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Reproduce example in Crypto.JWT
by Pietro Grandinetti 26 Feb '23

26 Feb '23

I am try to reproduce the first example here: https://hackage.haskell.org/package/jose-0.10/docs/Crypto-JWT.html I can't seem to succeed with importing the function `runJOSE` ghci> import Crypto.JWT ghci> :t runJOSE <interactive>:1:1: error: Variable not in scope: runJOSE ghci> import Crypto.JOSE.Error --https://github.com/frasertweedale/hs-jose/blob/master/src/Crypto/JOSE/Error.hs ghci> :t runJOSE <interactive>:1:1: error: Variable not in scope: runJOSE Can you point out what I am missing? Thanks.

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Understanding reason for Monad
by Pietro Grandinetti 22 Feb '23

22 Feb '23

Hello-- I am going to use the function 'hashPassword' in [1] and I am not able to understand why the result is MonadRandom ByteArray instead of simply being a ByteString (or, ByteArray, or similar). Looking at [2], the only useful thing I can do with a MonadRandom a is to convert it to IO a. Why would a result of this determistic computation be a IO? [1] - https://hackage.haskell.org/package/cryptonite-0.30/docs/Crypto-KDF-BCrypt.… [2] - https://hackage.haskell.org/package/cryptonite-0.30/docs/Crypto-Random-Type…

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Attoparsec.ByteString.Char8 or Attoparsec.ByteString for diff output?
by Pedro B. 20 Feb '23

20 Feb '23

Dear Listers, I am developing a program to parse dif output taken from stdin (as in diff file1 file2 | myApp) or from a file. I am reading the input as ByteString in either case and I am parsing it Attoparsec. My question is, Should I use Data.Attoparsec.ByteString.Char8 or Data.Attoparsec.ByteString? So far, I've been using Data.Attoparsec.ByteString.Char8 and it works for my sample files, which are in utf8 or, latin1, or the default Windows encoding. What do you suggest? Regards, Pedro Borges

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Generic use of queries postgresql-simple
by Pietro Grandinetti 09 Feb '23

09 Feb '23

Hello-- I use postgresql-simple and for selecting rows from one table I have the following function. runMyQuery :: Connection -> String -- query with '?' params to bind -> Int -- first param -> Int -- second param -> IO [MyTable1] runMyQuery conn dbQuery param1 param2 = query conn dbQuery $ (param2, param2) data MyTable1 = MyTable1 {col1 :: Int, col2 :: Int, col3 :: String} instance FromRow MyTable1 where fromRow = MyTable1 <$> field <*> field <*> field <*> field There are some things I'd like to improve: 1. I will have to write "MyTable1", "MyTable2", ... datatypes, one for each table, but would like to have only one function runMyQuery where the return type is a generic IO [MyModel]. My problem is that I don't know how to write it for tables with different number of columns and different SQL types. 2. The 2nd to the penultimate arguments of runMyQuery are the arguments to bind into the SQL string query. This means that I should write several different version of "runMyQuery" each with a different number of arguments, but would like to have only one. Can you recommend some approaches? Thanks.

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ways to use local ghc without installing packages via cabal install --lib
by Dennis Raddle 17 Jan '23

17 Jan '23

I have a situation where I build some projects with stack (using VS Code and HLS) and build some projects with ghc installed via ghcup. For the latter I need to install the needed packages with cabal install --lib <package> which are some of the same packages specified in my stack.yaml. I've been playing with Windows, macOS, various versions of the resolver, various versions of GHC, cabal, and stack. At one point I had both the HLS and my local ghc builds working. Recently I'm not sure what changed but I found myself needing to run cabal install --lib on many packages. At that point HLS stopped working with the message that it could not determine the local GHC version. After poking around the web, it seems that similar problems were happening for other people when they were running both a stack project and compiling via ghc separately, and that cabal install --lib was a problem. So I'm wondering if there's an alternative way of using separate ghc builds. Alternatively I can try to move everything under stack, that is doable, but not as convenient for some things

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Question on fromIntegral
by Jonathan Drews 01 Dec '22

01 Dec '22

Hi Folks: I am using Glasgow Haskell Compilation System, version 9.2.4 on OpenBSD 7.2. I am stumped by the following problem. If I run the following program in ghci, then it works fine. Look: fact :: Integer -> Integer fact n = product [1..n] term :: Double -> Double term x = x**4/fromIntegral(fact(4)) $ ghci GHCi, version 9.2.4: https://www.haskell.org/ghc/ :? for help ghci> :l seriesTermTest.hs [1 of 1] Compiling Main ( seriesTermTest.hs, interpreted ) Ok, one module loaded. ghci> term 1.0 4.1666666666666664e-2 However if I use fromIntegral inside a list comprehension like so: fact :: Integer -> Integer fact n = product [1..n] expon :: Double -> Double expon x = sum [x**i/fromIntegral(fact(i)) | i <- [0..50]] then I get the following error message: $ ghci GHCi, version 9.2.4: https://www.haskell.org/ghc/ :? for help ghci> :l ePowerSeries.hs [1 of 1] Compiling Main ( ePowerSeries.hs, interpreted ) ePowerSeries.hs:6:40: error: * Couldn't match expected type `Integer' with actual type `Double' * In the first argument of `fact', namely `(i)' In the first argument of `fromIntegral', namely `(fact (i))' In the second argument of `(/)', namely `fromIntegral (fact (i))' | 6 | expon x = sum [x**i/fromIntegral(fact(i)) | i <- [0..50]] | ^ Failed, no modules loaded. What am I doing wrong? -- Kind regards, Jonathan

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How to create a monad in GHC 7.10 or newer
by Ahmad Ismail 13 Nov '22

13 Nov '22

In the book "Haskell programming from first principles" it is said that: If you are using GHC 7.10 or newer, you’ll see an Applicative constraint in > the deﬁnition of Monad, as it should be: class Applicative m => Monad m where > (>>=) :: m a -> (a -> m b) -> m b > (>>) :: m a -> m b -> m b > return :: a -> m a I have created the following applicative functor. data WhoCares a = ItDoesnt | Matter a | WhatThisIsCalled deriving (Eq, Show) instance Functor WhoCares where fmap _ ItDoesnt = ItDoesnt fmap _ WhatThisIsCalled = WhatThisIsCalled fmap f (Matter a) = Matter (f a) instance Applicative WhoCares where pure = Matter Matter f <*> Matter a = Matter (f a) main = do -- fmap id == id let funcx = fmap id "Hi Julie" let funcy = id "Hi Julie" print(funcx) print(funcy) print(funcx == funcy) -- fmap (f . g) == fmap f . fmap g let funcx' = fmap ((+1) . (*2)) [1..5] let funcy' = fmap (+1) . fmap (*2) $ [1..5] print(funcx') print(funcy') print(funcx' == funcy') -- pure id <*> v = v print(pure id <*> (Matter 10)) -- pure (.) <*> u <*> v <*> w = u <*> (v <*> w) let appx = pure (.) <*> (Matter (+1)) <*> (Matter (*2)) <*> (Matter 10) let appy = (Matter (+1)) <*> ((Matter (*2)) <*> (Matter 10)) print(appx) print(appy) print(appx == appy) -- pure f <*> pure x = pure (f x) let appx' = pure (+1) <*> pure 1 :: WhoCares Int let appy' = pure ((+1) 1) :: WhoCares Int print(appx') print(appy') print(appx' == appy') -- u <*> pure y = pure ($ y) <*> u let appx'' = Matter (+2) <*> pure 2 let appy'' = pure ($ 2) <*> Matter (+ 2) print(appx'') print(appy'') print(appx'' == appy'') Due to lack of examples, I am not understanding how to implement >>= and >>. The code I came up with so far is: instance Monad (WhoCares a) where (>>=) :: Matter a -> (a -> Matter b) -> Matter b (>>) :: Matter a -> Matter b -> Matter b return :: a -> Matter a return = pure So, that I can do stuff like: half x = if even x then Matter (x `div` 2) else ItDoesnt incVal :: (Ord a, Num a) => a -> WhoCares a incVal x | x + 1 <= 10 = return (x + 1) | otherwise = ItDoesnt decVal :: (Ord a, Num a) => a -> WhoCares a decVal x | x - 1 >= 0 = return (x - 1) | otherwise = ItDoesnt main = do print (Matter 7 >>= incVal >>= incVal >>= incVal) print (Matter 7 >>= incVal >>= incVal >>= incVal >>= incVal) print (Matter 7 >>= incVal >>= incVal >>= incVal >>= incVal >>= decVal >>= decVal) print (Matter 2 >>= decVal >>= decVal >>= decVal) print(Matter 20 >>= half >>= half) With Output: 10 ItDoesnt ItDoesnt ItDoesnt 5 Please help. *Thanks and Best Regards,Ahmad Ismail*

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Invoking ghci with hackage modules
by Lists0 16 Jun '22

16 Jun '22

Hi all, a very basic question, that I cannot solve alone: How do I load modules from hackage into ghci ? Say, I have a module arrows.hs : {-# LANGUAGE Arrows #-} module Main where import Control.Arrow import Control.Monad import qualified Control.Category as Cat import Data.List import Data.Maybe import System.Random I get the error in ghci: % ghci arrows.hs GHCi, version 8.10.6: https://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( arrows.hs, interpreted ) arrows.hs:9:1: error: Could not find module ‘System.Random’ Use -v (or `:set -v` in ghci) to see a list of the files searched for. | 9 | import System.Random | ^^^^^^^^^^^^^^^^^^^^ Failed, no modules loaded. Prelude> System.Random is in the package "random" from hackage, where the other modules are in the package base. I cannot find a solution in the doku that works :-( Thanks for help!

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Utrecht Summer School on Advanced Functional Programming 2022
by Swierstra, W.S. (Wouter) 08 Apr '22

08 Apr '22

# Call for Participation SUMMER SCHOOL ON ADVANCED FUNCTIONAL PROGRAMMING Utrecht, the Netherlands, 04 July – 08 July 2022 http://www.afp.school ## ABOUT The Advanced Functional Programming summer school has been running for more than ten years. We aim to educate aspiring Haskell programmers beyond the basic material covered by many textbooks. After running remotely for the past two years, we're thrilled to announce that this year's edition will take place on site in Utrecht. We are monitoring the pandemic closely - and will make contingency plans if we cannot meet in person; at the moment, we do not have plans for remote participation. The lectures will cover several more advanced topics regarding the theory and practice of Haskell programming, including topics such as: * lambda calculus; * monads and monad transformers; * lazy evaluation; * generalized algebraic data types; * type families and type-level programming; * concurrency and parallelism. The summer school consists of a mix of lectures, labs, and a busy social program. ## LECTURERS * Gabriele Keller * Trevor McDonell * Wouter Swierstra ## PREREQUISITES We expect students to have a basic familiarity with Haskell already. You should be able to write recursive functions over algebraic data types, such as lists and trees. There is a great deal of material readily available that covers this material. If you've already started learning Haskell and are looking to take your functional programming skills to the next level, this is the course for you. ## DATES Soft registration deadline: 24 June, 2022 School: 04 July – 08 July 2022 ## COSTS 750 euro - Profession registration only 250 euro - Student registration fee 200 euro - Housing fee We will charge a registration fee of 750 euros (or 250 euros for students) to cover our expenses. If this is problematic for you for any reason at all, please email the organisers and we can try to offer you a discounted rate or a fee waiver. We have a limited number of scholarships or discounts available for students that would not be able to attend otherwise, especially for women and under-represented minorities. ## FURTHER INFORMATION Further information, including instructions on how to register, is available on our website: http://www.afp.school

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Evaluation oddity
by Torsten Otto 06 Feb '22

06 Feb '22

Hi all, maybe one of you sees a difference, I sure don’t… The full code for this is below. When I call *Main> diffieH 23 16 20 13 and use the result in *Main> encipher "abc" 13 „nop" I get the letters to be expected. However, when I combine these into > start text p a b = encipher text (diffieH p a b) *Main> start "abc" 23 16 20 „efg" the result changes. Any hints are most welcome. Thank you and regards Torsten > import Data.Char > diffieH p a b = (b ^ a) `mod` p > number2letter number = chr (number + 96) > letter2number letter = (ord letter) - 96 > start text p a b = encipher text (diffieH p a b) > encipher [] _ = [] > encipher (b:rest) key = (number2letter (((letter2number b)+key) `mod` 26)) : (encipher rest key)

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