- Beginners - Haskell.org

Type signatures returned with :t
by Galaxy Being 19 Sep '21

19 Sep '21

> :t Just True Just True :: Maybe Bool > :t Left True Left True :: Either Bool b > :t Right False Right False :: Either a Bool What am I being told here? It seems data Maybe a = Just a | Nothing data Either a b = Left a | Right b are both straightforward parameterized types, but Maybe doesn't give me a type parameter back, while Either does, and in different order, different names (a becomes b; b becomes a) depending on which variable I invoke. What deeper lore am I not seeing here? -- ⨽ Lawrence Bottorff Grand Marais, MN, USA borgauf(a)gmail.com

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Re: [Haskell-beginners] A one- or two-page diagram of how Haskell works?
by Anthony Clayden 16 Sep '21

16 Sep '21

Thank you Tarik, you're spot-on correct. > It's better to try to model execution using lambda calculus. > The most dominant factor in languages like Haskell is lambda calculus. > Think through lambda calculus for a mental model. I presume it's this advice that Michael is complaining about in his long comment on the cafe yesterday. In particular, Haskell is unusual even amongst functional programming languages in its lazy evaluation. So to try to correct Michael's misunderstanding: >> When I write C, or even C++, I have a mental model of how execution will proceed. Firstly, 'execution' in Haskell does not proceed in anything like the sequence-of-steps and update-of-locations/frame-on-the-stack of C or C++ (or Fortran or ALGOL or COBOL or ...). In particular, 'variables' in Haskell are not overwritable locations in the sense of a procedural language. Variables are just a name for an expression (might be a lambda expression). This is what we mean by 'referential transparency' that procedural languages do not have. We can replace any variable by the expression it denotes, or by any other equivalent expression. Secondly, there's an important theorem (Church-Rosser) in Lambda calculus [see wikipedia], that if Beta-reduction terminates, it'll produce the same 'reduced form' of the initial expression whether it proceeds inside-out or outside-in or both-sides-to-the-middle. You don't need to second-guess how the compiler proceeds, because you don't need to know. You do need to understand Beta-reduction (that's easy). You also need to understand why Alpha-renaming is needed hand-in-hand with Beta-reduction (not so easy, but work a few examples with a recursive function, such as stepping through a list). And you need to stop immediately trying to imagine Haskell semantics as some sort of crazy Turing machine: you'll go mad. Turing himself [1937] proved the semantics are equivalent; but that paper is impenetrable for ordinary mortals like me.

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Explanation of elem data type
by Galaxy Being 04 Aug '21

04 Aug '21

According to this <http://zvon.org/other/haskell/Outputprelude/elem_f.html> elem is Eq a => a -> [a] -> Bool but according to my ghci :t it's this elem :: (Foldable t, Eq a) => a -> t a -> Bool I understand the first, but not the second, especially with the t. What is this saying extra, different from the first one? ⨽ Lawrence Bottorff Grand Marais, MN, USA borgauf(a)gmail.com

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Re: [Haskell-beginners] Beginners Digest, Vol 157, Issue 1
by Michael Turner 03 Aug '21

03 Aug '21

"I recently wrote a blog post which explains how expressions are evaluated: https://coot.me/posts/containers-strict-foldr.html" OK, but what I was asking for is a one-or-two-page diagram of the dependencies among Haskell concepts. A diagram. Do you know of any such thing? 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 On Sun, Aug 1, 2021 at 9:07 PM <beginners-request(a)haskell.org> wrote: > > Send Beginners mailing list submissions to > beginners(a)haskell.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > or, via email, send a message with subject or body 'help' to > beginners-request(a)haskell.org > > You can reach the person managing the list at > beginners-owner(a)haskell.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Beginners digest..." > > > Today's Topics: > > 1. Re: A one- or two-page diagram of how Haskell works? > (coot(a)coot.me) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Sat, 31 Jul 2021 14:49:16 +0000 > From: coot(a)coot.me > To: The Haskell-Beginners Mailing List - Discussion of primarily > beginner-level topics related to Haskell <beginners(a)haskell.org> > Subject: Re: [Haskell-beginners] A one- or two-page diagram of how > Haskell works? > Message-ID: > <EZfK5cGJHh6cQSJdxYXSllCLYfcKlKaNZ901czluMR9OpPv1PUXkB50Ti5u_RKom9_14czzQZERMj1_ySn3P50uTZ4GIwSg137PpsJ9m1no=(a)coot.me> > > Content-Type: text/plain; charset="utf-8" > > I recently wrote a blog post which explains how expressions are evaluated: https://coot.me/posts/containers-strict-foldr.html > > Best regards, > Marcin > > Sent with ProtonMail Secure Email. > > ‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐ > > On Thursday, June 24th, 2021 at 05:41, Michael Turner <michael.eugene.turner(a)gmail.com> wrote: > > > 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 > > > > > Beginners mailing list > > > > > Beginners(a)haskell.org > > > > > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners >

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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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Ratio data constructor
by Galaxy Being 21 Jul '21

21 Jul '21

I'm investigating rational numbers with Haskell. This is the source I've found data Ratio a = !a :% !a deriving (Eq) reduce :: (Integral a) => a -> a -> Ratio a {-# SPECIALISE reduce :: Integer -> Integer -> Rational #-} reduce _ 0 = ratioZeroDenominatorError reduce x y = (x `quot` d) :% (y `quot` d) where d = gcd x y (%) :: (Integral a) => a -> a -> Ratio a x % y = reduce (x * signum y) (abs y) The Ratio data type would seem to be a parameterized type with two parameters of the same type that must be "settled" in that they're not to be lazily dealt with. Then the :% is the data constructor, the : meaning it's a data constructor and not just an operation function. So this could have been data Ratio a = :% !a !a deriving (Eq) correct? But then what confuses me is in reduce, why reduce x y = (x `quot` d) :% (y `quot` d) and not just %? We have :% defined in the data type and then (%) defined as a function. What is going on here? -- ⨽ Lawrence Bottorff Grand Marais, MN, USA borgauf(a)gmail.com

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Can't build project properly
by Galaxy Being 03 Jul '21

03 Jul '21

I'm trying to do a basic stack project named http-lesson (Chapter 39 from *Get Programming with Haskell) *which starts out with this in Main.hs module Main where import qualified Data.ByteString as B import qualified Data.ByteString.Char8 as BC import qualified Data.ByteString.Lazy as L import qualified Data.ByteString.Lazy.Char8 as LC import Network.HTTP.Simple After doing stack new http-lesson, the books says to modify the http-lesson.cabal file in the project, adding bytestring and http-conduit for the imports noted above executable http-lesson-exe main-is: Main.hs other-modules: Paths_http_lesson hs-source-dirs: app ghc-options: -threaded -rtsopts -with-rtsopts=-N build-depends: base >=4.7 && <5 , http-lesson , bytestring , http-conduit default-language: Haskell2010 Being a beginner, I follow the cookbook-like directions, i.e., I next run stack build . . . and it errors out, not finding the imported stuff. But then when I check the http-lesson.cabal file, the extra packages I added have been removed. Hunting around, I find this <https://schooloffp.co/2020/12/05/whirlwind-tour-of-stack-for-beginners.html> that tells me to put the extra packages in package.yaml. Very confused at this point. ⨽ Lawrence Bottorff Grand Marais, MN, USA borgauf(a)gmail.com

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Date type needs helper function?
by Galaxy Being 01 Jul '21

01 Jul '21

I am using the built-in data type Day (Data.Time) in two ways data PDate = PDate Day or type PDate Day doesn't seem to matter. But then this doesn't work testrec1 = PDate 2021 7 1 I always must use the "helper function" fromGregorian testrec0 = PDate (fromGregorian 2021 7 1) ... PDate 2021-07-01 Looking at Real World Haskell examples data BookInfo = Book Int String [String] deriving (Show) ... myInfo = Book 9780135072455 "Algebra of Programming" ["Richard Bird", "Oege de Moor"] I know there's a great Haskell lesson to learn here, so why can Book take everything naked but my Day version not? ⨽ Lawrence Bottorff Grand Marais, MN, USA borgauf(a)gmail.com

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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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