June 2021 - Beginners

A one- or two-page diagram of how Haskell works?
by Michael Turner 31 Jul '21

31 Jul '21

When I write C, or even C++, I have a mental model of how execution will proceed. When I write Prolog, but get confused, I run a kind of skeletal inference algorithm in my head and the confusion usually clears up. I can imagine how things are stored and what's done with them. I can see /through/ the code to the machine. With Haskell, I still feel blind. Has anyone summarized it all in a chart where I can look at it and think, "Ah, OK, GHC is taking this line and thinking of it THIS way"? If someone wanted to write an interpreter for Haskell, would there be a way for them to see how it would basically need to work, in one chart? Regards, Michael Turner Executive Director Project Persephone 1-25-33 Takadanobaba Shinjuku-ku Tokyo 169-0075 Mobile: +81 (90) 5203-8682 turner(a)projectpersephone.org Understand - http://www.projectpersephone.org/ Join - http://www.facebook.com/groups/ProjectPersephone/ Donate - http://www.patreon.com/ProjectPersephone Volunteer - https://github.com/ProjectPersephone "Love does not consist in gazing at each other, but in looking outward together in the same direction." -- Antoine de Saint-Exupéry

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Simple data type for dates?
by Galaxy Being 30 Jun '21

30 Jun '21

I've looked at Data.Time, but I'm not seeing examples. I'd like to figure out how to simply have use of similar to this: type MyMonth = String type MyDay = String type MyYear = String data MyDate = MyDate MyMonth MyDay MyYear deriving Show storeDate :: MyDate storeDate = "June" "29" "2021" which will be in a bigger data structure I'm building. I can't believe this isn't reinventing some long-established wheel. Is there such a type ready to use? ⨽ Lawrence Bottorff Grand Marais, MN, USA borgauf(a)gmail.com

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Partial results with megaparsec
by Dmitriy Matrosov 28 Jun '21

28 Jun '21

Hi. I've used attoparsec before and it can return 'Partial' result containing continuation, which is able to continue parsing if provided with more input. E.g. > let A.Partial c1 = A.parse (A.string "mac address") "mac "; in c1 "address" Done "" "mac address" Does the same thing possible with megaparsec? After looking through library API, i haven't find a way to do this. 'runParser' (which seems the only way to run megaparsec parser) fails on incomplete (text) input. And if that's not possible, how should i deal with chunked input? E.g. i read from a socket with 'recv' and i don't want to read all data at once. How can i suspend parsing for a time, read next chunk and continue? Thanks.

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DataKinds and GADTs question?
by Velichko Lefterov 12 Jun '21

12 Jun '21

I tried to understand DataKinds and GADTs... Is there a difference between: data MyType :: Type -> Type where MyTypeConstructor :: Int -> MyType Int data MyType2 a where MyTypeConstructor2 :: Int -> MyType2 Int

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Designing TimeOut for Search Algorithm
by Leonhard Applis 07 Jun '21

07 Jun '21

Good Morning, I currently want to write a search algorithm with 3 end conditions: a) Solution Found b) N-iterations done c) x minutes timeout I would like to split the function in two pieces, one which is pure with a seed and no timeout, and one time-outed in a monad (IO?). So pureSearch :: Seed -> Iterations -> Maybe Result and search :: Timeout -> Seed -> Iterations -> IO Maybe Result Is this a known Pattern? Do you know good examples for this behavior? Any other recommendations before I start implementing? Best Leonhard

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Better intro to the All About Monads tutorial?
by Michael Turner 05 Jun '21

05 Jun '21

I've done some work on this page: https://wiki.haskell.org/All_About_Monads I could use some help in improving my improvements 1. Beginners -- if you haven't looked at monads yet, does this new intro boost your confidence? 2. Those who help beginners here -- have I committed any technical errors? I was a little frustrated about the text as it stood. For example: "... a monad is a way to structure computations in terms of values and sequences of computations using typed values. Monads allow the programmer to build up computations using sequential building blocks, which can themselves be sequences of computations. The monad determines how combined computations form a new computation and frees the programmer from having to code the combination manually each time it is required." The problem with this was that you could substitute "compiler" or "C++ class" and it would be not much less true and not much more uninformative. The approach I took to rewriting (aside from where I take the reader's hand gently at the beginning) was to try to anchor the treatment more in concepts that the reader might already understand. The two main concepts were were pipelines (as in shell programming) and "cross-cutting concerns" (with links to Wikipedia articles for readers who might not know one or the other or both.) I've left almost all of the original wording intact and just wrote around it. Perhaps it could be trimmed, maybe even a lot. I don't understand monads well enough to write one myself yet. But I think a newbie perspective can help in writing for newbies: unlike the expert, the sources of confusion and incomprehension are fresh in your mind, as are the ways you got past them. Regards, Michael Turner Executive Director Project Persephone 1-25-33 Takadanobaba Shinjuku-ku Tokyo 169-0075 Mobile: +81 (90) 5203-8682 turner(a)projectpersephone.org Understand - http://www.projectpersephone.org/ Join - http://www.facebook.com/groups/ProjectPersephone/ Donate - http://www.patreon.com/ProjectPersephone Volunteer - https://github.com/ProjectPersephone "Love does not consist in gazing at each other, but in looking outward together in the same direction." -- Antoine de Saint-Exupéry

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import not working
by Galaxy Being 05 Jun '21

05 Jun '21

I go to a directory. I run stack install Safe I start up ghci and get the Prelude prompt. I run import Safe and I get import Safe | ^^^^^^^^^^^ Failed, no modules loaded. I'm once again frustrated. I try to load this hs file {-# LANGUAGE ViewPatterns #-} {-# LANGUAGE NoMonomorphismRestriction #-} import Safe lookupDefault :: Eq a => a -> b -> [(a,b)] -> b lookupDefault k _ (lookup k -> Just s) = s lookupDefault _ d _ = d headTup :: (a, [t]) -> [t] headTup (headMay . snd -> Just n) = [n] headTup _ = [] headNil :: [a] -> [a] headNil (headMay -> Just x) = [x] headNil _ = [] with the same result. What do I need to do? -- ⨽ Lawrence Bottorff Grand Marais, MN, USA borgauf(a)gmail.com

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Re: [Haskell-beginners] import not working
by David McBride 05 Jun '21

05 Jun '21

Spam detection software, running on the system "mail.haskell.org", has identified this incoming email as possible spam. The original message has been attached to this so you can view it (if it isn't spam) or label similar future email. If you have any questions, see @@CONTACT_ADDRESS@@ for details. Content preview: safe is a non core package. You will have to install it by using the command stack exec --package safe ghci You can do that with multiple packages. And I believe you can write a shebang at the top of you're script so that these packages are installed automatically when you run the script. [...] Content analysis details: (5.0 points, 5.0 required) pts rule name description ---- ---------------------- -------------------------------------------------- 0.0 FREEMAIL_FROM Sender email is commonly abused enduser mail provider (toad3k[at]gmail.com) -0.0 SPF_PASS SPF: sender matches SPF record 5.0 UNWANTED_LANGUAGE_BODY BODY: Message written in an undesired language -1.9 BAYES_00 BODY: Bayes spam probability is 0 to 1% [score: 0.0000] 0.0 HTML_MESSAGE BODY: HTML included in message 0.0 T_DKIM_INVALID DKIM-Signature header exists but is not valid 1.9 FREEMAIL_REPLY From and body contain different freemails The original message was not completely plain text, and may be unsafe to open with some email clients; in particular, it may contain a virus, or confirm that your address can receive spam. If you wish to view it, it may be safer to save it to a file and open it with an editor.

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Data constraint?
by Galaxy Being 04 Jun '21

04 Jun '21

If I have a type type WaterChem = CaHardness NaturalChem | Alkalinity NaturalChem and I want to have the values of CaHardness and Alkalinity constrained to positive Int and between certain high and low values, I could do a newtype to creater a NaturalChem number, thus never less than 0, but what is the best practice to insure these values are between a certain range? Types in Haskell can't go that far, can they? Reading this <https://www.haskell.org/tutorial/moretypes.html> tells me I can have some of what I want. How are type values that need to be constrained handled best practices? Again, the type world of Haskell seems to do some of the lifting, but I'd like some advice if I want to have both of my constraints. -- ⨽ Lawrence Bottorff Grand Marais, MN, USA borgauf(a)gmail.com

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