April 2020 - Beginners

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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Why the compiler does not commit to a type class?
by Alexander Chen 17 Apr '20

17 Apr '20

hi, Prelude> u = undefined Prelude> k :: (Ord a, Num b) => a -> b -> a; k = u Prelude> :t k 1 2 k 1 2 :: (Ord a, Num a) => a Prelude> :t k (1 :: Integer) 2 k (1 :: Integer) 2 :: Integer In the first version the compiler is not really pushed so it simply gives back the type class constriction it has to adhere to. While in the second it is pushed to give an more specific answer and thus the :: Integer. Question: Is this train of thought correct?

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'/' instead use 'DIV'. why?
by Alexander Chen 17 Apr '20

17 Apr '20

Hi, Prelude> :t (/) (/) :: Fractional a => a -> a -> a Prelude> :t div div :: Integral a => a -> a -> a Prelude> 6 / length [23,34,45] error Prelude> 6 / 3 2.0 Could somebody explain to me why this is? thanks, Alexander Chen

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