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Can I specify the a in a phantom type to be limitedto a sum type?

haskell-cafe - Fri, 05/15/2015 - 7:47am
How can I create Answers of type Gender, Race, or Age? These should be possible: λ> Answer Male λ> Answer White λ> Answer Black λ> Answer 28 Others such as using a string should not be possible: λ> Answer "a string" -- should throw type error {-# LANGUAGE UndecidableInstances #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE GADTs #-} {-# LANGUAGE StandaloneDeriving #-} module Tutorial where data Gender = Male | Female deriving (Show) data Race = White | Black deriving (Show) type Age = Int data Answer a where Answer :: Gender -> Answer Gender deriving instance Show (Answer w)
Categories: Offsite Discussion

Equality Contstraint?

haskell-cafe - Fri, 05/15/2015 - 2:17am
I am poking around in Arrow to understand it better. There is the following definition: second :: a b c -> a (d,b) (d,c) second f = arr swap >>> first f >>> arr swap where swap :: (x,y) -> (y,x) swap ~(x,y) = (y,x) Can someone explain what the ~ is? Searching the net resulted in frustration. I kind of think it is some kind of equality constraint but can’t find documentation. Perhaps if I knew what it was called I might succeed in finding something. If it is an equality constraint, please provide a reference if you have one. I did not find anything in a search including GHC, etc. Thanks
Categories: Offsite Discussion

Comparing Functions for Equality?

Haskell on Reddit - Thu, 05/14/2015 - 8:20pm

I read in A Book of Abstract Algebra, by Prof. Charles Pinter, that:

If f and g are functions from Real to Real, then f and g are equal if and only if f(x) = g(x) for every real number x.

When I tried to compare functions in ghci, I saw this compile-time error:

Prelude> let f x = "foo" Prelude> let g x = "foo" Prelude> f == g <interactive>:7:3: No instance for (Eq (t0 -> [Char])) arising from a use of `==' In the expression: f == g In an equation for `it': it = f == g

Is it simply too much work, not possible, to compare every input for the compiler? Or it's out of the compiler's scope?

submitted by kevin_meredith
[link] [19 comments]
Categories: Incoming News

Data constructor ‘Minus’ comes from an un-promotable type ‘Ints’

haskell-cafe - Thu, 05/14/2015 - 5:36pm
Hello, I clearly don’t really know what I’m doing…but at least I know it…. Here we defined the Naturals…and then attempt to construct the Integers…. naturals I can now define + and * and prove things about them…1 * x == x etc….nice..but lets put that on one side. Borrow bits and bobs from singletons Borrow bits and bobs from singletons..i.e. the isomorphic values…my proofs in nat now map to SNat…double nice. Create the integers by following my nose……(the integers are the equivalence class of pairs of naturals….) i.e. we have “positive” or “negative” or “zero”… Ok….this works as a set of values….but…. I can’t prove anything about these because the data constructors for my integers aren’t “promotable”… I cant do the same trick I did with Nat. “:k Zero” ….. Data constructor ‘Zero’ comes from an un-promotable type ‘Ints’ In a type in a GHCi command: Zero I’ve tried rejigging this in various futile and ignorant m
Categories: Offsite Discussion

Why does the inferred type of this expression change when I bind it to a varid?

Haskell on Reddit - Thu, 05/14/2015 - 4:03pm

I'd like a function that takes an Integer and gives back the number of digits in it. Here's the output from my ghci session:

GHCi, version 7.6.3: :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Prelude> :t (length . show) (length . show) :: Show a => a -> Int Prelude> let digits = length . show Prelude> :t digits digits :: () -> Int Prelude>

So, length . show has the right type on its own, but when I bind it to the name digits, it suddenly gets a much more restrictive type. What causes this, and how can I avoid it?

Sure, it works if I just specify the type I want, but I'm surprised that the type inference gets it right some of the time and wrong other times.

submitted by penguinland
[link] [5 comments]
Categories: Incoming News

Question: What's the most Haskell way to implement this? (Card game)

Haskell on Reddit - Thu, 05/14/2015 - 3:14pm

So I basicly have two implementations in mind and im not sure about which of them is the proper Haskell implementation. I have differents cards, entities of those cards, players who can be the owner of those cardentities, and locations on a board where the cardentities can be.

My first idea was:

data Player = Player {name :: String, ..} data Card = Card {name :: String, desc :: String, uri :: String} data CardEntity = CardEntity {card :: Card, rotation :: Direction, orientation :: Orientation, ..} data Location = Location {id :: Int}

But i wasn't sure where to put the information where a cardentity is and who it owns. Should an entity have an attribute who it owner is and where it is, or should a Location and a Player have a List of cardentities they have/own?

Then i came up with another idea. Maybe I could just give an ID to all players, entities and cards and make a function for each of their attributes:

data Player = Player {id :: Int} (or just type Player = Int) data CardEntity = CardEntity {id :: Int}


cardEntityRotation :: CardEntity -> Direction cardEntityOrientation :: CardEntity -> Orientation playerName :: Player -> String


that way i could easily give cardentities the owner attribute and players the cards attribute:

playerCardEntities :: Player -> [CardEntity] cardEntityOwner :: CardEntity -> Player cardName :: Card -> String

but cards are getting loaded from disk in the beginning so I'd probably had to make a hashmap for every single attribute..

Which one is better, or is there an better implementation than those two?

Thanks in advance.

submitted by Ecyoph
[link] [9 comments]
Categories: Incoming News

Which free monad?

Haskell on Reddit - Thu, 05/14/2015 - 2:57pm

It seems that there are two free-like monads.

data Free f r = Pure r | Free (f (Free f r)) data Free' f r = Done r | forall s. s :>>= (s -> f (Free' f r))

since s is existentially quantified, it can't be observed, so they should act the same. Is there ever a reason to use the second version instead of the (more popular) first? I'm not sure how haskell handles implicit state in closures, but I imagine it might be more efficient to reify the state in some cases.

submitted by dogodel
[link] [12 comments]
Categories: Incoming News

mightybyte: LTMT Part 3: The Monad Cookbook

Planet Haskell - Thu, 05/14/2015 - 11:52am

The previous two posts in my Less Traveled Monad Tutorial series have not had much in the way of directly practical content. In other words, if you only read those posts and nothing else about monads, you probably wouldn't be able to use monads in real code. This was intentional because I felt that the practical stuff (like do notation) had adequate treatment in other resources. In this post I'm still not going to talk about the details of do notation--you should definitely read about that elsewhere--but I am going to talk about some of the most common things I have seen beginners struggle with and give you cookbook-style patterns that you can use to solve these issues.

Problem: Getting at the pure value inside the monad

This is perhaps the most common problem for Haskell newcomers. It usually manifests itself as something like this:

main = do lineList <- lines $ readFile "myfile.txt" -- ... do something with lineList here

That code generates the following error from GHC:

Couldn't match type `IO String' with `[Char]' Expected type: String Actual type: IO String In the return type of a call of `readFile'

Many newcomers seem puzzled by this error message, but it tells you EXACTLY what the problem is. The return type of readFile has type IO String, but the thing that is expected in that spot is a String. (Note: String is a synonym for [Char].) The problem is, this isn't very helpful. You could understand that error completely and still not know how to solve the problem. First, let's look at the types involved.

readFile :: FilePath -> IO String lines :: String -> [String]

Both of these functions are defined in Prelude. These two type signatures show the problem very clearly. readFile returns an IO String, but the lines function is expecting a String as its first argument. IO String != String. Somehow we need to extract the String out of the IO in order to pass it to the lines function. This is exactly what do notation was designed to help you with.

Solution #1 main :: IO () main = do contents <- readFile "myfile.txt" let lineList = lines contents -- ... do something with lineList here

This solution demonstrates two things about do notation. First, the left arrow lets you pull things out of the monad. Second, if you're not pulling something out of a monad, use "let foo =". One metaphor that might help you remember this is to think of "IO String" as a computation in the IO monad that returns a String. A do block lets you run these computations and assign names to the resulting pure values.

Solution #2

We could also attack the problem a different way. Instead of pulling the result of readFile out of the monad, we can lift the lines function into the monad. The function we use to do that is called liftM.

liftM :: Monad m => (a -> b) -> m a -> m b liftM :: Monad m => (a -> b) -> (m a -> m b)

The associativity of the -> operator is such that these two type signatures are equivalent. If you've ever heard Haskell people saying that all functions are single argument functions, this is what they are talking about. You can think of liftM as a function that takes one argument, a function (a -> b), and returns another function, a function (m a -> m b). When you think about it this way, you see that the liftM function converts a function of pure values into a function of monadic values. This is exactly what we were looking for.

main :: IO () main = do lineList <- liftM lines (readFile "myfile.txt") -- ... do something with lineList here

This is more concise than our previous solution, so in this simple example it is probably what we would use. But if we needed to use contents in more than one place, then the first solution would be better.

Problem: Making pure values monadic

Consider the following program:

import Control.Monad import System.Environment main :: IO () main = do args <- getArgs output <- case args of [] -> "cat: must specify some files" fs -> liftM concat (mapM readFile fs) putStrLn output

This program also has an error. GHC actually gives you three errors here because there's no way for it to know exactly what you meant. But the first error is the one we're interested in.

Couldn't match type `[]' with `IO' Expected type: IO Char Actual type: [Char] In the expression: "cat: must specify some files"

Just like before, this error tells us exactly what's wrong. We're supposed to have an IO something, but we only have a String (remember, String is the same as [Char]). It's not convenient for us to get the pure result out of the readFile functions like we did before because of the structure of what we're trying to do. The two patterns in the case statement must have the same type, so that means that we need to somehow convert our String into an IO String. This is exactly what the return function is for.

Solution: return return :: a -> m a

This type signature tells us that return takes any type a as input and returns "m a". So all we have to do is use the return function.

import Control.Monad import System.Environment main :: IO () main = do args <- getArgs output <- case args of [] -> return "cat: must specify some files" fs -> liftM concat (mapM readFile fs) putStrLn output

The 'm' that the return function wraps its argument in, is determined by the context. In this case, main is in the IO monad, so that's what return uses.

Problem: Chaining multiple monadic operations import System.Environment main :: IO () main = do [from,to] <- getArgs writeFile to $ readFile from

As you probably guessed, this function also has an error. Hopefully you have an idea of what it might be. It's the same problem of needing a pure value when we actually have a monadic one. You could solve it like we did in solution #1 on the first problem (you might want to go ahead and give that a try before reading further). But this particular case has a pattern that makes a different solution work nicely. Unlike the first problem, you can't use liftM here.

Solution: bind

When we used liftM, we had a pure function lines :: String -> [String]. But here we have writeFile :: FilePath -> String -> IO (). We've already supplied the first argument, so what we actually have is writeFile to :: String -> IO (). And again, readFile returns IO String instead of the pure String that we need. To solve this we can use another function that you've probably heard about when people talk about monads...the bind function.

(=<<) :: Monad m => (a -> m b) -> m a -> m b (=<<) :: Monad m => (a -> m b) -> (m a -> m b)

Notice how the pattern here is different from the first example. In that example we had (a -> b) and we needed to convert it to (m a -> m b). Here we have (a -> m b) and we need to convert it to (m a -> m b). In other words, we're only adding an 'm' onto the 'a', which is exactly the pattern we need here. Here are the two patterns next to each other to show the correspondence.

writeFile to :: String -> IO () a -> m b

From this we see that "writeFile to" is the first argument to the =<< function. readFile from :: IO String fits perfectly as the second argument to =<<, and then the return value is the result of the writeFile. It all fits together like this:

import System.Environment main :: IO () main = do [from,to] <- getArgs writeFile to =<< readFile from

Some might point out that this third problem is really the same as the first problem. That is true, but I think it's useful to see the varying patterns laid out in this cookbook style so you can figure out what you need to use when you encounter these patterns as you're writing code. Everything I've said here can be discovered by carefully studying the Control.Monad module. There are lots of other convenience functions there that make working with monads easier. In fact, I already used one of them: mapM.

When you're first learning Haskell, I would recommend that you keep the documentation for Control.Monad close by at all times. Whenever you need to do something new involving monadic values, odds are good that there's a function in there to help you. I would not recommend spending 10 hours studying Control.Monad all at once. You'll probably be better off writing lots of code and referring to it whenever you think there should be an easier way to do what you want to do. Over time the patterns will sink in as form new connections between different concepts in your brain.

It takes effort. Some people do pick these things up more quickly than others, but I don't know anyone who just read through Control.Monad and then immediately had a working knowledge of everything in there. The patterns you're grappling with here will almost definitely be foreign to you because no other mainstream language enforces this distinction between pure values and side effecting values. But I think the payoff of being able to separate pure and impure code is well worth the effort.

Categories: Offsite Blogs

CFP : Extended deadline : Functional Art, Music,Modelling and Design (FARM 2015)

General haskell list - Thu, 05/14/2015 - 11:12am
************************************************************ Call for Papers and Demos : FARM 2015 The 3rd ACM SIGPLAN International Workshop on Functional Art, Music, Modelling and Design Vancouver, Canada, 5 September, 2015 affiliated with ICFP 2015 EXTENTED Submission Deadline : 27 May, 2015 (optional abstract submission : 17 May, 2015) ************************************************************ The ACM SIGPLAN International Workshop on Functional Art, Music, Modelling and Design (FARM) gathers together people who are harnessing functional techniques in the pursuit of creativity and expression. Functional Programming has emerged as a mainstream software development paradigm, and its artistic and creative use is booming. A growing number of software toolkits, frameworks and environments for art, music and design now employ functional programming languages and techniques. FARM is a forum for expl
Categories: Incoming News

Suggestion: "Sizable" super class for Storable

haskell-cafe - Thu, 05/14/2015 - 9:50am
Storable instances have a size, given by sizeOf. In many cases, we're not interested in peeking/poking data but only passing it opaquely via the FFI. A common use case is when the C API offers an "init" function such as: void mycontext_init(mycontext *context); For these cases it would be useful to know the size of "mycontext", so we could malloc it and pass a pointer to mycontext_init. Also, it allows Haskell-side code to decide how it wants to allocate the data, perhaps using some other (external) mechanism not related to the specific API that the FFI bindings are wrapping. c2hs would benefit by allowing users to use the '+' notation in function parameters (which generate malloc-and-pass style code), without having to guess the size of the structure. Instead, it could simply use the Sizable (TM) instance to get the size, and the user will define Sizable in any way they want (for example, using the {#sizeof#} macro, which is somewhat unreliable, or by hard-coding or manually entering the size or b
Categories: Offsite Discussion

maintaining pre-AMP+FTP-Prelude in external package

libraries list - Wed, 05/13/2015 - 12:50pm
The Prelude of GHC-7.10/base-4.8 introduces several name clashes mostly due to the AMP and FTP: (<*) clashes with Accelerate, (<*>) clashes with NumericPrelude, (and (<>) would clash with HMatrix if added to Prelude), 'join', 'pure', 'traverse', 'fold' clash with custom defined functions. "import Prelude hiding (pure)" is not yet supported by GHC-7.4 (as shipped with Ubuntu 12.04) and in newer GHC versions it generates an annoying warning. The only remaining option is to explicitly import identifiers from Prelude. What about maintaining the pre-AMP+FTP-Prelude in a package on Hackage? Then we could maintain compatibility with a range of GHC versions by disabling import of Prelude and importing preamplified (so to speak) Prelude. The base-compat package seems to support the other way round, that is, providing new 'base' functions to old compilers.
Categories: Offsite Discussion

Fedora ghc-7.10.1 repo

glasgow-user - Wed, 05/13/2015 - 4:28am
Hi, It is a bit later than I wanted but I have prepared a Fedora Copr repo for ghc-7.10.1. (note the EPEL7 and F20 builds are currently quick builds and the F21+ builds are perf builds) I will probably add a cabal-install build soon. Let me know if you find any problems or if it is useful. :) Jens
Categories: Offsite Discussion

Proposal: Generalize forever to Applicative

libraries list - Tue, 05/12/2015 - 6:05am
This looks like a no-brainer to me: forever :: Applicative f => f a -> f b forever a = let x = a *> x in x _______________________________________________ Libraries mailing list Libraries< at >
Categories: Offsite Discussion

mapM /= traverse?

libraries list - Mon, 05/11/2015 - 8:15pm
I was hoping that in GHC 7.10 we would make mapM = traverse for lists, but it appears this isn't the case: the Traversable instance for lists overrides mapM to be the manually-defined version in terms of foldr. Why is this? Fusion? Unfortunately since I want mapM = traverse (for Haxl) I'll need to continue to redefine it in our custom Prelude. Cheers, Simon
Categories: Offsite Discussion

RFC: "Native -XCPP" Proposal

glasgow-user - Wed, 05/06/2015 - 12:08pm
Hello *, As you may be aware, GHC's `{-# LANGUAGE CPP #-}` language extension currently relies on the system's C-compiler bundled `cpp` program to provide a "traditional mode" c-preprocessor. This has caused several problems in the past, since parsing Haskell code with a preprocessor mode designed for use with C's tokenizer has caused already quite some problems[1] in the past. I'd like to see GHC 7.12 adopt an implemntation of `-XCPP` that does not rely on the shaky system-`cpp` foundation. To this end I've created a wiki page to describe the actual problems in more detail, and a couple of possible ways forward. Ideally, we'd simply integrate `cpphs` into GHC (i.e. "plan 2"). However, due to `cpp`s non-BSD3 license this should be discussed and debated since affects the overall-license of the GHC code-base, which may or may not be a problem to GHC's user-base (and that's what I hope this discussion will help to find out). So please go ahead and
Categories: Offsite Discussion

Looking for retainers of PINNED objects

glasgow-user - Wed, 04/29/2015 - 5:37am
Hi all, I'm profiling a fairly large program which seems to have a space leak. The heap profiling (-hc) shows that PINNED objects are accumulated over time. In order to check the retainers of the objects, I ran the retainer profiling. Unfortunately it didn't output anything with -hr -hcPINNED. Also, this is just a guess though, the retainer profiling without any filters (I mean just -hr) doesn't seem to include PINNED objects at all. Is there a way to check what retains the PINNED objects? Thanks, Mitsutoshi _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users< at >
Categories: Offsite Discussion


glasgow-user - Sat, 04/11/2015 - 5:44pm
What’s the story with this? I tried to follow the instructions here: <> but I get Dominic Steinitz dominic< at > _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users< at >
Categories: Offsite Discussion

New gtk2hs 0.12.4 release

gtk2hs - Wed, 11/21/2012 - 12:56pm

Thanks to John Lato and Duncan Coutts for the latest bugfix release! The latest packages should be buildable on GHC 7.6, and the cairo package should behave a bit nicer in ghci on Windows. Thanks to all!


Categories: Incoming News