February 2020 - Haskell-Cafe

Re: [Haskell-cafe] [Job] Cloudseal seeks Haskell Software Engineers
by tomas jose gallegos 12 Feb '20

12 Feb '20

Hello there, My name is Tomas Gallegos and I specialize in functional programming using Haskell. I am very interested in this Haskell job position. It would be great if we could talk to discuss this position further and maybe schedule an interview. Thank you for your time and consideration, I look forward to hearing from you.(Attached to this email is my resume). Regards, Tomas Gallegos Sent with [ProtonMail](https://protonmail.com) Secure Email.

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help designing a function
by PICCA Frederic-Emmanuel 12 Feb '20

12 Feb '20

Hello, I try to write a software, which takes a stack of images and produce a data cube. Depending on the user input obtain via an .ini file, these cube are computed from different kind of projection. For now I have only two projection QxQyQzProjection and HklProjection. I prepare the InputXXX for the processXXX functions from the BinocularConfiguration and a function (InputType -> a) mkInputQxQyQz :: FramesQxQyQzP a => BinocularsConfig -> (InputType -> a) -> IO (InputQxQyQz a) processQxQyQz :: FramesQxQyQzP a => InputQxQyQz a -> IO () mkInputHkl :: FramesHklP a => BinocularsConfig -> (InputType -> a) -> IO (InputHkl a b) processHkl :: FramesHklP a => InputHkl a b -> IO () now I want to write this kind of function, which use the right Projection depending on the type find in the BinocularsConfig processProjection :: Show a => BinocularsConfig -> (InputType -> a) -> IO () processProjection c' f = do print c' case (ptype . bProjection $ c') of QxQyQzProjection -> do i <- mkInputQxQyQz c' f print i processQxQyQz i HklProjection -> do i <- mkInputHkl c' f print i processHkl i return () Indeed when I compile this code, I have an Error explaining that a FramesHklP a constrain is missing from the processProjection function. and idem for the FramesQxQyQzP. so I can add both constrains, but in that case I need to implement the two interfaces for each type a. Indeed this is wrong the `a` is for a specific projection thus I can not create an instance for each kind of Projection. I do not know If I am clear, but I do not know how to organise my code in order to be able to write this processProjection method. any help would be appreciate. thanks Frederic

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[Job] Hackworth are hiring a Haskell developer, full time, on-site in London
by Drew Hess 11 Feb '20

11 Feb '20

[Hi Haskellers! Apologies if you’ve already seen this on Reddit or Hacker News.] Hackworth are hiring a full-time Haskell developer in London. This is an on-site role, at our office in Shoreditch. Salary range: £70,000-£80,000 per year In this role, you'll help lead the research phase of our first product: a programming language optimized for learning to program, rooted in functional programming concepts, with a particular focus on the visualization of program execution. You'll be responsible for designing several prototype languages, exploring the programming language and computer education research literature for ideas, and explaining these designs and their trade-offs to your colleagues. Once the research phase concludes, you'll help lead the development phase of the product, the result of which will be a novel interactive programming environment for children. We're targeting the end of 2021 for initial beta testing with schools and external organizations. We expect to open-source the product at some point during the development phase. You'll have the option of leading that open source effort, if you wish. This role does not include any on-call, devops, or customer support requirements. You'll be focused exclusively on writing Haskell and creating an innovative programming environment for kids. For more information, please see the full job posting on our website: https://www.hackworthltd.uk/jobs/20200204/. ## Tech Our software stack is entirely Haskell and Nix. We use macOS on the desktop, and NixOS and AWS everywhere else. If you prefer Linux on your desktop, we can probably arrange that. ## About us Hackworth Ltd is a well-financed, bootstrapped, private limited company based in London. Our purpose is to make programming relevant to the interests of children of all backgrounds. We're committed to building a sustainable, ethical, socially responsible business. We serve children and schools; therefore, we’ll never fund our business by selling ad placements or exploiting compulsion loops via loot boxes or micro-transactions. See here for more: https://www.hackworthltd.uk/about/ ## Applying To apply for this role, you should meet the following requirements: * The equivalent of (approximately) 2 years' experience writing Haskell, in either an industrial or research setting. It need not have been all at one organization, nor one contiguous block of time. * You can be on-site at our London office at least 4 days per week. * You're eligible to work in the United Kingdom. If you're familiar with any of the following topics, that's a definite plus, and you should mention it in your application; but please note that these are not required for the role: * Dependent types * Gradual typing * Refinement types * Total functional programming * Program synthesis * Structured editing a la Hazel or similar * Block-based programming (e.g., Scratch) * Virtual machines designed for functional languages, e.g., Landin's SECD. ## To apply To apply for this role, please submit a resume/CV to careers(a)hackworthltd.com. Even if you’re unsure about your fit, or if you have any additional questions about the role, or about working at Hackworth in general, please get in touch! Hackworth are committed to diversity. We are an equal opportunities employer and do not discriminate on the grounds of gender, sexual orientation, marital or civil partner status, pregnancy or maternity, gender reassignment, race, colour, nationality, ethnic or national origin, religion or belief, disability or age.

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OpenBSD stack executable
by mike thomas 08 Feb '20

08 Feb '20

Hi there. I want to use stack on OpenBSD 6.6. In the absence of an official executable, I've been looking for instructions on how to bootstrap stack *without* a preexisting stack executable, with no luck. I've got GHC 8.6.4, Cabal 2.4.0 and Happy 1.19.5. Any thoughts? Cheers Mike Thomas

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MonadThrow and Either instance
by PICCA Frederic-Emmanuel 08 Feb '20

08 Feb '20

Hello, I try to parse a config file and extract a (Path Abs Dir) from these files So I have a parser as defined in config-ini So I need a function with this signature pathAbsDir :: Text -> Either String (Path Abs Dir) pathAbsDir t = do d <- try $ parseAbsDir (unpack . uncomment $ t) case d of Right v -> Right v Left e -> Left (show e) since I am using parseAbsDir wi this signature parseAbsDir :: MonadThrow m => FilePath -> m (Path Abs Dir) I would like to know how to write the patAbsDir method for now I have this error message src/Hkl/Binoculars/Config.hs:82:8-49: error: • Couldn't match type ‘[Char]’ with ‘GHC.Exception.SomeException’ arising from a use of ‘try’ • In a stmt of a 'do' block: d <- try $ parseAbsDir (unpack . uncomment $ t) In the expression: do d <- try $ parseAbsDir (unpack . uncomment $ t) case d of Right v -> Right v Left e -> Left (show e) In an equation for ‘pathAbsDir’: pathAbsDir t = do d <- try $ parseAbsDir (unpack . uncomment $ t) case d of Right v -> Right v Left e -> Left (show e) | 82 | d <- try $ parseAbsDir (unpack . uncomment $ t) | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ I understand that the MonadThrow instance of Either is for Either Exception a But I want (Either String a). Cheers Frederic

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Seeking help to generalize this ..
by Debasish Ghosh 07 Feb '20

07 Feb '20

Hi - How can I generalize the following pattern to an arbitrary list of functions ? compose :: (Monad m) => (Foo -> m Foo) -> (Foo -> m Foo) -> (Foo -> m Foo) -> Foo -> m Foo compose f1 f2 f3 acc = do a <- f1 acc b <- f2 a f3 b Any help please .. regards. -- Debasish Ghosh http://manning.com/ghosh2 http://manning.com/ghosh Twttr: @debasishg Blog: http://debasishg.blogspot.com Code: http://github.com/debasishg

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sharing
by Tom Schouten 05 Feb '20

05 Feb '20

Hi, Is there currently a consensus on what is the best way to embed a pure dataflow language while keeping control over value sharing? I have a stream processing language that has a monadic interface (a -> b -> m c) to track variable sharing so it can compile down to C or Verilog, but I would like to create an expression interface layer on top (s a -> s b -> s c) that still allows me to recover the underlying monad representation to implement sharing. I got used to writing in monadic form but I really can't sell this to EEs... Other than a TH or syntax frontend, unsafe tricks, or Conal's compiling to categories which requires a plugin (I believe), is there another way? Cheers, Tom

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Re: [Haskell-cafe] sharing
by Tom Schouten 05 Feb '20

05 Feb '20

(Reply from Sebastiaan Joosten) I've tried something similar before but ran into a lot of cases where verbose intermediate type annotations where necessary to resolve double-wrapping ambiguities because the monad is abstract in my case: xor :: DSL m r => r S -> r S -> m (r S)) Where r is representation wrapper, m is the associated compiler/interpreter, and S is a phantom tag. On 2/5/20 10:05 AM, Sebastiaan Joosten wrote: > Hi Tom, > > It wasn't entirely clear to me what you're looking for, so I'm > probably way off here > >> On 5 Feb 2020, at 08:59, Tom Schouten <tom(a)zwizwa.be >> <mailto:tom@zwizwa.be>> wrote: >> >> asking people to give up expressions while they work just fine in C >> or Verilog > > If all you care about is writing hardware expressions, how about > providing a handful of self-lifting operators? Below is an example of > what I mean (you'd replace the list applicative with your own > applicative). I'm freely combining pureed and unpureed Booleans in the > 'test' example, which would presumably be what your EE's write (I've > only given them a single self-lifting operator, so they're still stuck > writing pretty boring code). Does this help? > > Best, > Sebastiaan > > PS: I'm not replying to haskell-cafe only because my mails bounce > there due to a technical issue that's entirely on my side, feel free > to add this back onto the haskell-cafe thread > > {-# LANGUAGE FlexibleInstances, FlexibleContexts, > MultiParamTypeClasses #-} > class Pureable a b where > maybePure :: a -> b > instance Pureable a [a] where > maybePure = pure > instance Pureable [a] [a] where > maybePure = id > (.&&) :: (Pureable a [Bool], Pureable b [Bool]) > => a -> b -> [Bool] > a .&& b = (&&) <$> maybePure a <*> maybePure b > test :: [Bool] -> [Bool] > test x = (True .&& x) .&& (False .&& x)

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Re: [Haskell-cafe] ListT version leveraging Traversable
by Olaf Klinke 04 Feb '20

04 Feb '20

Dear Juan, Traversables indeed are intimately connected with monad transformers: Two monads are known to commute if one monad (say t) yields a functor on the Kleisli category of the other monad (say m). If you write down what this means for fmap, you get the following type signature: (a -> m b) -> t a -> m (t b) which is the type of mapM and traverse. So does every Traversable monad t yield a monad transformer? Not quite. A functor has to obey the law fmap f.fmap g = fmap (f.g) and this is not given simply by the existence of traverse, you need: traverse f <=< traverse g = traverse (f <=< g). As Li-yao Xia pointed out, it is fairly easy to find a counterexample to <*> = ap for TravListT. But you could of course define <*> = ap and the problem is gone. However, a lawful monad satisfies join . join = join . fmap join and there is a counterexample for TravListT: type M a = TravListT [] a -- [] is not commutative x :: M Int x = TravListT ([[1,2],[3,4]]) -- ((join.join) xxx) /= ((join.fmap join) xxx) xxx :: M (M (M Int)) xxx = let xx = [[x, x],[x]] in TravListT [[TravListT xx,TravListT (reverse xx)]] I have not attempted to make this counterexample minimal, so please forgive the long term. Your claim (3) that [] is the monad of non-determinism is at least not entirely true. Lists modulo ordering and multiplicity are a model of non-determinism. In other words, if Data.Set was a monad then this was your model of non-determinism. Indeed, we have ((sort.runTravListT.join.join) xxx) == ((sort.runTravListT.join.fmap join) xxx) so the counterexample is only a counterexample because the ordering matters for equality. If you're modelling non-determinism, you may safely use the deprecated ListT knowing that the monad laws are violated only due to ordering and multiplicity. Olaf

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ANN:: provenience-0.1.0.0
by Olaf Klinke 04 Feb '20

04 Feb '20

Dear Haskellers, I just uploaded the first release of provenience to hackage [1]. The provenience package contains a monad transformer that lets you automatically track intermediate values and data dependencies in a computation. The philosophy behind it is that computation is done on one machine, while review of the algorithm is done by a third party. We make only minimal assumptions about the third party. Typically, we target static html as documentation format. The resulting data flow graph can be exported as various document formats via Pandoc. If desired, a machine-readable representation of each node is recorded as well. At my company we use provenience to generate invoices paired with Excel files out of the same Haskell application. The provenience Package will be presented at BOB conference [2] in Berlin end of this month. Regards, Olaf [1] https://hackage.haskell.org/package/provenience-0.1.0.0 [2] https://bobkonf.de/2020/klinke.html

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