- Haskell-Cafe - Haskell.org

Injective type classes?
by Tom Smeding 30 Mar '21

30 Mar '21

Hi Cafe, With this class definition with this instance: class C a where instance C b => C (a, b) GHC can of course infer, given 'C b', that 'C (a, b)'. This is nothing more than the meaning of the instance declaration above. However, GHC cannot infer the inverse: {-# LANGUAGE FlexibleContexts #-} foo :: C a => a -> Int foo = undefined bar1 :: C (a, b) => a -> b -> Int bar1 _ = foo This gives an error on the right-hand side of 'bar1': "Could not deduce (C b) arising from a use of ‘foo’; from the context: C (a, b)". The same happens in similar cases: {-# LANGUAGE GADTs #-} bar2 :: (C p, p ~ (a, b)) => a -> b -> Int bar2 _ = foo data Thing a where Tup :: a -> b -> Thing (a, b) bar3 :: C a => Thing a -> Int bar3 (Tup x y) = foo y Both these usages of 'foo' yield the same error. My use-case is 'bar3', where I would like GHC to determine that the call to 'foo' is valid. (I don't actually care directly about bar1 and bar2.) Is there a way to make 'bar3' compile? Note that: - In my actual use case, 'C' is of course not empty. - In my actual use case, my type class instances _are_ in fact injective, even though I do enable FlexibleInstances to be able to write e.g. 'instance C (T a Int)'. - Above, the dictionary for 'C (a, b)' includes a dictionary for 'C b', doesn't it? So if inference can resolve 'C b', then the compilation to Core can find the right dictionary, I think? (Not sure about this part.) Thanks a lot for your help. Cheers, Tom

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Maybe won't let me count
by Galaxy Being 30 Mar '21

30 Mar '21

I've got this import Data.Maybe data MyList a = Empty | Cons a (MyList a) deriving (Eq,Ord,Show) data BaconOrIndex = Bacon | Indx Int deriving (Eq,Ord,Show) whereIsBM Empty = Nothing whereIsBM (Cons idx lx) = if (idx == Bacon) then Just 1 else (whereIsBM lx) which I would like to tell me where the Bacon is (index), not just if there's Bacon, which is what it does now. That is, I need this to happen > whereIsBM (Cons (Indx 5) (Cons Bacon (Cons (Indx 2) (Cons (Indx 8) Empty)))) Just 2 So I need to traverse a BaconOrIndex list and count how deep I went to find the Bacon variable. I get the above code to evaluate error-free, but obviously I'm only returning a Just 1 when it sees Bacon. What I need is to have the last part be . . . else (1 + whereIsBM lx) work; but it keeps giving the error Non type-variable argument in the constraint: Num (Maybe a) (Use FlexibleContexts to permit this) • When checking the inferred type whereIsBM :: forall a. (Num a, Num (Maybe a)) => MyList BaconOrIndex -> Maybe a I haven't a clue what this means. Eventually, I'll wrap this in something that handles the Nothing and does fromJust on the alternative. This whole effort is because if I didn't use the Maybe strategy, and said whereIsBM Empty = 0 ... it would never give back 0 if it didn't find Bacon, rather, it would simply return the whole countdown to Empty. What can I do to make Maybe work here? LB

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Why Haskell?
by Clinton Mead 30 Mar '21

30 Mar '21

I’m looking for recommendations of videos/articles to show to a software development manager about why one should use Haskell, focused more from a benefits to business perspective. Naturally this may involve some code, this manager isn’t completely clueless when it comes to programming languages, but basically something that explains why I’m raving about this “Haskell” thing all the time and why it’s a good idea. Thanks, Clinton

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Arrays in base... (was: ... indexing lists)
by Viktor Dukhovni 29 Mar '21

29 Mar '21

On Mon, Mar 29, 2021 at 10:58:49AM +0200, Henning Thielemann wrote: > On Mon, 29 Mar 2021, Viktor Dukhovni wrote: > > > Thus I applaud Michael Snoyman's quest to address the absense of a basic > > array type in the `base` library. Perhaps more users would stop abusing > > lists (memoisable iterators) as an indexed store. > > Data.Array actually _was_ part of base-3. > > However, I think we should split 'base' in more smaller parts rather than > making it bigger. FWIW, I don't think that splitting base into multiple libraries would achieves much, it would likely raise the cost of coordinating versioning. I do however agree that perhaps separating base from GHC could be a good idea, if GHC could ship a smaller foundational library with primops, ... and `base` evolved somewhat independently. However that too has potential drawbacks, because packages would be more likely to have non-overlapping version bounds on a separately evolving base... Probably manageable, but something to keep in mind. -- Viktor.

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Munich Virtual Haskell Meeting, 2021-03-31 @ 19:30
by Leah Neukirchen 29 Mar '21

29 Mar '21

Dear all, This week, our monthly Munich Haskell Meeting will take place again on Wednesday, March 31 on Google Meet at 19:30 CEST. **Due to meetup limitations in Bavaria, this meeting will take place online!** For details see here: https://muenchen.haskell.bayern/dates.html A Google Meet link to join the room is provided on the page. Everybody is welcome, especially the Haskellers from Bavaria that do not usually come to our Munich meetings due to travel distance! cu, -- Leah Neukirchen <leah(a)vuxu.org> https://leahneukirchen.org/

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Why does my module take so long to compile?
by Troels Henriksen 28 Mar '21

28 Mar '21

I have a program with about 220 modules. One of these modules takes _much_ longer to compile than the others (several minutes longer). I don't understand why. It compiled much faster with GHC 8.8.3, but got orders of magnitude slower when I changed to GHC 8.10.2. I don't understand why, since I don't think there is nothing special about the module - e.g. no type-level programming. Is there any good way to "profile" the stack or cabal compilation process to at least get an intuition for what takes so long? Note that the module is in the middle of the dependency tree of my program, so it is not viable to separate it out and compile it manually with GHC. It's slow even when I pass '--fast' to stack which should disable the optimiser, so I guess the type checker may be spending a lot of time on handling the admittedly large amount of typeclass constraints I have on some of my definitions. I'm mostly looking for techniques and tools I can also use next time this problem occurs, not the specific problem here, but if anyone's curious, the module in question is this one: https://github.com/diku-dk/futhark/blob/master/src/Futhark/Pass/ExtractKern… -- \ Troels /\ Henriksen

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Some questions about Fudgets library
by Antonio 28 Mar '21

28 Mar '21

I wonder why there is no Fudgets package in Hackage. I found this, but the fudgets link is broken: http://hackage.haskell.org/user/ThomasHallgren Nevertheless, there is a cabalized package in the Fudgets homepage, so it's strange it's not in Hackage. Also, is there an arrowized version of Fudgets? If not, why? Is there some fundamental reason that makes it impossible to port it to the arrows framework or simply nobody cared to port it? (Also, the license seems to not allow modifications.) I'm trying to write a bittorrent client in Haskell. I decided to use arrows for it and took a look at Fudgets and Yampa. I haven't decided yet which one to use. Fudgets seems more suitable and I like it more, but the fact that it doesn't use the arrow notation and classes is a bit annoying. I read that Fruit is more or less a mix of Fudgets and Yampa that uses arrows. Sadly, it's unmaintained, the web doesn't exist and it's not in Hackage. Is there any other Fudgets-like option? Greetings.

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Install on Ubuntu
by Galaxy Being 28 Mar '21

28 Mar '21

I've found a Haskell setup guide here, but after the stack setup it wants to install cabal curl -sSL https://get.haskellstack.org | sh then stack install cabal-install cabal update Why do I want to install cabal after stack? Any advice welcome on the "definitive setup." Just started a new machine with Ubuntu 20.10 on it. LB

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Does Haskell have this SML syntax?
by Galaxy Being 27 Mar '21

27 Mar '21

I saw this in *The Little MLer*, a book on SML datatype 'a list = Empty | Cons of 'a * 'a list fun subst_c (pred) = fn (n,Empty) => Empty | (n,Cons(e,t)) => if pred (e) then Cons (n,subst_c (pred) (n,t)) else Cons (e,subst_c (pred) (n,t)) The data type is just a homemade list, and the function subst_c takes a predicate ('a -> Bool) and determines whether an incoming list's elements pass or fail. What is interesting is the fn ... => ... part which takes in more parameters, namely an 'a and an 'a list. Technically this fn... is an anonymous, nameless function, and it seems bizarre to me that it's nested inside the named function but still taking in parameters as if it were at the top level. Here's a previous version showing all three parameters at the top level fun subst_c (pred) (n,Empty) = Empty ... The purpose of the first function was to demonstrate currying. IOW, what the second unnamed function is doing behind the scenes can be broken down to two-stages (currying) of the first with named, then unnamed functions. So my question is, is there anything like this in Haskell where a function inside a function -- named or anonymous -- can take incoming parameters as if it were top level? LB

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Query regarding GSOC
by Shubham Mittal 25 Mar '21

25 Mar '21

Hello I am Shubham from India currently pursuing BTech IT 2nd year. I would like to contribute to Haskell-Cafe as a part of GSOC. Can you please guide me how I can i do so? Thanks and Regards, Shubham Mittal

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