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Deriving Show instance with ADT that has phantom type
by Cody Goodman 15 May '15

15 May '15

Going through the Data Types Ala Carte paper I found this definition: data Expr f = In (f (Expr f)) I tried to use "deriving Show"... data Expr f = In (f (Expr f)) deriving Show I found out that doesn't work with ADT's that have phantom types. I tried StandaloneDeriving too since GHC recommended it: {-# LANGUAGE StandaloneDeriving #-} data Expr f = In (f (Expr f)) deriving Show instance deriving Show (Expr f) Got this error: src/Main.hs:32:1-31: No instance for (Show (f (Expr f))) … arising from a use of ‘showsPrec’ In the second argument of ‘(.)’, namely ‘(showsPrec 11 b1)’ In the second argument of ‘showParen’, namely ‘((.) (showString "In ") (showsPrec 11 b1))’ In the expression: showParen ((a >= 11)) ((.) (showString "In ") (showsPrec 11 b1)) When typechecking the code for ‘showsPrec’ in a standalone derived instance for ‘Show (Expr f)’: To see the code I am typechecking, use -ddump-deriv Compilation failed. Here is the derived instance output from ghc -ddump-simple: Derived instances: instance GHC.Show.Show (Main.Expr f_aq1) where GHC.Show.showsPrec a_azP (Main.In b1_azQ) = GHC.Show.showParen ((a_azP GHC.Classes.>= 11)) ((GHC.Base..) (GHC.Show.showString "In ") (GHC.Show.showsPrec 11 b1_azQ)) GHC.Show.showList = GHC.Show.showList__ (GHC.Show.showsPrec 0) Didn't work. Can this be derived? Will I need to manually make instances?

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cast to specific Typeclass
by Teodor Vlasov 15 May '15

15 May '15

Hello everyone, I'm trying to implement traversal over data structures and found Data.Typeable very useful for that. But it seems to be impossible to cast to specific typeclass, so that we can cast Typeable to something that is Show'able for example. It'd be usefull to have something like: data CanShow = forall a. CanShow a toConcrete :: (Typeable a) => a -> Maybe CanShow Implementing such datatypes for needed classes would allow to do some kind of class casting. Is it possible to implement this somehow? Is it a limitation of the implementation or a principal one? Thanks! Regards, Teodor.

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Transforming ASTs
by anakreon 15 May '15

15 May '15

This might be a duplicated message. The first time I posted it I had not subscribed to haskell cafe. Suppose the following data type for encoding Boolean expressions: data BExpr a = BTrue | BFalse | Id String | Not a | And a a | Or a a | BEq a a deriving (Functor) type Expr = Fix BExpr It is easy to produce a string representation of an expression or evaluate it: estr :: BExpr String -> String eval :: BExpr Bool -> Bool with the cata function from Data.Functor.Fixedpoint. Could you suggest a solution for transforming trees encoded as Exp into equivalent Expr (e.g Not Not a ~> a)? cata does not work since it expects a function f a -> a while a transformation would be f a -> f a.

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Can I specify the a in a phantom type to be limited to a sum type?
by Cody Goodman 15 May '15

15 May '15

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) -- λ> -- This is pretty useful and accepts Answers with the Gender -- λ> Answer Male -- Answer Male -- λ> :t Answer Male -- Answer Male :: Answer Gender -- λ> -- how do I also accept Race and Age? -- below I try to sort out how to use type families (unsuccessfully) before bed -- type family Answer :: * -- type instance Answer Gender = Answer Gender

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Fwd: Transforming ASTs
by Sergey Vinokurov 15 May '15

15 May '15

Hi anakreon, You should look at paramorphism and histomorphism recursion schemes. Paramorphism has type f (a, Fix f) -> a, and thus allows you to pattern match on the subtree that produced the result. But it's not enough to write simplification of (Not (Not x)) -> x because it'll have to access simplification result that is more than one level deep. But this recursion scheme would be helpful for simpler transformations like (And True x) -> x. As for original optimization of double negation I'd suggest to use histomorphism where you'll have access to all the expression subtrees annotated with intermediate results. E.g. {-# LANGUAGE DeriveFunctor #-} {-# LANGUAGE StandaloneDeriving #-} {-# LANGUAGE UndecidableInstances #-} data Cofree f a = a :< f (Cofree f a) deriving (Functor) strip :: (Functor f) => Cofree f a -> Fix f strip (_ :< x) = Fix $ fmap strip x newtype Fix f = Fix (f (Fix f)) deriving instance (Show (f (Fix f))) => Show (Fix f) unFix :: Fix f -> f (Fix f) unFix (Fix x) = x histo :: (Functor f) => (f (Cofree f a) -> a) -> Fix f -> a histo alg x = case histo' alg x of y :< _ -> y histo' :: (Functor f) => (f (Cofree f a) -> a) -> Fix f -> Cofree f a histo' alg = (\x -> alg x :< x) . fmap (histo' alg) . unFix simplify :: Fix BExpr -> Fix BExpr simplify = histo alg where alg :: BExpr (Cofree BExpr Expr) -> Expr alg (Not (_ :< (Not (x :< _)))) = x alg x = Fix $ fmap strip x main :: IO () main = do print $ simplify $ Fix (Not (Fix (Not (Fix (Not (Fix (And (Fix BTrue) (Fix BFalse)))))))) For more detailed exposition I'd suggest to look into https://github.com/willtim/recursion-schemes/raw/master/slides-final.pdf Sergey. On Fri, May 15, 2015 at 7:08 PM, anakreon <anakreonmejdi(a)gmail.com> wrote: > This might be a duplicated message. The first time I posted it I had not > subscribed to haskell cafe. > > Suppose the following data type for encoding Boolean expressions: > > data BExpr a = BTrue > | BFalse > | Id String > | Not a > | And a a > | Or a a > | BEq a a > deriving (Functor) > type Expr = Fix BExpr > > It is easy to produce a string representation of an expression or evaluate > it: > > estr :: BExpr String -> String > eval :: BExpr Bool -> Bool > > with the cata function from Data.Functor.Fixedpoint. > > Could you suggest a solution for transforming trees encoded as Exp into > equivalent Expr (e.g Not Not a ~> a)? > cata does not work since it expects a function f a -> a while a > transformation would be f a -> f a. > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >

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Haskell Weekly News
by Kim-Ee Yeoh 15 May '15

15 May '15

*Top Picks:* - Is Servant <http://haskell-servant.github.io/>the most type-safe HTTP server library ever? Are the type signatures hard to read <https://news.ycombinator.com/item?id=9519782#up_9520750>? In addition to its utmost relevance as a web library, Servant is also an awesome case study in the type safety vs type readability trade-off spectrum, brought to you by Alp Mestanogullari and Julian Arni. HN <https://news.ycombinator.com/item?id=9519782> and /r/haskell <http://www.reddit.com/r/haskell/comments/35hiie/servant_04_released/> - Ozan Sener compiles Pandoc into JS via GHCJS and creates a web interface to it using the Reflex FRP library <https://github.com/ryantrinkle/reflex>. Markup.Rocks <https://github.com/osener/markup.rocks> is much loved on /r/haskell <http://www.reddit.com/r/haskell/comments/35ax22/ive_compiled_pandoc_with_gh…>. See also HN <https://news.ycombinator.com/item?id=9512293>. - Is Haskell a "Startup Secret Weapon"? Alexandr Kurilin reveals adoption challenges <https://github.com/commercialhaskell/commercialhaskell/blob/master/usage/fr…> at Front Row Education <https://www.crunchbase.com/organization/front-row>. Among them, slow build times. Also, "senior developers [that] get very frustrated when something wouldn't compile for hours and they couldn't find any help to move forward." Comments on HN <https://news.ycombinator.com/item?id=9519924> and /r/haskell <http://www.reddit.com/r/haskell/comments/35h8vh/how_front_row_education_use…> . - At Facebook, Bryan O'Sullivan debugs aeson's gigabyte space leak <http://www.serpentine.com/blog/2015/05/13/sometimes-the-old-ways-are-the-be…> on decoding a JSON megabyte of non-stop backslashes. Culprit? The streaming interface didn't match the use case. In place of streaming, Bryan now blasts bytes into a single big buffer, gaining 27x speed and 42x memory reduction. Comments on HN <https://news.ycombinator.com/item?id=9540389>, Proggit <http://www.reddit.com/r/programming/comments/35udjh/fixing_a_haskell_perfor…>, /r/haskell <http://www.reddit.com/r/haskell/comments/35udfy/fixing_an_aeson_performance…> . - Paul Chiusano's Unison programming platform <http://unisonweb.org/2015-05-07/about.html> hits the HN <https://news.ycombinator.com/item?id=9512955> and /r/haskell <http://www.reddit.com/r/haskell/comments/35cop9/unison_a_nextgeneration_pro…> headlines. Features include a browser-based UI that constrains edits to those that are well-typed. Also, DRY-ness up the wazoo: every type and term is uniquely identified by a hash a la Git. - Joey Hess reports <http://www.reddit.com/r/haskell/comments/35bw0b/at_last_debian_unstable_has…> that Debian unstable now has a working GHCi for ARM. The Template Haskell challenges have been surmounted. - Garrison Jensen blows the whistle <http://www.garrisonjensen.com/2015/05/13/haskell-programs-are-lies.html> on the impostor sieve on the front page of Haskell.org. In jest. A festive one-upmanship of fondly treasured code ensues on /r/haskell <http://www.reddit.com/r/haskell/comments/35vc31/the_real_way_to_generate_a_…>. And since bad publicity is better than no publicity, we owe kudos to Garrison. HN-worthy <https://news.ycombinator.com/item?id=9541478>. - Michael Snoyman decries use of ExceptT IO <https://www.fpcomplete.com/user/commercial/content/exceptions-best-practices> for exception handling because the user exception data type creates misleading expectations of comprehensiveness. The gotcha is that it doesn't cover IO exceptions! Furthermore, distinct exception types mean that the corresponding code can't compose. Instead? Use MonadThrow. /r/haskell <http://www.reddit.com/r/haskell/comments/35sk6w/best_practices_for_using_ex…> - JP Moresmau steps down as chief of EclipseFP <http://jpmoresmau.blogspot.com/2015/05/eclipsefp-end-of-life-from-me-at-lea…> and the companion Haskell packages BuildWrapper, ghc-pkg-lib, and scion-browser. Without anyone to take his place, the sun sets on EclipseFP. But the sun continues to shine on ide-backend (previously reported <http://haskell.1045720.n5.nabble.com/Haskell-Weekly-News-td5768107.html>), a GHC API wrapper akin to BuildWrapper. JP spitballs <http://haskell.1045720.n5.nabble.com/IDE-backend-Atom-editor-td5809240.html> on how he might move on to the Atom editor, jiggering it to use ide-backend-client. /r/haskell <http://www.reddit.com/r/haskell/comments/35y5v1/jp_moresmaus_blog_eclipsefp…> - Tatsuya Hirose translates Go By Example into Haskell. GBE <https://gobyexample.com/> comprises code samples annotated for an experienced programmer new to Go. For this first cut <http://lotz84.github.io/haskellbyexample/>, Tatsuya stays close to the original and creates Go-ish, imperative Haskell. Already good for Go-to-Haskell crossovers. Potentially excellent when done in idiomatic Haskell. /r/haskell <http://www.reddit.com/r/haskell/comments/3586e3/haskell_by_example/> - Tomas Petricek observes <http://tomasp.net/blog/2015/against-types/> the diversity of type theories and type systems and posits harm in any attempt at a single all-encompassing capture of the meaning of 'types'. What about unsound types? He doesn't offer a way out for those stuck with the appellation. Comments on HN <https://news.ycombinator.com/item?id=9545540> and /r/haskell <http://www.reddit.com/r/haskell/comments/35zzvu/against_the_definition_of_t…> . - John De Goes launches a crowdfund <https://www.indiegogo.com/projects/record-all-lambdaconf-2015-talks> to solicit $15,000 for video-recording 70 talks at LambdaConf 2015, which features many known Haskell programmers. /r/haskell <http://www.reddit.com/r/haskell/comments/35n14p/indiegogo_record_all_lambda…> - A redditor asks ex-Lispers what it's like moving to Haskell <http://www.reddit.com/r/haskell/comments/35bybo/lispers_of_haskell_whats_yo…>. "I miss Lisp parens" is a frequent answer. Also, Lisp has a better REPL experience. A biggie upside? The refactoring afforded by Haskell's type system. *Tip of the Week:* - When programming fractals, use the LLVM backend <https://ghc.haskell.org/trac/ghc/ticket/10400#comment:3> because it's "usually good at optimizing this kind of non-allocating code" as Reid Barton advises. -- Kim-Ee

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Equality Contstraint?
by Michael Jones 15 May '15

15 May '15

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

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trying to go from FFT type to Vector.Storable
by briand＠aracnet.com 15 May '15

15 May '15

Hi, I'm using the FFT package (the FFTW bindings), and for a complex FFT I get back : (FFTWReal r, Ix i, Shapable i) => CArray i (Complex r) -> CArray i (Complex r) at first I thought I could use elems to get to a list and then do Vector.Storable.fromList to get back to a Vector.Storable. Unfortunately r is FFTWReal so i need to map over the returned list and get a haskell Double back from that. FFTWReal maps to RealFloat which i found, but i haven't found anything that will take a RealFloat and give me a Double. Was hoping someone might know how to do that, or maybe have a little cleaner way to get from the FFT data type over to Vector. Thanks. Brian

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evaluate arbitrary code + mutable variables
by Jeffrey Brown 14 May '15

14 May '15

I wrote a graph-like data type, with some functions for adding statements and relationships between statements. (It's different from a graph in that there's no edge/vertex distinction; there are only statements, but a statement might refer to other statements.) Currently I'm using them in GHCI. Because it does not allow one to change a variable, I keep having to do things like this: let g0 = emptyDocument let s1 = newStatement ... let g1 = addStatement s1 g0 let s2 = newStatement ... let g2 = addStatement s2 g1 ... If I wrote a standalone application, I could use mutable variables, so I would not have to define a new object every time I want to modify an existing one. However I like being able to type in arbitrary code into GHCI. Can one have both of those at once? That is, could I either (1) use MVars from within GHCI, or (2) write a standalone app that lets the user evaluate arbitrary Haskell?

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Suggestion: "Sizable" super class for Storable
by jones.noamle＠gmail.com 14 May '15

14 May '15

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 by some other method). Your thoughts are much appreciated!

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