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type T' = T a b c
by Imants Cekusins 22 Aug '16

22 Aug '16

*problem*: given type T with a few parameters e.g. T a b c some functions where this type is passed to, are agnostic to a, b, c i.e. accept T with any of a, b, c processT::T a b c -> () when either number of these parameters or T change (c is dropped or d is added: T a b c d), signatures of every function where T is passed to, need to be updated. *question*: is this possible to specify type synonym T' so that it may be passed to parameter (a,b,c) - agnostic functions so: processT::T' -> () ?

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help writing a simple api which queries for a resource and displays this info
by xu xiut 21 Aug '16

21 Aug '16

Hello, I am wanting to store something in a database, then retrieve it. That is my entire ambition for the day. Technologies chosen: spock: https://github.com/agrafix/Spock rethinkdb driver: https://github.com/AtnNn/haskell-rethinkdb I chose to go with Rethinkdb, but i'm open to using Postgresql. If someone requests for /companies, I want to list all the companies in the "companies" table. ``` -- ghci λ> companies <- run h $ table "companies" :: IO [Datum] λ> companies [{"name":"tesla","id":"7781ee7e-1e43-4608-bb96-fe10cac3b53a"}] λ> firstCompany <- run h $ table "companies" ! "name" :: IO [Datum] λ> firstCompany ["tesla"] λ> :info Datum data Datum = Null | Bool Bool | String Text | Number Double | Array Database.RethinkDB.Datum.Array | Object Database.RethinkDB.Datum.Object | Time time-1.5.0.1:Data.Time.LocalTime.LocalTime.ZonedTime | Point LonLat | Line GeoLine | Polygon GeoPolygon | Binary bytestring-0.10.6.0:Data.ByteString.Internal.ByteString -- Defined in ‘Database.RethinkDB.Datum’ instance Eq Datum -- Defined in ‘Database.RethinkDB.Datum’ instance Ord Datum -- Defined in ‘Database.RethinkDB.Datum’ instance Show Datum -- Defined in ‘Database.RethinkDB.Datum’ instance Expr Datum -- Defined in ‘Database.RethinkDB.ReQL’ instance Result Datum -- Defined in ‘Database.RethinkDB.Driver’ instance ToDatum Datum -- Defined in ‘Database.RethinkDB.Datum’ instance FromDatum Datum -- Defined in ‘Database.RethinkDB.Datum’ 15:40 < lpaste> xuxu revised “investigating the Datum data type”: “investigating the Datum data type” at http://lpaste.net/179391 ``` I don't know how to display the result. >From Spock's readme, here's an example where I don't need to query the database: ``` main = runSpock 3000 $ spockT id $ do get "companies" $ text $ T.concat ["tesla", "google"] ``` How to convert that into something a little more practial where I do utilize a db? Here's what I have right now, but I'm sure it doesn't compile ``` {-# LANGUAGE OverloadedStrings #-} import Web.Spock import qualified Data.Text as T import qualified Database.RethinkDB as R import qualified Database.RethinkDB.NoClash default (Datum, ReQL, String, Int, Double) main :: IO () main = -- 1. i don't know if the following line will work let h = connect "localhost" 28015 Nothing in runSpock 3000 $ spockT id $ -- 2. list all companies do get "companies" $ text $ T.concat -- something ``` I don't exactly know how "text" works. If I'm reading https://github.com/agrafix/Spock/blob/1ee54503ad67f62af795a31772040f56f7ae0… correctly, spockT is using a reader monad. I don't know how it works with the Text type, but I have a suspicion I need to convert Rethink's Datum type into a Text type. I would be absolutely thrilled if anyone is able to help with this.

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Automatic Differentiation
by Jonas Béal 17 Aug '16

17 Aug '16

Hello, I have some questions about the Automatic Differentiation package in Haskell (https://hackage.haskell.org/package/ad-4.3.2.1/docs/Numeric-AD.html ) I need to compute the jacobian with that method and I have a type problem, here a simplified example in order to focus on the error: According to documentation I need to write "*jacobian function values*". So I built the input *function. *Please notice this is important for me that my function may depend on an external parameter (named factor here) testFunctionForJacobian :: Fractional a => a -> [a] -> [a] testFunctionForJacobian factor inputs = [(sum inputs) * factor] Then, using *jacobian* function of Numeric.AD in terminal, as a test, it works perfectly >jacobian (testFunctionForJacobian 2) [1,2] < [[2.0,2.0]] No apparent type problem here > :t (testFunctionForJacobian 2) (testFunctionForJacobian 2) :: Fractional a => [a] -> [a] But, I would like to insert that in a bigger function computing the jacobian testJacobian :: Fractional a => a -> [a] -> [[a]] testJacobian factor inputs = jacobian (testFunctionForJacobian factor) inputs This time it generates an error message about *factor* Couldn't match expected type ‘Numeric.AD.Internal.Reverse.Reverse <http://numeric.ad.internal.reverse.reverse/> s a’ with actual type ‘a’ ‘a’ is a rigid type variable bound by the type signature for testJacobian :: Fractional a => a -> [a] -> [[a]] s a’ All in all, my type seems implicitly correct in the terminal example but I did not manage to write it explicitly in my function. Here the *jacobian* function signature to help you answer me: -- Jonas Béal -- *This message is intended only for the personal and confidential use of the designated recipient(s) named above. If you are not the intended recipient of this message you are hereby notified that any review, dissemination, distribution or copying of this message is strictly prohibited. Email transmission cannot be guaranteed to be secure or error-free. Therefore, we do not represent that this information is complete or accurate and it should not be relied upon as such. All information is subject to change without notice.*

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Bool newtype
by Imants Cekusins 14 Aug '16

14 Aug '16

for a Bool-like newtype: newtype B = B Bool , is there an easy way to use this newtype B in place of Bool? e.g. let b1 = B True in if b1 then 1 else 0 ?

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Problem getting haskell-mode working with emacs on windows
by George Taylor 13 Aug '16

13 Aug '16

Hello, Environment: Win7 32bit, Emacs 24.5.1, ghci 7.8.3 I have had Haskell working with Windows 7 and emacs previously, but have had issues, and am attempting to get everything working again. Problems. I searched for "Install Haskell Emacs", and found a link to the page https://wiki.haskell.org/Emacs/Installing_haskell-mode But this page was blanked on 11 April 2016, with no information as to where to go. I looked at the previous version of the page, and it said to "place the basic mode haskell-mode.el and the modules you want to use in [a suitable directory, I used "~/haskell-extensions"], add the following command to your init file (~/.emacs): (load "~/lib/emacs/haskell-site-file")" I did the above (but just with haskell-mode.el) and launched emacs, and got a message: "File error: Cannot open load file, no such file or directory, ~/haskell-extensions/haskell-site-file" in emacs, as an experiment, I did M-x Load file: C:\home/haskell-extensions/haskell-mode.el And got the message "Cannot open load file: no such file or directory, haskell-customize" I downloaded haskell-customize.el and placed it in ~/haskell-extensions, and tried again, but no joy. Questions: 0. Is haskell-mode depracated? It no longer seems to be available at MELPA. 1. Why was the page "https://wiki.haskell.org/Emacs/Installing_haskell-mode" blanked? 2. Is a similar, current set of instructions available? 3. Should I be able to load haskell-mode using M-x load-file? If so, what other .el (or other) files are required, and where should they be placed? 4. Is there a dissimilar set of instructions available? Any help will win thousands in prizes.

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My Continuation doesn't typecheck
by martin 08 Aug '16

08 Aug '16

Hello all, in order to gain some intuition about continuations, I tried the following: -- two functions accepting a continuation f1 :: Int -> (Integer->r) -> r f1 a c = c $ fromIntegral (a+1) f2 :: Integer -> (String -> r) -> r f2 b c = c $ show b -- combine the two functions into a single one run1 :: Int -> (String -> r) -> r run1 a = f1 a f2 -- *Main> run1 9 id -- "10" So far so good. Then I tried to write a general combinator, which does not have f1 and f2 hardcoded: combine a f g = f a g -- This also works -- *Main> combine 9 f1 f2 id -- "10" What confuses me is the the type of combine. I thought it should be combine :: Int -> (Int -> (Integer->r) -> r) -> -- f1 (Integer -> (String -> r) -> r) -> -- f2 ((String -> r) -> r) but that doesn't typecheck: Couldn't match expected type ‘(String -> r) -> r’ with actual type ‘r’ Can you tell me where I am making a mistake?

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reading lines with history from bash terminal in OS X
by Dennis Raddle 08 Aug '16

08 Aug '16

I wrote a program first in Windows, where it works as expected, and now I'm using it in OS X and getting undesired behavior. It reads lines from the terminal using the getLine function. In Windows (DOS, actually) the up and down arrows can be used to choose previously entered lines. However, this does not work in bash in OS X. What do I need to get the history available via the arrow keys? D

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Learning Haskell: More About Algebraic Data Types
by Manuel M T Chakravarty 08 Aug '16

08 Aug '16

We just published a new chapter in our online Haskell tutorial ”Learning Haskell”: http://learn.hfm.io This seventh chapter expands on algebraic data types by discussing parametric & recursive data types. It also includes a small graphics library to graphically render binary tries as you construct and manipulate them. Happy Learning! Manuel

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Generalised symbolic expressions
by Dmitrij Szamozvancev 07 Aug '16

07 Aug '16

Hello all, I'm learning about data types and type classes and tried implementing a simple symbolic expression evaluator, but generalised for non-numeric data types. data GenExpr a = Atom a | Sum (GenExpr a) (GenExpr a) | Prod (GenExpr a) (GenExpr a) | Neg (GenExpr a) What I am hoping to do is write a general evaluation function which can some GenExp expression and evaluate it. For example, giving it a GenExp Integer would evaluate it according to normal arighmetic, GenExp Bool would be evaluated using Boolean algebra etc. I've declared explicit type classes for types that can be summed, multiplied and negated: class HasSum a where (.+.) :: a -> a -> a class HasProd a where (.*.) :: a -> a -> a class HasNeg a where (.-) :: a -> a instance HasSum Integer where (.+.) = (+) instance HasProd Integer where (.*.) = (*) instance HasNeg Integer where (.-) = negate instance HasSum Bool where (.+.) = (||) instance HasProd Bool where (.*.) = (&&) instance HasNeg Bool where (.-) = not -- Evaluate generalised expression genEval :: (HasSum a, HasProd a, HasNeg a) => GenExpr a -> a genEval (Atom a) = a genEval (Sum ge1 ge2) = genEval ge1 .+. genEval ge2 genEval (Prod ge1 ge2) = genEval ge1 .*. genEval ge2 genEval (Neg ge) = (.-) $ genEval ge However, I would like it to work with types that don't support all three operations, e.g. natural numbers which don't have a negation operation or strings that only have concatenation as addition: instance HasSum Nat where (.+.) = (+) instance HasProd Nat where (.*.) = (*) instance HasSum String where (.+.) = (++) But these wouldn't be suitable inputs for genEval because they don't fulfil all three type constraints. The only solution I can think of is writing separate genEvalNat and genEvalString functions, which kind of defeats the purpose of the generalisation. Is there a more elegant way to solve this problem? Thank you in advance! Dima

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import Control.Lens.Tutorial
by Dan Phillips 04 Aug '16

04 Aug '16

Hi all I'm trying to follow this tutorial about lenses: https://hackage.haskell.org/package/lens-tutorial-1.0.1/docs/Control-Lens-T… Right at the top of the tutorial,it says do this in ghci: import Control.Lens.Tutorial I tried that, but I got an error: "Could not find module `Control.Lens.Tutorial'" So I tried this: D:\Sync\src\tutorials> stack install lens-tutorial Run from outside a project, using implicit global project config Using resolver: lts-6.9 from implicit global project's config file: C:\Users\dan\AppData\Roaming\stack\global-project\stack.yaml lens-tutorial-1.0.1: download lens-tutorial-1.0.1: configure lens-tutorial-1.0.1: build lens-tutorial-1.0.1: copy/register I'm not exactly sure what that did, but it looked promising. However, I still get the same error in ghci as before when I do the import. My question is, what do I need to do to get the import statement working in ghci so I can follow the tutorial? I would be grateful for any guidance. Thanks. Regards Dan Phillips

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