- Beginners - Haskell.org

send string to stack ghc
by Paum B. 24 Aug '18

24 Aug '18

Hi List, It is possibile to run: stack ghc -- --interactive -XOverloadedStrings and then send to it strings (commands) externally? Thanks paum

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Runtime error “Could not deduce (Integral Float) arising from a use of..”
by Jack Vice 23 Aug '18

23 Aug '18

I am trying to calculate the standard deviation for each index in a list of lists of floats. Using Ubuntu 18.04, ghc 8.0.2. I am getting the following runtime error which I have googled and still don't understand what "Integral Float" is exactly or even which parameter is causing the trouble. *Main> let z = stdDev 0 2 y x <interactive>:250:9: error: • Could not deduce (Integral Float) arising from a use of ‘stdDev’ from the context: Floating a bound by the inferred type of z :: Floating a => [a] at <interactive>:250:5-38 • In the expression: stdDev 0 (length (head (x))) y x In an equation for ‘z’: z = stdDev 0 (length (head (x))) y x Code: -- i is start index, l is length of each list, ms is list of means, -- xs is Matrix stdDev i l ms xs | i < l = sqrt(fromIntegral(sumOfMinusMeans i (ms!!i) xs) / fromIntegral(l)):(stdDev (i+1) l ms xs) | otherwise = [] --i is index, m is mean for the index sumOfMinusMeans i m (x:xs) | xs == [] = (x!!i - m)**2 | i < length x = (x!!i - m)**2 + (sumOfMinusMeans i m xs) | otherwise = 0 Types: *Main> :t stdDev stdDev :: (Floating a1, Floating a, Integral a1) => Int -> Int -> [a1] -> [[a1]] -> [a] *Main> :t sumOfMinusMeans sumOfMinusMeans :: (Eq t, Floating t) => Int -> t -> [[t]] -> t Variables: *Main> y [380.0,1.0] *Main> x [[600.0,1.0],[400.0,1.0],[170.0,1.0],[430.0,1.0],[300.0,1.0]]

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Hutton 2nd ed, ex 7.6
by trent shipley 22 Aug '18

22 Aug '18

1. I got through the problem without getting so stuck I had to ask this list. But, I am disappointed in how much I had to cheat by looking up solutions on Google after making my best attempt. This higher order functions chapter is giving more trouble than any chapter so far. 2. Since getting tutoring on how to read function declarations, I'm doing better at deciphering simple declarations, but writing them myself is beyond me more often than not. 3. I feel like Hutton introduces concepts well, but with more brevity and fewer examples than would be best for me. Unfortunately, he leaves out almost all the ancillary pragmatics as exemplified by the need for "null" and "const" below. My bet is that when Hutton 2nd ed is used as a textbook in college those practical lacks get covered in lecture, problem sessions, and in mutual aid between students. Does anyone have any idea on how I might improve my performance. I am not used to having difficulty learning a computer language. (Prolog and Java Swing being exceptions.) {-- 6. A higher-order function unfold that encapsulates a simple pattern of recursion for producing a list can be defined as follows: unfold p h t x | p x = [] | otherwise = h x : unfold p h t (t x) That is, the function unfold p h t produces the empty list if the predicate p is true of the argument value, and otherwise produces a non-empty list by applying the function h to this value to give the head, and the function t to generate another argument that is recursively processed in the same way to produce the tail of the list. For example, the function int2bin can be rewritten more compactly using unfold as follows: int2bin = unfold (==0) (‘mod‘ 2) (‘div‘ 2) Redefine the functions chop8, map f and iterate f using unfold. Hutton, Graham. Programming in Haskell (Kindle Locations 2830-2842). Cambridge University Press. Kindle Edition. Basically, what you do to develop and test this question is to copy and paste extended example 7.6 Binary String Transmittter from the chapter into your text editor. Then you alter large chunks of it to use the unfold function. --} import Data.Char import Prelude hiding (iterate, map) type Bit = Int -- I had to crib the type signature unfold :: (b -> Bool) -> (b -> a) -> (b -> b) -> b -> [a] unfold p h t x | p x = [] | otherwise = h x : unfold p h t (t x) -- Hutton, Graham. Programming in Haskell (Kindle Location 2329). Cambridge University Press. Kindle Edition. -- cribbed type signature -- Had to copy (f . head), my attempt was (f head). -- I had no idea the function "null" existed before Googling the solution. -- Is Googling to learn the existence of "null" accepted pedagological practice? -- Is it a lacuna on Hutton's part? map :: (a -> b) -> [a] -> [b] map f = unfold null (f . head) tail -- had to copy the signature, although I didn't try hard to come up with it on my own. -- I had tried to use (==[]). I got the solution "False" by copying from the internet. -- I would not have gotten the word "const" without cheating from the internet. It is nowhere in Hutton's book up to this point. iterate :: (a -> a) -> a -> [a] iterate f = unfold (const False) id f -- chop8 :: [Bit] -> [[Bit]] -- chop8 [] = [] -- chop8 bits = take 8 bits : chop8 (drop 8 bits) -- Hutton, Graham. Programming in Haskell (Kindle Locations 2681-2683). Cambridge University Press. Kindle Edition. -- I attempted this first, and i pretty much got it on my own. chop8 :: [Bit] -> [[Bit]] chop8 bits = unfold (==[]) (take 8) (drop 8) bits

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What should be inside the Monad or MonadTrans's type declaration? --Bound library question2.
by Anthony Lee 19 Aug '18

19 Aug '18

Hi, In Scope.hs there is one instance of Monad and one instance of MonadTrans for Scope, For the Monad instance, it is defined like this: Monad (Scope b f); For the MonadTrans instance, it is like this: MonadTrans (Scope b); Does it mean: In ">>=" the e represents (a) of (Scope b f a)? In lift function the m represents (f a) of (Scope b f a)? https://github.com/ekmett/bound/blob/master/src/Bound/Scope.hs ========================Scope.hs================================ instance Monad f => Monad (Scope b f) where #if !MIN_VERSION_base(4,8,0) return a = Scope (return (F (return a))) {-# INLINE return #-} #endif Scope e >>= f = Scope $ e >>= \v -> case v of B b -> return (B b) F ea -> ea >>= unscope . f {-# INLINE (>>=) #-} instance MonadTrans (Scope b) where lift m = Scope (return (F m)) {-# INLINE lift #-}

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Re: [Haskell-beginners] Bound library questions (David McBride)
by Anthony Lee 18 Aug '18

18 Aug '18

Now I understand it. Thanks a lot! Anthony On Sat, Aug 18, 2018 at 5:34 AM <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: Beginners Digest, Vol 122, Issue 6 (Anthony Lee) > 2. Re: Bound library questions (David McBride) > 3. Empty list and null (trent shipley) > 4. Re: Empty list and null (Ut Primum) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Fri, 17 Aug 2018 09:21:47 -0400 > From: Anthony Lee <anthonynlee(a)gmail.com> > To: beginners(a)haskell.org > Subject: Re: [Haskell-beginners] Beginners Digest, Vol 122, Issue 6 > Message-ID: > <CA+pBo5FSLqbgC= > L9bONcCb0y+UPSGq6jpVgq60ouKUNk4xNv3w(a)mail.gmail.com> > Content-Type: text/plain; charset="utf-8" > > The source code is here: https://github.com/ekmett/bound/ > > On Fri, Aug 17, 2018 at 8:41 AM <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. Bound library questions (Anthony Lee) > > > > > > ---------------------------------------------------------------------- > > > > Message: 1 > > Date: Fri, 17 Aug 2018 06:46:52 -0400 > > From: Anthony Lee <anthonynlee(a)gmail.com> > > To: beginners(a)haskell.org > > Subject: [Haskell-beginners] Bound library questions > > Message-ID: > > <CA+pBo5HVraf_8jbs15bR6QAy1S7=z_u-3KBwT= > > c3sqkAgc0m5Q(a)mail.gmail.com> > > Content-Type: text/plain; charset="utf-8" > > > > In Scope.hs there are some functions I feel difficult to understand, > > Why fmap/foldmap/traverse is applied three times? > > > > instance Functor f => Functor (Scope b f) where > > fmap f (Scope a) = Scope (fmap (fmap (fmap f)) a) > > {-# INLINE fmap #-} > > > > -- | @'toList'@ is provides a list (with duplicates) of the free > variables > > instance Foldable f => Foldable (Scope b f) where > > foldMap f (Scope a) = foldMap (foldMap (foldMap f)) a > > {-# INLINE foldMap #-} > > > > instance Traversable f => Traversable (Scope b f) where > > traverse f (Scope a) = Scope <$> traverse (traverse (traverse f)) a > > {-# INLINE traverse #-} > >

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Empty list and null
by trent shipley 18 Aug '18

18 Aug '18

Why does Haskell so often seem to treat [] as a general null. For example I know 0 : 1 : [] gives [0, 1]. But shouldn't it produce a type fault in a consistent world? Int:Int:List isn't properly a list. It mixes types. I expect something like: Let GSN mean general_scalar_null. 1 : 2 : GSN -- works 1 : 2 : [] -- type fault, cannot mix int and empty list in the same list. And why does [] == [] : [] instead of [[], []] == [] : [] What sorts of nullity are there in core Haskell? Trent.

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Bound library questions
by Anthony Lee 17 Aug '18

17 Aug '18

In Scope.hs there are some functions I feel difficult to understand, Why fmap/foldmap/traverse is applied three times? instance Functor f => Functor (Scope b f) where fmap f (Scope a) = Scope (fmap (fmap (fmap f)) a) {-# INLINE fmap #-} -- | @'toList'@ is provides a list (with duplicates) of the free variables instance Foldable f => Foldable (Scope b f) where foldMap f (Scope a) = foldMap (foldMap (foldMap f)) a {-# INLINE foldMap #-} instance Traversable f => Traversable (Scope b f) where traverse f (Scope a) = Scope <$> traverse (traverse (traverse f)) a {-# INLINE traverse #-}

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Re: [Haskell-beginners] Beginners Digest, Vol 122, Issue 6
by Anthony Lee 17 Aug '18

17 Aug '18

The source code is here: https://github.com/ekmett/bound/ On Fri, Aug 17, 2018 at 8:41 AM <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. Bound library questions (Anthony Lee) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Fri, 17 Aug 2018 06:46:52 -0400 > From: Anthony Lee <anthonynlee(a)gmail.com> > To: beginners(a)haskell.org > Subject: [Haskell-beginners] Bound library questions > Message-ID: > <CA+pBo5HVraf_8jbs15bR6QAy1S7=z_u-3KBwT= > c3sqkAgc0m5Q(a)mail.gmail.com> > Content-Type: text/plain; charset="utf-8" > > In Scope.hs there are some functions I feel difficult to understand, > Why fmap/foldmap/traverse is applied three times? > > instance Functor f => Functor (Scope b f) where > fmap f (Scope a) = Scope (fmap (fmap (fmap f)) a) > {-# INLINE fmap #-} > > -- | @'toList'@ is provides a list (with duplicates) of the free variables > instance Foldable f => Foldable (Scope b f) where > foldMap f (Scope a) = foldMap (foldMap (foldMap f)) a > {-# INLINE foldMap #-} > > instance Traversable f => Traversable (Scope b f) where > traverse f (Scope a) = Scope <$> traverse (traverse (traverse f)) a > {-# INLINE traverse #-} >

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Exercise 7.2a
by trent shipley 15 Aug '18

15 Aug '18

I am trying to solve Hutton 2016 exercise 7.2a. My attempt gets the same error message as the one below. The problem with the error message below is that it is for the book's solved example, so I have no idea how to make it work. I suspect I have GHCi configured wrong, or an old version that is buggy on Mac, or such. Help is appreciated. Also, at this point I am having trouble reading these function declarations. all :: (a -> Bool) -> [Bool] -> Bool all b ls = and (map b ls) My try: all :: -- name of function defined (a -> -- maps to "all" Bool -- maps to boolean function b ) -> [Bool] -- maps to input ls -> Bool -- maps to return type of "all" all b ls = and (map b ls) OR (right?) all :: -- name of function defined (a -> Bool) function of type a returns bool -> [Bool] -- maps to input ls -> Bool -- maps to return type of "all" all b ls = and (map b ls) ------ Book example all :: (a -- maps to "all" -> Bool -- input function type ) -> [Bool] -- implied list of things mapped over -> Bool -- returned all p = and . map p OR (right?) all :: -- name of defined function (a -> Bool) -- input function of type a returns type Bool -> [Bool] -- implied list to work on -> Bool -- return type all p = and . map p ============================ -- CODE {-- 2. Without looking at the definitions from the standard prelude, define the following higher-order library functions on lists. a.Decide if all elements of a list satisfy a predicate: all :: (a -> Bool) -> [Bool] -> Bool b.Decide if any element of a list satisfies a predicate: any :: (a -> Bool) -> [Bool] -> Bool c.Select elements from a list while they satisfy a predicate: takeWhile :: (a -> Bool) -> [a] -> [a] d.Remove elements from a list while they satisfy a predicate: dropWhile :: (a -> Bool) -> [a] -> [a] Note: in the prelude the first two of these functions are generic functions rather than being specific to the type of lists. Hutton, Graham. Programming in Haskell (Kindle Locations 2806-2819). Cambridge University Press. Kindle Edition. --} import Prelude hiding (all) {-- My attempt. Presumably bad. -- I swear GHCi was happy with it yesterday. all :: (a -> Bool) -> [Bool] -> Bool all b ls = and (map b ls) --} -- from book all :: (a -> Bool) -> [Bool] -> Bool all p = and . map p {-- >From GHCi on Mac Prelude> :r [1 of 1] Compiling Main ( ex7_2a.hs, interpreted ) ex7_2a.hs:30:9: error: • Couldn't match type ‘a’ with ‘Bool’ ‘a’ is a rigid type variable bound by the type signature for: all :: forall a. (a -> Bool) -> [Bool] -> Bool at ex7_2a.hs:29:1-36 Expected type: [Bool] -> Bool Actual type: [a] -> Bool • In the expression: and . map p In an equation for ‘all’: all p = and . map p • Relevant bindings include p :: a -> Bool (bound at ex7_2a.hs:30:5) all :: (a -> Bool) -> [Bool] -> Bool (bound at ex7_2a.hs:30:1) | 30 | all p = and . map p | ^^^^^^^^^^^ Failed, no modules loaded. --}

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How to write the FromJSON instance for CUid
by PICCA Frederic-Emmanuel 10 Aug '18

10 Aug '18

Hello, I try to write the instance for the CUid[1] type But I do not know what to do. instance FromJSON CUid where parseJSON = ... {-# INLINE parseJSON #-} Thansk for your help [1] https://hackage.haskell.org/package/base-4.11.1.0/docs/System-Posix-Types.h…

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