August 2017 - Haskell-Cafe

Prevent a type parameter from sharing some of its parameters with another parameter
by Timotej Tomandl 21 Aug '17

21 Aug '17

As the title says. Motivation: in a mutable code I want to keep some parameters strictly immutable, i.e. not sharing s with the whole mutable type I think this is best illustrated by 2 examples: 1. abstract example: imagine I have a type X a b c and another type Y a b and I want to prevent any case where the first parameter of X "a" is shared with it's third parameter "c" i.e. X a b (Y a a), X a b (Y a b), X a b (Y b a), X a b (Y a d) etc., any nestings in Y referencing a etc. but allow X a b (Y b b), X a b (Y b c) and any other combination not ruled out above 2. concrete example: imagine I have a type: Item s element key where s is used in the same way as in STRef s a, i.e. it cannot leak outside of a certain context, but I want the key to be immutable i.e. independent of s under any circumstences, so I can't write a type like: Item s element (STRef s refType) this is equivalent to disallowing X a b (Y a d) in 1. Is there any way to write a restriction like this in Haskell? -- Timo

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Ternary infix expression parser
by Jeffrey Brown 19 Aug '17

19 Aug '17

I want to write a parser that lets someone write "(a #uses b) #because (c #needs d)" and read it into a tree: data Tree = Tree String Tree Tree | Leaf String magic "(a #uses b) #because (c #needs d)" = Tree "because" (Tree "uses" (Leaf "a") (Leaf "b")) (Tree "needs" (Leaf "c") (Leaf "d")) I thought I could use Text.Megaparsec.Expr for it, but now I realize it hadnles binary operations, and this is a ternary one. Suggestions? -- Jeff Brown | Jeffrey Benjamin Brown Website <https://msu.edu/~brown202/> | Facebook <https://www.facebook.com/mejeff.younotjeff> | LinkedIn <https://www.linkedin.com/in/jeffreybenjaminbrown>(spammy, so I often miss messages here) | Github <https://github.com/jeffreybenjaminbrown>

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haskell-src-exts 8.2 compatible release
by Matthew Pickering 18 Aug '17

18 Aug '17

Hi all, I realise that people will be starting to use haskell-src-exts libaries with the new GHC 8.2 release. However, I do not have time in the foreseeable future (~ 1-2 months) to be able to implement the necessary parts to support the new language features. The way to make an 8.2 compatible release happen sooner is to send me some patches! There are some pull requests which I would like to merge but require minor changes or rebasing and also a few language features which need implementing. https://github.com/haskell-suite/haskell-src-exts/milestones/1.18 None of them look very difficult to implement unlike other language changes in past releases. I started to work on what was necessary a few months ago but didn't make a large amount of progress. https://github.com/haskell-suite/haskell-src-exts/tree/ghc-8.2 Anyone's help contributing to this important but menial library would be greatly appreciated. Thanks, Matt

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[ANN] Speculate: automatically discover properties about Haskell functions generalization of counter-examples
by Rudy Matela 17 Aug '17

17 Aug '17

Hello, I am happy to announce Speculate. A library to automatically discover properties about Haskell functions. https://hackage.haskell.org/package/speculate Quick example -- discovering properties about addition and multiplication: $ cabal update $ cabal install speculate $ ghci > import Test.Speculate > speculate args{constants = [constant "+" (+), constant "*" (*)]} _ :: Integer (+) :: Integer -> Integer -> Integer (*) :: Integer -> Integer -> Integer x + y == y + x x * y == y * x (x + y) + z == x + (y + z) (x * y) * z == x * (y * z) (x + x) * y == x * (y + y) x <= x * x Speculate is similar to QuickSpec, but uses a different algorithm to produce inequalities and coditional equations -- see README for details. Speculate is available on both Stackage (nightly) and Hackage. -- Rudy

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Re: [Haskell-cafe] cabal update problem?
by Jonathon Delgado 17 Aug '17

17 Aug '17

01-index.tar.gz is also 4 times the size of 00-index.tar.gz

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DataHaskell impromptu workshop at ICFP'17 ?
by Marco Zocca 16 Aug '17

16 Aug '17

Dear all, among those that will attend the last three days of ICFP (i.e. the Haskell symposium and CUFP), are there people who would be interested in meeting to discuss possible future directions for the DataHaskell project? Looking forward to your feedback; Best, Marco

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cabal update problem?
by yi lu 16 Aug '17

16 Aug '17

Hi all, I find cabal 2.0.0.0 brings new change both in downloading package list and the hackage mirror. 1) 00-index.tar.gz has be replaced by 01-index.tar.gz 2) If timestamp.json exists and it is new enough, 01-index.tar.gz won't download, even though I don't have the latest copy of 01-index.tar.gz. So my questions are 1) What else has been changed to hackage.haskell.org mirror? I found four more files: mirrors.json root.json snapshot.json timestamp.json 2) Is it a feature or a bug? Yours, Yi

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123
by Dmitriy Matrosov 16 Aug '17

16 Aug '17

Hi. Is there a way to avoid `UndecidableInstances` in following code: data A f = A {_a1 :: f String} instance Show (f String) => Show (A f) where it does not compile with 1.hs:4:10: error: • The constraint ‘Show (f String)’ is no smaller than the instance head (Use UndecidableInstances to permit this) • In the instance declaration for ‘Show (A f)’ Though, initially, this was {-# LANGUAGE RankNTypes #-} data A f = A {_a1 :: f String} instance forall f a. Show (f a) => Show (A f) where which also does not compile with 1.hs:5:10: error: • Variable ‘a’ occurs more often in the constraint ‘Show (f a)’ than in the instance head (Use UndecidableInstances to permit this) • In the instance declaration for ‘Show (A f)’ The error is different and i don't sure, that this two cases are related. I want these instances to make a type with many records parametrized by `Alternative` type, e.g. data Volume t = Volume { _volName :: t Name , _volSize :: t Size , _volPath :: t Path , _pool :: t Pool } When i try to make instances, which require `*` type, i will end with above cases. -- Dmitriy

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Re: [Haskell-cafe] Precedence of plus and bit-wise-and
by Doug McIlroy 16 Aug '17

16 Aug '17

Sorry for hitting return prematurely. The final sentence below tells the tale. The reason for the low precedence of & and | in PL/I and C is that those were used as Boolean operators in conditions. For example lessequal(a,b) { return(a<b | a==b); } That usage was baked in before the short-cut operators && and || were introduced in C. doug

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Re: [Haskell-cafe] Precedence of plus and bit-wise-and
by Doug McIlroy 16 Aug '17

16 Aug '17

The reason for the low precedence of & and | in PL/I and C is that those were used as Boolean operators in conditions. For example lessequal(a,b) { return(a<b | a==b); }

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