (in other terms i can't or do not want to use Read, but i'd like to implement a default version of bar for those instances that also implement Read. is there a way to do so? _______________________________________________ Beginners mailing list Beginners(a)haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

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How do I map a String and a IO String?
by Jona Ekenberg 29 Jun '17

29 Jun '17

I want to replace lines with the content of a file if the line is a filename > {-# LANGUAGE LambdaCase #-} > test = do > putStrLn $ concatMap (\l -> l ++ "\n") > $ map (\case l > | (isPrefixOf "./" l) -> readFile l > | otherwise -> l) But since l is a String and readFile gives an IO String this doesn't work. How should I get around this? Kind regards, Jona

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order of things in a file.hs?
by Silent Leaf 29 Jun '17

29 Jun '17

hi, what is your personal practice of ordering data/functions/main/hidden functions, etc in a haskell file? like all trivial things they are very hard to decide upon, given that either order of helpers then main function / value, or the other way round, has tempting pros and cons. thanks in advance for your opinion!

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where do non-beginners haskellers go to expose code to the community?
by Silent Leaf 29 Jun '17

29 Jun '17

hi, let's say i'm a real-deal but typically open-source/not-employed haskeller, i have a really well-done library/program. i want to publish it so it's available, but i also want to just broadcast to at least a part of the haskell community basically saying "hiya, made that cool code, go check it out". both to get returns and ideas, but not so much for code-review, since it's about reviewing a finished product, or at least an alpha/beta version. where would i go to do that? (again, it's hypothetical, i'm not real-deal yet) i mean what's the point of coding if nobody knows what you do, right? (of course the question is not serious, but still, it must count for a bit right? if you don't get paid or you don't sell your code and it's not only for your own usage?) but maybe there are ways by which most haskellers go check the latest published packages? or maybe the community is not that tight-knit? or maybe it does not depend on the language, and there are places where to make your code known to potential users? i admit i don't actually know how someone with an opensource program would go to advertize it. even creating a website for it does not tell anybody that it exists. was just wondering. it'd be somewhat good if there were a centralized place where to review any package published on official repositories, both wrt the code and the functionalities. a kind of message board automatically plugged to the flux of published packages.

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