- Beginners - Haskell.org

Maybe?
by Alexander Chen 13 May '20

13 May '20

Hi, safeHead.hs safeHead :: [a] -> Maybe a safeHead [] = Nothing safeHead (x:_) = Just x prelude> safeHead [23,1,4,2,4,2] Just 23 Question: what am i doing wrong? best, Alexander

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formatting?
by Alexander Chen 08 May '20

08 May '20

Hi, tensDigit.hs tensDigit :: Integral a => a ->a tensDigit x = d where xLast = x `div` 10 d = xLast `mod` 10 prelude> load tensDigit.hs parse error on input ‘=’ Perhaps you need a 'let' in a 'do' block? e.g. 'let x = 5' instead of 'x = 5' 4 | d = xLast `mod` 10 | ^ [1 of 1] Compiling Main ( chapter7_8.hs, interpreted ) Failed, no modules loaded. I suspect this has something to do with the spacing (since this is example code) if so could someone explain to me what the rules on spacing are so I can get past this. I am using a simple text editor so if someone has a recommendation for an IDE that simply takes care of this, much obliged!! best,

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Re: [Haskell-beginners] formatting? (Alexander Chen)
by nowsnow 08 May '20

08 May '20

> I am using a simple text editor > so if someone has a recommendation for an IDE that simply takes care of this, much obliged!! Geany is a powerful, stable and lightweight programmer's text editor that provides tons of useful features without bogging down your workflow. It runs on Linux, Windows and MacOS is translated into over 40 languages, and has built-in support for more than 50 programming languages.

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why does the superclass not work?
by Alexander Chen 29 Apr '20

29 Apr '20

Hi, superclass.hs myX = 1 :: Int sigmund :: Int -> Int sigmund x = myX prelude> :l superclass.hs [1 of 1] Compiling Main Ok, one module loaded. ================================================================================== superclass.hs myX = 1 :: Int sigmund :: Num -> Num sigmund x = myX prelude> :l superclass.hs typed_checked.hs:3:13: error: • Expecting one more argument to ‘Num’ Expected a type, but ‘Num’ has kind ‘* -> Constraint’ • In the type signature: sigmund :: Num -> Num | 3 | sigmund :: Num -> Num | ^^^ typed_checked.hs:3:20: error: • Expecting one more argument to ‘Num’ Expected a type, but ‘Num’ has kind ‘* -> Constraint’ • In the type signature: sigmund :: Num -> Num =================================================================================== I would think since Num is a superclass of Int defining the type in the superclass would be OK, also in the error message it refers to (see black) what does that refer to? thanks!

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It needs a binder?
by Alexander Chen 29 Apr '20

29 Apr '20

Hi, binder.hs i:: Num a => a prelude>:l binder.hs typed_checked.hs:1:1: error: The type signature for ‘i’ lacks an accompanying binding | 1 | i:: Num a=> a | ^ binder.hs i:: Num a => a i = 2 prelude>:l binder.hs [1 of 1] Compiling Main Ok, one module loaded. Why does it need a binder to make it work?

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Unsubscribe
by Christopher Ledbetter 27 Apr '20

27 Apr '20

Unsubscribe Sent from my iPad > On Apr 26, 2020, at 8:51 AM, Ken Overton <ken.overton(a)gmail.com> wrote: > > > Hello all, > > I recently came across this function which made me realize I don't understand list comprehensions well. I hope someone can help me understand them better by understanding this example better. The function takes a list of Eq and returns the list of unique elements from it: > > unique :: Eq a => [a] -> [a] > unique xs = [x | (x,y) <- zip xs [0..], x `notElem` (take y xs)] > > It's using a list comprehension with multiple 'generators' (hope I have the term correctly). My understanding of multiple generators in a list comprehension is that they refine the results of the previous generator. > > So the first generator should produce [(Eq,Int)] as input to the second generator? And the second generator should produce [Bool]? > > My understanding must be wrong though; how do we end up with just the items where the second generator produced True? > > Thanks, > > > -- > Ken Overton > (917) 863-3937 > ken.overton(a)gmail.com > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

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List comprehensions with multiple generators
by Ken Overton 26 Apr '20

26 Apr '20

Hello all, I recently came across this function which made me realize I don't understand list comprehensions well. I hope someone can help me understand them better by understanding this example better. The function takes a list of Eq and returns the list of unique elements from it: unique :: Eq a => [a] -> [a] unique xs = [x | (x,y) <- zip xs [0..], x `notElem` (take y xs)] It's using a list comprehension with multiple 'generators' (hope I have the term correctly). My understanding of multiple generators in a list comprehension is that they refine the results of the previous generator. So the first generator should produce [(Eq,Int)] as input to the second generator? And the second generator should produce [Bool]? My understanding must be wrong though; how do we end up with just the items where the second generator produced True? Thanks, -- Ken Overton (917) 863-3937 ken.overton(a)gmail.com

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instance 2
by Alexander Chen 22 Apr '20

22 Apr '20

hi, data StringOrInt = TisanInt Int | TisAString String instance Eq (StringOrInt) where (==) (TisanInt v) (TisanInt v') = v == v' (==) (TisAString s) (TisAString s') = s == s' prelude> Instance_testen.hs:4:37: error: Ambiguous occurrence ‘==’ It could refer to either ‘Prelude.==’, imported from ‘Prelude’ at Instance_testen.hs:1:1 (and originally defined in ‘GHC.Classes’) or ‘Main.==’, defined at Instance_testen.hs:4:1 | 4 | (==) (TisanInt v) (TisanInt v') = v == v' | ^^ Instance_testen.hs:5:41: error: Ambiguous occurrence ‘==’ It could refer to either ‘Prelude.==’, imported from ‘Prelude’ at Instance_testen.hs:1:1 (and originally defined in ‘GHC.Classes’) or ‘Main.==’, defined at Instance_testen.hs:4:1 | 5 | (==) (TisAString s) (TisAString s') = s == s' | ^^ [1 of 1] Compiling Main ( Instance_testen.hs, interpreted ) Failed, no modules loaded. what gives?

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instance
by Alexander Chen 22 Apr '20

22 Apr '20

Hi, data TisanInteger = Tisan Integer instance Eq (Tisan Integer) where (Tisan v) == (Tisan v') = v == v' prelude> Instance_testen.hs:7:14: error: Not in scope: type constructor or class ‘Tisan’ A data constructor of that name is in scope; did you mean DataKinds? | 7 | instance Eq (Tisan Integer) where (Tisan v) == (Tisan v') = v == v' | ^^^^^ [1 of 1] Compiling Main ( Instance_testen.hs, interpreted ) Failed, no modules loaded. what am i doing wrong? best,

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[ANN] Haskell Beginners WT.Social Community: "Haskell: The Haskell-Beginners Community"
by Benjamin L. Russell 18 Apr '20

18 Apr '20

Twenty-eight days ago (on March 23, 2020 JST), I created a new Haskell beginners WT.Social community, "Haskell: The Haskell-Beginners Community" (see https://pashpost.com/group/haskell-the-haskell-beginners-community) and migrated a substantial portion of the posts originally from my former corresponding pashPOST (since renamed to "theTUNDRA") community of the same name thereto (sansposts with missing links or irrelevant content). Feel free to join and contribute! -- Benjamin L. Russell -- Benjamin L. Russell / DekuDekuplex at Yahoo dot com http://dekude kuplex.wordpress.com/ Computer Science Document Translator/Editor "Furuike ya, kawazu tobikomu mizu no oto." -- Matsuo Basho^

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