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• Haskell's cabal dependency problem with happy

  - by wirrbel

  I have problems installing ghc-mod on my linux machine. cabal worries about "happy" not being available in versione = 1.17: $ cabal install ghc-mod Resolving dependencies... [1 of 1] Compiling Main ( /tmp/haskell-src-exts-1.14.0-1357/haskell-src-exts-1.14.0/Setup.hs, /tmp/haskell-src-exts-1.14.0-1357/haskell-src-exts-1.14.0/dist/setup/Main.o ) Linking /tmp/haskell-src-exts-1.14.0-1357/haskell-src-exts-1.14.0/dist/setup/setup ... Configuring haskell-src-exts-1.14.0... setup: The program happy version =1.17 is required but it could not be found. Failed to install haskell-src-exts-1.14.0 cabal: Error: some packages failed to install: ghc-mod-3.1.3 depends on haskell-src-exts-1.14.0 which failed to install. haskell-src-exts-1.14.0 failed during the configure step. The exception was: ExitFailure 1 hlint-1.8.53 depends on haskell-src-exts-1.14.0 which failed to install. However, it even is installed in v. 1.19, as you can see here: $ cabal install happy Resolving dependencies... [1 of 1] Compiling Main ( /tmp/happy-1.19.0-1124/happy-1.19.0/Setup.lhs, /tmp/happy-1.19.0-1124/happy-1.19.0/dist/setup/Main.o ) Linking /tmp/happy-1.19.0-1124/happy-1.19.0/dist/setup/setup ... Configuring happy-1.19.0... Building happy-1.19.0... Preprocessing executable 'happy' for happy-1.19.0... [ 1 of 18] Compiling NameSet ( src/NameSet.hs, dist/build/happy/happy-tmp/NameSet.o ) [ 2 of 18] Compiling Target ( src/Target.lhs, dist/build/happy/happy-tmp/Target.o ) [ 3 of 18] Compiling AbsSyn ( src/AbsSyn.lhs, dist/build/happy/happy-tmp/AbsSyn.o ) [ 4 of 18] Compiling ParamRules ( src/ParamRules.hs, dist/build/happy/happy-tmp/ParamRules.o ) [ 5 of 18] Compiling GenUtils ( src/GenUtils.lhs, dist/build/happy/happy-tmp/GenUtils.o ) [ 6 of 18] Compiling ParseMonad ( src/ParseMonad.lhs, dist/build/happy/happy-tmp/ParseMonad.o ) [ 7 of 18] Compiling Lexer ( src/Lexer.lhs, dist/build/happy/happy-tmp/Lexer.o ) [ 8 of 18] Compiling Parser ( dist/build/happy/happy-tmp/Parser.hs, dist/build/happy/happy-tmp/Parser.o ) [ 9 of 18] Compiling AttrGrammar ( src/AttrGrammar.lhs, dist/build/happy/happy-tmp/AttrGrammar.o ) [10 of 18] Compiling AttrGrammarParser ( dist/build/happy/happy-tmp/AttrGrammarParser.hs, dist/build/happy/happy-tmp/AttrGrammarParser.o ) [11 of 18] Compiling Grammar ( src/Grammar.lhs, dist/build/happy/happy-tmp/Grammar.o ) [12 of 18] Compiling First ( src/First.lhs, dist/build/happy/happy-tmp/First.o ) [13 of 18] Compiling LALR ( src/LALR.lhs, dist/build/happy/happy-tmp/LALR.o ) [14 of 18] Compiling Paths_happy ( dist/build/autogen/Paths_happy.hs, dist/build/happy/happy-tmp/Paths_happy.o ) [15 of 18] Compiling ProduceCode ( src/ProduceCode.lhs, dist/build/happy/happy-tmp/ProduceCode.o ) [16 of 18] Compiling ProduceGLRCode ( src/ProduceGLRCode.lhs, dist/build/happy/happy-tmp/ProduceGLRCode.o ) [17 of 18] Compiling Info ( src/Info.lhs, dist/build/happy/happy-tmp/Info.o ) [18 of 18] Compiling Main ( src/Main.lhs, dist/build/happy/happy-tmp/Main.o ) Linking dist/build/happy/happy ... Installing executable(s) in /home/hope/.cabal/bin Installed happy-1.19.0 Any ideas? cabal-install version 1.16.0.2 using version 1.16.0 of the Cabal library

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• Haskell vs Erlang for web services

  - by Zachary K

  I am looking to start an experimental project using a functional language and am trying to decide beween Erlang and Haskell, and both have some points that I really like. I like Haskell's strong type system and purity. I have a feeling it will make it easier to write really reliable code. And I think that the power of haskell will make some of what I want to do much easier. On the minus side I get the feeling that some of the Frameworks for doing web stuff on Haskell such as Yesod are not as advanced as their Erlang counter parts. I rather like the Erlang approach to threads and to fault tollerence. I have a feeling that the scalability of Erlang could be a major plus. Which leeds to to my question, what has people's exerience been in implementing web application backends in both Haskell and Erlang. Are there packages for Haskell to provide some of the lightweight threads and actors that one has in Erlang?

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• What's the progress on Haskell records?

  - by mmh

  Recently I stumbled once again on the issues of Haskells records, in particular the uniqueness of field names (it's a pain ...) I already read A proposal for records in Haskell from SPJ and Greg Morrisett but it's last update was 2003. Another paper Lightweight Extensible Records for Haskell from SPJ and Mark Jones is even older: It's from a Haskell workshop in 1999. Now I wonder if the process of giving Haskell new records made any progress. Does anybody know something about it or can point me to some further reading ?

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• What kind of things are easy in Haskell and hard in Scala, and vice versa?

  - by Daniel C. Sobral

  There has been some intermingling of Scala and Haskell communities, and I have noticed now and then people commenting on stuff that's supposed to be easy in Haskell and hard and Scala. Less often (maybe because I read Scala questions, not Haskell ones), I see someone mentioning that something in Scala is easier than in Haskell. So. I'd like to know from people who are knowledgeable in both what kind of things are easy in Haskell and hard in Scala, and, conversely, what kind of things are easy in Scala and hard in Haskell.

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• How to implement early exit / return in Haskell?

  - by Giorgio

  I am porting a Java application to Haskell. The main method of the Java application follows the pattern: public static void main(String [] args) { if (args.length == 0) { System.out.println("Invalid number of arguments."); System.exit(1); } SomeDataType d = getData(arg[0]); if (!dataOk(d)) { System.out.println("Could not read input data."); System.exit(1); } SomeDataType r = processData(d); if (!resultOk(r)) { System.out.println("Processing failed."); System.exit(1); } ... } So I have different steps and after each step I can either exit with an error code, or continue to the following step. My attempt at porting this to Haskell goes as follows: main :: IO () main = do a <- getArgs if ((length args) == 0) then do putStrLn "Invalid number of arguments." exitWith (ExitFailure 1) else do -- The rest of the main function goes here. With this solution, I will have lots of nested if-then-else (one for each exit point of the original Java code). Is there a more elegant / idiomatic way of implementing this pattern in Haskell? In general, what is a Haskell idiomatic way to implement an early exit / return as used in an imperative language like Java?

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• Type checking and recursive types (Writing the Y combinator in Haskell/Ocaml)

  - by beta

  When explaining the Y combinator in the context of Haskell, it's usually noted that the straight-forward implementation won't type-check in Haskell because of its recursive type. For example, from Rosettacode [1]: The obvious definition of the Y combinator in Haskell canot be used because it contains an infinite recursive type (a = a -> b). Defining a data type (Mu) allows this recursion to be broken. newtype Mu a = Roll { unroll :: Mu a -> a } fix :: (a -> a) -> a fix = \f -> (\x -> f (unroll x x)) $ Roll (\x -> f (unroll x x)) And indeed, the “obvious” definition does not type check: ?> let fix f g = (\x -> \a -> f (x x) a) (\x -> \a -> f (x x) a) g <interactive>:10:33: Occurs check: cannot construct the infinite type: t2 = t2 -> t0 -> t1 Expected type: t2 -> t0 -> t1 Actual type: (t2 -> t0 -> t1) -> t0 -> t1 In the first argument of `x', namely `x' In the first argument of `f', namely `(x x)' In the expression: f (x x) a <interactive>:10:57: Occurs check: cannot construct the infinite type: t2 = t2 -> t0 -> t1 In the first argument of `x', namely `x' In the first argument of `f', namely `(x x)' In the expression: f (x x) a (0.01 secs, 1033328 bytes) The same limitation exists in Ocaml: utop # let fix f g = (fun x a -> f (x x) a) (fun x a -> f (x x) a) g;; Error: This expression has type 'a -> 'b but an expression was expected of type 'a The type variable 'a occurs inside 'a -> 'b However, in Ocaml, one can allow recursive types by passing in the -rectypes switch: -rectypes Allow arbitrary recursive types during type-checking. By default, only recursive types where the recursion goes through an object type are supported. By using -rectypes, everything works: utop # let fix f g = (fun x a -> f (x x) a) (fun x a -> f (x x) a) g;; val fix : (('a -> 'b) -> 'a -> 'b) -> 'a -> 'b = <fun> utop # let fact_improver partial n = if n = 0 then 1 else n*partial (n-1);; val fact_improver : (int -> int) -> int -> int = <fun> utop # (fix fact_improver) 5;; - : int = 120 Being curious about type systems and type inference, this raises some questions I'm still not able to answer. First, how does the type checker come up with the type t2 = t2 -> t0 -> t1? Having come up with that type, I guess the problem is that the type (t2) refers to itself on the right side? Second, and perhaps most interesting, what is the reason for the Haskell/Ocaml type systems to disallow this? I guess there is a good reason since Ocaml also will not allow it by default even if it can deal with recursive types if given the -rectypes switch. If these are really big topics, I'd appreciate pointers to relevant literature. [1] http://rosettacode.org/wiki/Y_combinator#Haskell

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• Multithreaded Haskell

  - by Ingdas

  Hi I'm learning Haskell and I'd like to write some multithreaded programs now to see the performance gains from that in a functional language. I can find some references to it on the internet but never a proper introduction to it. Can anyone point me to a guide which is understandable for someone who knows the syntax fairly well, but is not a wizard in Haskell?

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• Inspirational software for end-users written in Haskell?

  - by Lenny222

  I think great technology ist invisible. Besides the usual suspects (GHC, Xmonad, proprietary trading software) what great examples are there for end-user software written in Haskell? I think good examples are FreeArc, Hledger and "Nikki And The Robots". Do you have more examples (full blown GUI apps, small CLI tools, etc)? Edit: For example i am fascinated by Wings3D, because while written in Erlang, users can not tell that. It just works. Among Haskell's weak spots are cross-platform GUIs. There are not many GUI apps written in Haskkel in general and most of them are no easy to use, install or even compile. What are good examples to learn from how to make hard things look easy?

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• in memory datastore in haskell

  - by Simon

  I want to implement an in memory datastore for a web service in Haskell. I want to run transactions in the stm monad. When I google hash table steam Haskell I only get this: Data. BTree. HashTable. STM. The module name and complexities suggest that this is implemented as a tree. I would think that an array would be more efficient for mutable hash tables. Is there a reason to avoid using an array for an STM hashtable? Do I gain anything with this stem hash table or should I just use a steam ref to an IntMap?

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• Learning Haskell: How to remove an item from a List in Haskell

  - by BM

  Trying to learn Haskell. I am trying to write a simple function to remove a number from a list without using built-in function (delete...I think). For the sake of simplicity, let's assume that the input parameter is an Integer and the list is an Integer list. Here is the code I have, Please tell me what's wrong with the following code areTheySame :: Int -> Int-> [Int] areTheySame x y | x == y = [] | otherwise = [y] removeItem :: Int -> [Int] -> [Int] removeItem x (y:ys) = areTheySame x y : removeItem x ys

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• Problem with do construct in haskell

  - by Anonymous Coward

  Hello everyone I'm trying to learn Haskell and want to write a small program which prints the content of a file to the screen. When I load it into GHCi I get the following error: The last statement in a 'do' construct must be an expression I know this question has be asked already here: Haskell — “The last statement in a 'do' construct must be an expression”. Even though my code is very similar I still can't figure out the problem. If anyone could point out the problem to me I'd be very thankful. module Main (main) where import System.IO import System(getArgs) main :: IO() main = do args <- getArgs inh <- openFile $ ReadMode head args printFile inh hClose inh printFile :: Handle -> IO () printFile handle = do end <- hIsEOF handle if end then return () else do line <- hGetLine handle putStrLn line printFile handle

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• Preferred way to do locales in the Haskell Platform

  - by user355466

  The Haskell platform includes two obsolete libraries, old-time and old-locale. For old-time, it also includes the preferred alternative (namely time), but I can't figure out what the recommended alternative for old-locale is. Is this simply a shortcoming of the Platform as of now (version 2010.1.0.0), or something I've overlooked?

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• Using Data.Heap in Haskell, or reading Haskell docs for a beginner

  - by Masse

  I'm trying to use Haskells Data.Heap module, but I'm incapable of even using it with integers. The only heap I've been capable of using is "empty", which does not take any arguments. Later on I'll figure out how to instance for my needs, but for now I'd be glad if I was even able to test it with numbers.

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• Multiple actions upon a case statement in Haskell

  - by Schroedinger

  One last question for the evening, I'm building the main input function of my Haskell program and I have to check for the args that are brought in so I use args <- getArgs case length args of 0 -> putStrLn "No Arguments, exiting" otherwise -> { other methods here} Is there an intelligent way of setting up other methods, or is it in my best interest to write a function that the other case is thrown to within the main? Or is there an even better solution to the issue of cases. I've just got to take in one name.

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• Haskell web frameworks survey

  - by Phuc Nguyen

  There are several web frameworks for Haskell like Happstack, Snap, and Yesod, and probably a few more. In what aspects do they differ from each other? For example: features (e.g. server only, or also client scripting, easy support for different kinds of database) maturity (e.g. stability, documentation quality) scalability (e.g. performance, handy abstraction) main targets Also, what are examples of real-world sites / web apps using these frameworks? Many thanks.

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• Is Haskell worth learning?

  - by Jason K

  I am looking at this question primarily from a career point of view, so I hope you answer it accordingly. I am fairly proficient with Python, can write C++ and I am a final year student of computer science engineering I am looking to learn Haskell because I have heard a lot about it. My question is: apart from learning it because of all the good I have heard about it, is it any good for my career? Is it used in the industry? I am curious to learn it but unless it helps me somehow in my career, I am not willing to make that change at this stage. Looking for some personal experiences here. Thanks!

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• Haskell: Best tools to validate textual input?

  - by Ana

  In Haskell, there are a few different options to "parsing text". I know of Alex & Happy, Parsec and Attoparsec. Probably some others. I'd like to put together a library where the user can input pieces of a URL (scheme e.g. HTTP, hostname, username, port, path, query, etc.) I'd like to validate the pieces according to the ABNF specified in RFC 3986. In other words, I'd like to put together a set of functions such as: validateScheme :: String -> Bool validateUsername :: String -> Bool validatePassword :: String -> Bool validateAuthority :: String -> Bool validatePath :: String -> Bool validateQuery :: String -> Bool What is the most appropriate tool to use to write these functions? Alex's regexps is very concise, but it's a tokenizer and doesn't straightforwardly allow you to parse using specific rules, so it's not quite what I'm looking for, but perhaps it can be wrangled into doing this easily. I've written Parsec code that does some of the above, but it looks very different from the original ABNF and unnecessarily long. So, there must be an easier and/or more appropriate way. Recommendations?

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• Using foldr to append two lists together (Haskell)

  - by Luke Murphy

  I have been given the following question as part of a college assignment. Due to the module being very short, we are using only a subset of Haskell, without any of the syntactic sugar or idiomatic shortcuts....I must write: append xs ys : The list formed by joining the lists xs and ys, in that order append (5:8:3:[]) (4:7:[]) => 5:8:3:4:7:[] I understand the concept of how foldr works, but I am only starting off in Functional programming. I managed to write the following working solution (hidden for the benefit of others in my class...) : However, I just can't for the life of me, explain what the hell is going on!? I wrote it by just fiddling around in the interpreter, for example, the following line : foldr (\x -> \y -> x:y) [] (2:3:4:[]) which returned [2:3:4] , which led me to try, foldr (\x -> \y -> x:y) (2:3:4:[]) (5:6:7:[]) which returned [5,6,7,2,3,4] so I worked it out from there. I came to the correct solution through guess work and a bit of luck... I am working from the following definition of foldr: foldr = \f -> \s -> \xs -> if null xs then s else f (head xs) (foldr f s (tail xs) ) Can someone baby step me through my correct solution? I can't seem to get it....I already have scoured the web, and also read a bunch of SE threads, such as How foldr works

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• Interchange structured data between Haskell and C

  - by Eonil

  First, I'm a Haskell beginner. I'm planning integrating Haskell into C for realtime game. Haskell does logic, C does rendering. To do this, I have to pass huge complexly structured data (game state) from/to each other for each tick (at least 30 times per second). So the passing data should be lightweight. This state data may laid on sequential space on memory. Both of Haskell and C parts should access every area of the states freely. In best case, the cost of passing data can be copying a pointer to a memory. In worst case, copying whole data with conversion. I'm reading Haskell's FFI(http://www.haskell.org/haskellwiki/FFICookBook#Working_with_structs) The Haskell code look specifying memory layout explicitly. I have a few questions. Can Haskell specify memory layout explicitly? (to be matched exactly with C struct) Is this real memory layout? Or any kind of conversion required? (performance penalty) If Q#2 is true, Any performance penalty when the memory layout specified explicitly? What's the syntax #{alignment foo}? Where can I find the document about this? If I want to pass huge data with best performance, how should I do that? *PS Explicit memory layout feature which I said is just C#'s [StructLayout] attribute. Which is specifying in-memory position and size explicitly. http://www.developerfusion.com/article/84519/mastering-structs-in-c/ I'm not sure Haskell has matching linguistic construct matching with fields of C struct.

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• Why doesn't Haskell have type-level lambda abstractions?

  - by Petr Pudlák

  Are there some theoretical reasons for that (like that the type checking or type inference would become undecidable), or practical reasons (too difficult to implement properly)? Currently, we can wrap things into newtype like newtype Pair a = Pair (a, a) and then have Pair :: * -> * but we cannot do something like ?(a:*). (a,a). (There are some languages that have them, for example, Scala does.)

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• A programming language that does not allow IO. Haskell is not a pure language

  - by TheIronKnuckle

  (I asked this on Stack Overflow and it got closed as off-topic, I was a bit confused until I read the FAQ, which discouraged subjective theoratical debate style questions. The FAQ here doesn't seem to have a problem with it and it sounds like this is a more appropriate place to post. If this gets closed again, forgive me, I'm not trying to troll) Are there any 100% pure languages (as I describe in the Stack Overflow post) out there already and if so, could they feasibly be used to actually do stuff? i.e. do they have an implementation? I'm not looking for raw maths on paper/Pure lambda calculus. However Pure lambda calculus with a compiler or a runtime system attached is something I'd be interested in hearing about.

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• Is Haskell's type system an obstacle to understanding functional programming?

  - by FarmBoy

  I'm studying Haskell for the purpose of understanding functional programming, with the expectation that I'll apply the insight that I gain in other languages (Groovy, Python, JavaScript mainly.) I choose Haskell because I had the impression that it is very purely functional, and wouldn't allow for any reliance on state. I did not choose to learn Haskell because I was interested in navigating an extremely rigid type system. My question is this: Is a strong type system a necessary by-product of an extremely pure functional language, or is this an unrelated design choice particular to Haskell? If it is the latter, I'm curious what would be the most purely functional language that is dynamically typed. I'm not particularly opposed to strong typing, it has its place, but I'm having a hard time seeing how it benefits me in this educational endeavor.

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• Box2D Platform body not moving player body along with it

  - by onedayitwillmake

  I am creating a game using Box2D (Javascript implementation) - and I added the ability to have a static platform, that is moved along an axis as a function of a sine. My problem is when the player lands on the platform, as the platform moves along the X axis - the player is not moved along with it, as you visually would expect. The player can land on the object, and if it hits the side of the object, it does colide with it and is pushed. This image might explain better than I did: After jumping on to the red platform the player character will fall off as the platform moves to the right UPDATE: Here is a live demo showing the problem: http://onedayitwillmake.com/ChuClone/slideexample.php

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• Is Haskell's type system an obstacle to understanding functional programming?

  - by Eric Wilson

  I'm studying Haskell for the purpose of understanding functional programming, with the expectation that I'll apply the insight that I gain in other languages (Groovy, Python, JavaScript mainly.) I choose Haskell because I had the impression that it is very purely functional, and wouldn't allow for any reliance on state. I did not choose to learn Haskell because I was interested in navigating an extremely rigid type system. My question is this: Is a strong type system a necessary by-product of an extremely pure functional language, or is this an unrelated design choice particular to Haskell? If it is the latter, I'm curious what would be the most purely functional language that is dynamically typed. I'm not particularly opposed to strong typing, it has its place, but I'm having a hard time seeing how it benefits me in this educational endeavor.

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• Is return-type-(only)-polymorphism in Haskell a good thing?

  - by dainichi

  One thing that I've never quite come to terms with in Haskell is how you can have polymorphic constants and functions whose return type cannot be determined by their input type, like class Foo a where foo::Int -> a Some of the reasons that I do not like this: Referential transparency: "In Haskell, given the same input, a function will always return the same output", but is that really true? read "3" return 3 when used in an Int context, but throws an error when used in a, say, (Int,Int) context. Yes, you can argue that read is also taking a type parameter, but the implicitness of the type parameter makes it lose some of its beauty in my opinion. Monomorphism restriction: One of the most annoying things about Haskell. Correct me if I'm wrong, but the whole reason for the MR is that computation that looks shared might not be because the type parameter is implicit. Type defaulting: Again one of the most annoying things about Haskell. Happens e.g. if you pass the result of functions polymorphic in their output to functions polymorphic in their input. Again, correct me if I'm wrong, but this would not be necessary without functions whose return type cannot be determined by their input type (and polymorphic constants). So my question is (running the risk of being stamped as a "discussion quesion"): Would it be possible to create a Haskell-like language where the type checker disallows these kinds of definitions? If so, what would be the benefits/disadvantages of that restriction? I can see some immediate problems: If, say, 2 only had the type Integer, 2/3 wouldn't type check anymore with the current definition of /. But in this case, I think type classes with functional dependencies could come to the rescue (yes, I know that this is an extension). Furthermore, I think it is a lot more intuitive to have functions that can take different input types, than to have functions that are restricted in their input types, but we just pass polymorphic values to them. The typing of values like [] and Nothing seems to me like a tougher nut to crack. I haven't thought of a good way to handle them. I doubt I am the first person to have had thoughts like these. Does anybody have links to good discussions about this Haskell design decision and the pros/cons of it?

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