- Beginners - Haskell.org

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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tail recursion optimizations and monads
by Silent Leaf 28 Jun '17

28 Jun '17

Hi, If i do this: f :: (Monad m) => a -> m a f a = ma >>= f or maybe this: f :: (Monad m) => Foo -> m a f foo = mb >>= \foo' -> mc >> f foo' can i be sure it's tail recursive (as in, with optimization)? is it in fact ever the case? does it depend on the monad? esp obviously i'm very interested in the IO monad in those cases. esp, even if it's tail recursive on the level of haskell, does it entail an execution of the underlying IO action in any way tail-recursively optimized? or does it not even matter because at the level of the execution of the IO value, there's no recursion to speak of anymore? just wondering. thanks in advance!

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which typefor this FFI call
by PICCA Frederic-Emmanuel 28 Jun '17

28 Jun '17

Hello I would like to create a binding for this function [1]. herr_t H5Literate( hid_t group_id, H5_index_t index_type, H5_iter_order_t order, hsize_t *idx, H5L_iterate_t op, void *op_data ) My "only" problem is with the op and op_data part. Here the C op signature[2] op -> herr_t (*H5L_iterate_t)( hid_t g_id, const char *name, const H5L_info_t *info, void *op_data) What I would like to do is to create an Haskell function which allows to return what is contained under the op_data. What I understand from this is that I give the H5Literate function an op function which is executed for each group of my hdf5 file. So the op method is called with the releavant parameters. The op_data can be use to accumulate things during the traversal. H5Literate :: Hid -> Index -> Order -> Maybe HSize -> H5Iterate -> _ -> _ type H5Iterate = Hid -> ByteString -> Info -> _ -> _ So it seems to me that I need to use a State inorder to do the accumulation. For exemple,I want to collect each group names and accumulate then in a list. Somy questioniswhat should be the signature of both function. I have also the fealing that the return type MUST have a Storableinstance. Thanks for your help Frederic [1] https://support.hdfgroup.org/HDF5/doc/RM/RM_H5L.html#Link-Iterate [2] https://support.hdfgroup.org/HDF5/doc/RM/RM_H5L.html#Link-Visit

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