- Haskell-Cafe - Haskell.org

problems with "stack ghc"
by Dennis Raddle 13 Jul '17

13 Jul '17

I'm using Haskell stack in the normal way (i.e. "stack build", "stack exec") for most of my application. However, I also need to build files that aren't in the stack source tree. My stack source tree is at ~/stack/cac/src/ Let's say I want to build the program /Users/Dennis/test.hs. So far I have been using stack ghc --stack-yaml ~/stack/cac/stack.yaml -- --make /Users/Dennis/test.hs I am confused. I thought this was working until today. I made some modifications in various places, including to my cac.cabal (but NOT my stack.yaml) and I'm having a problem. The libraries that test.hs imports are not getting rebuilt when they change. Are they *supposed* to be rebuilt? Any ideas how to debug this? D

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persistent in types
by Dmitry Olshansky 13 Jul '17

13 Jul '17

Hello cafe, I am trying to develop a library PersT <https://github.com/odr/perst/tree/a5c59833742f4cde35bbeb80394094fa1c0972a7> for working with database persistent in types (instead of e.g. template-haskell in persistent library). Now there are base DDL/DML functionality and additional DML functionality for tree-like structures. Small dirty example of using it is located here <https://github.com/odr/perst/blob/a5c59833742f4cde35bbeb80394094fa1c0972a7/…> . The main problem which I have is compile time. Compiling of my small example (five tables with five fields) needs 7 minutes and more than 6Gb RAM (!) I compile using stack and ghc 8.0.2. To check you need additionally only sqlite installed. Are there any ways to reduce resource utilization in compile time? Is it some problem in ghc which can be changed in future? What do you think? I also will be thankful for any feedback about library. Regards, Dima

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What are the essential parts of a binary GHC package
by Jan von Löwenstein 12 Jul '17

12 Jul '17

Hi, if I plan to compile Haskell code on the customers side, which parts of https://www.haskell.org/ghc/download_ghc_8_0_2.html#linux_x86_64 could I remove in order to reduce the size of my delivery? Some things are pretty clear: - GHC has to be in - LICENSE has to be in - libraries have to be in - docs can probably be left out But what about - compiler/stage1/** - compiler/stage2/** - libraries/Cabal/Cabal/** Any advice? Thanks and best regards Jan

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Encrypting streamed data
by Ivan Lazar Miljenovic 12 Jul '17

12 Jul '17

I have a use case for needing to use public key cryptography to encrypt a large amount of data in a streaming fashion (get it out of a DB, encrypt, put into an AWS S3 bucket). The command-line gpg tool seems to be able to encrypt/decrypt data from stdin to stdout in a streaming fashion, but in my attempts to use it it seems very file-based for things like the keys to use (whereas I would prefer to be able to pass the public key as an actual value rather than a file; if nothing else because this is for tools that don't have email addresses to use and base their keys on for addressing). Is there an existing library that can achieve this using conduit/pipes/whatever? cryptonite-conduit only covers hashing, hOpenPGP is poorly documented and I can't work out how to use it ("just follow the types" is difficult when Haddock docs don't link to the required types (seems to be because it uses the "import Module as X" trick for re-exporting everything but then everything from those modules isn't available). Can anyone recommend a solution? -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com http://IvanMiljenovic.wordpress.com

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Higher order functor package?
by Clinton Mead 11 Jul '17

11 Jul '17

Consider the illustrative code below: {-# LANGUAGE GADTs #-} {-# LANGUAGE DataKinds #-} {-# LANGUAGE KindSignatures #-} {-# LANGUAGE RankNTypes #-} {-# LANGUAGE PolyKinds #-} {-# LANGUAGE StandaloneDeriving #-} {-# LANGUAGE UndecidableInstances #-} data Param = Param1 | Param2 data T (p :: Param) where TInt :: Int -> T Param1 TInteger :: Integer -> T Param1 TBool :: Bool -> T Param2 data U (p :: Param) where UDouble :: Double -> U Param1 UString :: String -> U Param2 data F (t :: Param -> *) where F :: t Param1 -> t Param2 -> F t f :: T a -> U a f (TInt x) = UDouble (fromIntegral x) f (TInteger x) = UDouble (fromIntegral x) f (TBool x) = UString (show x) class MyFunctor f where myFmap :: (forall a. t a -> u a) -> f t -> f u instance MyFunctor F where myFmap f (F x1 x2) = F (f x1) (f x2) deriving instance Show (U a) deriving instance (Show (t Param1), Show (t Param2)) => Show (F t) main = print $ myFmap f (F (TInt 42) (TBool False)) Basically this is a sort of "higher order" functor, but I can't seem to fit it into an ordinary functor. But it seems like I'm reinventing the wheel, as my code is suspiciously like `Functor` but only slightly different. Has this sort of class already been created and if so what package is it in?

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I'm afraid of OverloadedLabels.
by Nikita Churaev 11 Jul '17

11 Jul '17

So gi-gtk is already using OverloadedLabels to fight the namespacing problem. What I'm afraid is that eventually people will pick up on this and stop using normal functions altogether. My problem with OverloadedLabels is that it is a class: if people start using it widely, the original modules of types are going to become even more privileged than they already are: only it will be canonically able to define functions which can be conveniently called, and everybody else will have to only define "second-class" normal functions or risk instance collisions. I mean yes, it's the same as in most OOP languages, but isn't it also bad? There must be a better way... Is there any alternative approach to name collisions coming?

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Fwd: constraint inference
by Dmitry Olshansky 10 Jul '17

10 Jul '17

Sorry, I didn't copy to cafe... ---------- Forwarded message ---------- From: Dmitry Olshansky <olshanskydr(a)gmail.com> Date: 2017-07-07 9:24 GMT+03:00 Subject: Re: [Haskell-cafe] constraint inference To: Ivan Lazar Miljenovic <ivan.miljenovic(a)gmail.com> I am not sure why but when I didn't set RankNTypes I got an appropriate error. So I specified it. But if I write let { f :: (forall n. KnownSymbol n => Proxy n) -> String; f p = symbolVal p } then nothing is changed. 2017-07-07 9:11 GMT+03:00 Ivan Lazar Miljenovic <ivan.miljenovic(a)gmail.com>: > You've specified RankNTypes but aren't using it; did you mean "(forall > n. KnownSymbol n => Proxy n)" ? > > Typically in these situations you also can't partially apply it, so > you might need to make it "f p = symbolVal p". > > On 7 July 2017 at 15:50, Dmitry Olshansky <olshanskydr(a)gmail.com> wrote: > > Hello, cafe! > > > > If we have many constraints then compiling is sometimes too slow. > Especially > > if there are some type calculations. > > And I don't know how to divide this time among different source files > > because all constraints are checked only when we call a function. > > > > I tried to add constraint info into argument definition instead of the > > function signature but this trick is not working (see below). > > > > Is it (at least theoretically) possible to infer the constraint inside a > > function from the type of function's argument? > > > > ------------------------------------ > > $ stack ghci > > ---- GHCi, version 8.0.2: > > Prelude> import Data.Proxy > > Prelude Data.Proxy> import GHC.TypeLits > > Prelude Data.Proxy GHC.TypeLits> :set -XRankNTypes > > Prelude Data.Proxy GHC.TypeLits> let { f :: (KnownSymbol n => Proxy n) -> > > String; f = symbolVal } > > > > <interactive>:4:54: error: > > • No instance for (KnownSymbol n) arising from a use of ‘symbolVal’ > > Possible fix: > > add (KnownSymbol n) to the context of > > the type signature for: > > f :: (KnownSymbol n => Proxy n) -> String > > • In the expression: symbolVal > > In an equation for ‘f’: f = symbolVal > > ------------------------------------ > > > > Best regards, > > Dmitry > > > > > > _______________________________________________ > > Haskell-Cafe mailing list > > To (un)subscribe, modify options or view archives go to: > > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > > Only members subscribed via the mailman list are allowed to post. > > > > -- > Ivan Lazar Miljenovic > Ivan.Miljenovic(a)gmail.com > http://IvanMiljenovic.wordpress.com >

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[ANN] [Reminder] Budapest Haskell Hackathon 2017, 29-30th July 2017
by Daniel Berecz 10 Jul '17

10 Jul '17

Hi everybody, Just a last reminder about our hackathon in Budapest at the end of July: Our local user group will hold a Haskell Hackathon in Budapest this year (this will be the second)! We will hold it from the 29th to the 30th of July (Saturday & Sunday). You can find out more on the following link: https://wiki.haskell.org/Budapest_Hackathon_2017 We will reguraly update the site with new information. If you decide to come, please fill out the following Google Form: https://goo.gl/forms/QjTNdXTD1aEOMEQU2 If you have a project, demo, or talk that you want to bring to the event please contact us, and we can talk about the details. You can find our contact info the events wiki site, or you can just contact me directly. The event will be open to people of all experience levels, from beginners to gurus. The only requisite is that you’re interested in the Haskell language, and want to hang out with us, and have a good time! Greetings from Daniel, and the other BP-HUG organizers.

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Re: [Haskell-cafe] I'm afraid of OverloadedLabels.
by Anthony Clayden 10 Jul '17

10 Jul '17

> On Mon Jul 10 02:57:02 UTC 2017, Nikita Churaev wrote: > > You can maybe fix that up with FunDeps > > or TypeFamilies, but then you lose most of TDNR's > > benefits (esp not coping with existential quanted fields > > , which is typically a requirement to cope with > > OOP-style). > > What benefits exactly are lost? Hi Nikita, I'm not sure I (or anybody) has the energy for another round of the "records debate". The TDNR (ex-)proposal seems so attractive because it just doesn't consider in detail all the complexities. They're magically swept under the carpet with the special dot syntax. So you might expect TDNR would support all of the functionality with H98 field names, plus having multiple fields of the same name. But how? Note the comment on TDNR section 'The Proposal' "TDNR is not "overloading" in the sense of type classes." And there were plenty of complaints that it's confusingly similar-but-different to type classes. You could look at SPJ's re-working of TDNR [2010] as SORF [2013]: https://ghc.haskell.org/trac/ghc/wiki/Records/OverloadedRecordFields/SORF SORF took advantage of the new Symbol Kinds (not available in 2010), and tried to be more type-class-like, with the `Has` class. One of the motivations with SORF is "Records with polymorphic fields are very important in practice, so it would be a major wart not to support them." TDNR just didn't discuss higher-ranked types. It turned out that was just too hard, even for SORF. (Or rather for OverloadedRecordFields, which is what's being delivered from SORF.) SORF tries to tackle record update, for which TDNR said "However I propose not to apply it [TDNR] to record update. " I don't think our cousins in OOP would be impressed by a records system in which you can't write polymorphic record updates. (I.e. a method to update the `firstName` in any record that has that field.) Actually delivering improvements in GHC around records is being painfully slow. `DuplicateRecordFields` has eased the pain a little. Ironically `OverloadedRecordFields` doesn't tackle higher-ranked types, nor record updates. And needs special syntax `#firstName`. So perhaps TDNR's "benefits" were never achievable? ANtC

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Re: [Haskell-cafe] I'm afraid of OverloadedLabels.
by Anthony Clayden 10 Jul '17

10 Jul '17

> On Mon Jul 10 00:16:02 UTC 2017, Richard A. O'Keefe wrote: > ... > As a native speaker of an SVO language (English) > in a country where one of the official languages is VSO (Māori), > I sometimes find the OVS structure of > "mainstream OO languages" a little uncomfortable... > "Backward ran sentences until reeled the mind." lol. That's a cute comparison. I don't think "mainstream OO" is OVS. In: > ... button.colour <- red Gloss: 'button's colour goes red'. `button's colour` is the grammatical Subject. (Yes it represents an OOP 'Object.method', but that's because everything in OOP is an 'Object'; and the only way to access is by methods.) > ... button.reset Again `button` is the grammatical Subject. (It must be, because there's no grammatical Object.) Then `reset` must be the verb, used intransitively. (In French you'd use a reflexive: `the button resets itself`.) This dot-suffixing is already in Haskell: Data.ByteString.Lazy.Builder.Extras 'Data's ByteString's Lazy's Builder's Extras'. To me it seemed natural to carry on ....Extras.untrimmedStrategy.buffSize That is, a field is just a component that might have (sub-)sub-component(s). The field 'belongs to' the structure, so dot represents English possessive. And that's just how lenses look, deliberately. (But that's a mystifying pun on use of the dot: the dot changes from compound name separator to function composition.) But the outcry against TDNR's proposal for dot really was voiciferous. (And nobody paid so much attention to the semantics. So the flaw in TDNR wasn't discovered for a long time.) AntC

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