June 2022 - Haskell-Cafe

ANN: hledger-1.26
by Simon Michael 05 Jun '22

05 Jun '22

Aloha! I am pleased to announce hledger 1.26: https://github.com/simonmichael/hledger/releases/1.26 https://hledger.org/install https://hledger.org/release-notes.html#hledger-1-26 1.26 is a modest maintenance release; highlights include faster register reports and clearer error messages. Thank you to core contributor Stephen Morgan. What is hledger ? - free, high quality Plain Text Accounting[1] software: - a fast and robust multicurrency double-entry accounting system - built around human-readable, version-controllable plain text files - inspired by and partly compatible with Ledger CLI[2] - convertible to and from Beancount[3] - written in Haskell for correctness and longevity - easy to install on unix, mac and windows. For help getting started, or more info, see https://hledger.org, and join our chat via Matrix or IRC: https://hledger.org/support. Newcomers, experts, contributors, sponsors, feedback are always welcome! Best wishes, -Simon [1] https://plaintextaccounting.org [2] https://ledger-cli.org [3] https://beancount.github.io

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Re: [Haskell-cafe] Hiding type variables from the signature?
by Olaf Klinke 03 Jun '22

03 Jun '22

> λ f = deasilsB . deasilsA > > <interactive>:2:1: error: > • Could not deduce: Dress seed0 peels0 ~ Dress seed peels > ``` > > What is going on? How can I have my `deasils`? > > I tried attaching all kinds of type annotations, but nothing works so far. I do > not even understand the problem. How is it that an expression can be typed, but > cannot be assigned to a name? > > You can see the complete code at the gist [1]. > > [1]: https://gist.github.com/kindaro/6056f753bcf356ced96b817fee72533c/2823ec2792… The given type annotation for desilsA uses forall, which suggests you want to use ScopedTypeVariables here. But for that feature to kick in, you need to actually mention the scoped type variables in the function body, which you don't. :t deasil . first squeeze deasil . first squeeze :: (ForAll peels Functor, Squeezy' left a peels (Strip left a == '[])) => (a, right) -> Squeezed peels (left, right) You must use ScopedTypeVariables to constrain the type variable `a' to the intended (Dress seed peels). Olaf

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Hiding type variables from the signature?
by Ignat Insarov 02 Jun '22

02 Jun '22

Hello café, hope you all doing well! I have this code [1]. Its purpose is to provide an `fmap` that works at any depth. Here is the gist of the gist: ```Haskell fmapz ∷ forall seed fruit seed' fruit' peels recursive. Squeezish seed seed' fruit fruit' peels recursive ⇒ (seed → seed') → fruit → fruit' fmapz function = stretch . fmap @(Squeezed (Strip seed fruit)) function . squeeze ``` As the examples show, it works, and even has some type inference. However, the type signature is unwieldy. It seems like it should have only 4 variables, but I have to also do some type level book-keeping for which the other 2 variables are needed. These book-keeping type variables are determined by type families from the other 4, so surely I can hide them inside a type synonym or something like that? I tried giving a quantified constraint, like so: ```Haskell type Squeezish ∷ ★ → ★ → ★ → ★ → Constraint type Squeezish seed seed' fruit fruit' = forall peels recursive. Squeezish' seed seed' fruit fruit' peels recursive class Squeezish' seed seed' fruit fruit' (peels ∷ [★ → ★]) (recursive ∷ Bool) instance ( peels ~ Strip seed fruit , ForAll peels Functor , Squeezy seed' fruit' peels recursive , Squeezy seed fruit peels recursive ) ⇒ Squeezish' seed seed' fruit fruit' peels recursive ``` — But then the constraints from the context of `Squeezish'` fail to make it to the use site and numerous errors arise. What can I do to solve this problem? Or is it theoretically impossible? [1]: https://gist.github.com/kindaro/6056f753bcf356ced96b817fee72533c

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403 trying to upload hackage package candidate
by ☂Josh Chia (謝任中) 01 Jun '22

01 Jun '22

Hi, After running 'cabal sdist', I tried to upload a package candidate for direct-sqlite and got an inexplicable error. Any idea how I should proceed? You can see that I am one of the maintainers here: https://hackage.haskell.org/package/direct-sqlite/maintainers/ $ cabal upload -u jchia -P 'cat /tmp/pw' /home/jchia/gh/direct-sqlite/dist-newstyle/sdist/direct-sqlite-2.3.27.tar.gz Uploading /home/jchia/gh/direct-sqlite/dist-newstyle/sdist/direct-sqlite-2.3.27.tar.gz... Error uploading /home/jchia/gh/direct-sqlite/dist-newstyle/sdist/direct-sqlite-2.3.27.tar.gz: http code 403 Error: Forbidden No access for this resource.

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Keith Hanna
by chris done 01 Jun '22

01 Jun '22

Hi all, Does anyone know whether Keith Hanna is still active in the Haskell community? He's apparently no longer at the University of Kent (I called and asked) and has no discernible web presence besides research papers that refer to his Kent email address. He was mentioned in the community activities report in 2004 for his work on Vital, which I'd like to get in touch with him about. Cheers, Chris

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