June 2017 - Beginners

mempty and "No instance for (Monoid Int)"
by Baa 07 Jun '17

07 Jun '17

Maybe a is the Monoid: instance Monoid a => Monoid (Maybe a) -- Defined in ‘GHC.Base’ so I can compare its values with empty value: mempty == Nothing => True But if I try: mempty == Just 4 I get: <interactive>:1:1: error: • Ambiguous type variable ‘a0’ arising from a use of ‘mempty’ prevents the constraint ‘(Monoid a0)’ from being solved. Probable fix: use a type annotation to specify what ‘a0’ should be. These potential instances exist: instance Monoid a => Monoid (IO a) -- Defined in ‘GHC.Base’ instance Monoid Ordering -- Defined in ‘GHC.Base’ instance Monoid a => Monoid (Maybe a) -- Defined in ‘GHC.Base’ ...plus 7 others (use -fprint-potential-instances to see them all) • In the first argument of ‘(==)’, namely ‘mempty’ In the expression: mempty == Just 4 In an equation for ‘it’: it = mempty == Just 4 OK, I try: mempty::Maybe Int and get: <interactive>:1:1: error: • No instance for (Monoid Int) arising from a use of ‘mempty’ • In the expression: mempty :: Maybe Int In an equation for ‘it’: it = mempty :: Maybe Int so, how is related Int to Monoid, why does ghc expect from mempty::Maybe Int, Int to be Monoid?! As I understand, this means only that I mean "mempty" from (Maybe Int) type, which is Monoid and exists sure. Interesting is, that: mempty::Maybe [Int] => Nothing but how is related "monoidality" of "Maybe a" with "monoidality of "a" ??? Initial idea was to make comparison: mempty :: Maybe Int == Just 4 => False /Best regards, Paul

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Got "parse error on input ‘=’” in Emacs Intero REPL when define any function
by 乃潇张 07 Jun '17

07 Jun '17

Hi, I am trying to define some functions in a REPL window of emacs intero. However, I always got "parse error on input ‘=’” as blow: Starting: stack ghci --with-ghc intero "--docker-run-args=--interactive=true --tty=false" --no-build --no-load --ghci-options -odir=/Users/spike/Code/haskell/learn-haskell/baby/.stack-work/intero/intero445zea --ghci-options -hidir=/Users/spike/Code/haskell/learn-haskell/baby/.stack-work/intero/intero445zea baby Intero 0.1.20 (GHC 8.0.2) Type :intro and press enter for an introduction of the standard commands. addTwo x y = x + y <interactive>:2:12: error: parse error on input ‘=’ Perhaps you need a 'let' in a 'do' block? e.g. 'let x = 5' instead of 'x = 5’ I did some research, seems like “let” is not a must here for ghci version above 8.0. And my ghc version is 8.0.2. So is it a issue of ghci or intero? Thanks, Sid

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help designing types for a gsl fit
by PICCA Frederic-Emmanuel 02 Jun '17

02 Jun '17

Hello I imagine that I have two types A and B I want to create a ADT data C = C A B 1) the toGsl class class ToGsl a where toGsl :: a -> [Double] this class allow to convert a type to a Vector of double in order to use gsl for a fit so I can create instance for A and B (I do not write the instance but the result of the toGsl function) A -> [a1, ... an] B -> [b1, ... bm] Now I want this to work also with C C -> [a1..., an] ++ [b1, .... bm] So the C instance is juste the (toGsl a) ++ (toGsl b) Is there a way to write this generically for other type construct like (data D = D C A) etc.... Functor, Applicative, Monoid ? 2) the fromGsl case now the inverse method is cladd FromGsl a where fromGsl :: a -> [Double] -> a so now the question is if I have a [Double], I need t osplit this list for each composant of the ADT. in the case of C fromGSL (C a b) v = ...split the vector... and apply fromGsl to a and b So my question is the symetic of the previous one. Is it possible to create a generic function inorder to do this. thanks for your help. Frederic

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Attoparsec parser question
by PICCA Frederic-Emmanuel 01 Jun '17

01 Jun '17

Hello I have this kind ot type data PoniEntry a = PoniEntry { poniEntryHeader :: [Text] , poniEntryDetector :: (Maybe (Detector a)) -- ^ Detector Name , poniEntryPixelSize1 :: (Length Double) -- ^ pixels size 1 , poniEntryPixelSize2 :: (Length Double) -- ^ pixels size 1 , poniEntryDistance :: (Length Double) -- ^ pixels size 2 , poniEntryPoni1 :: (Length Double) -- ^ poni1 , poniEntryPoni2 :: (Length Double) -- ^ poni2 , poniEntryRot1 :: (Angle Double) -- ^ rot1 , poniEntryRot2 :: (Angle Double) -- ^ rot2 , poniEntryRot3 :: (Angle Double) -- ^ rot3 , poniEntrySpline :: (Maybe Text) -- ^ spline file , poniEntryWavelength :: WaveLength -- ^ wavelength } deriving (Show) type Poni a = [PoniEntry a] And I try to create a Parser for the Poni type lengthP :: Text -> Parser (Length Double) lengthP key = do value <-doubleP key pure $ value *~ meter detectorP ∷ ToPyFAI a ⇒ a → Parser a detectorP a = do _ ← "Detector: " *> string (toPyFAI a) <* endOfLine pure a poniEntryP ∷ Detector a ⇒ a → Parser (PoniEntry a) poniEntryP a = PoniEntry <$> headerP <*> optional (detectorP a) <*> lengthP "PixelSize1: " <*> lengthP "PixelSize2: " <*> lengthP "Distance: " <*> lengthP "Poni1: " <*> lengthP "Poni2: " <*> angleP "Rot1: " <*> angleP "Rot2: " <*> angleP "Rot3: " <*> optional ("SplineFile: " *> takeTill isEndOfLine <* endOfLine) <*> lengthP "Wavelength: " <?> "poniEntryP" poniP :: Detector a ⇒ a → Parser (Poni a) poniP a = some (poniEntryP a) But when I compile this I get this error message. src/Hkl/PyFAI/Poni.hs:(118,16)-(131,24): Couldn't match type `attoparsec-0.10.4.0:Data.Attoparsec.Internal.Types.Parser Text (PoniEntry a)' with `a -> Parser (PoniEntry a)' Expected type: a -> Parser (PoniEntry a) Actual type: Parser (PoniEntry a) In the expression: PoniEntry <$> headerP <*> optional (detectorP a) <*> lengthP "PixelSize1: " <*> lengthP "PixelSize2: " <*> lengthP "Distance: " <*> lengthP "Poni1: " <*> lengthP "Poni2: " <*> angleP "Rot1: " <*> angleP "Rot2: " <*> angleP "Rot3: " <*> optional ("SplineFile: " *> takeTill isEndOfLine <* endOfLine) <*> lengthP "Wavelength: " <?> "poniEntryP" In an equation for `poniEntryP': poniEntryP a = PoniEntry <$> headerP <*> optional (detectorP a) <*> lengthP "PixelSize1: " <*> lengthP "PixelSize2: " <*> lengthP "Distance: " <*> lengthP "Poni1: " <*> lengthP "Poni2: " <*> angleP "Rot1: " <*> angleP "Rot2: " <*> angleP "Rot3: " <*> optional ("SplineFile: " *> takeTill isEndOfLine <* endOfLine) <*> lengthP "Wavelength: " <?> "poniEntryP" src/Hkl/PyFAI/Poni.hs:134:11-29: Couldn't match type `[Parser (PoniEntry a)]' with `Parser (Poni a)' Expected type: a -> Parser (Poni a) Actual type: a -> [Parser (PoniEntry a)] In the return type of a call of `some' In the expression: some (poniEntryP a) In an equation for `poniP': poniP a = some (poniEntryP a) I must say, that I do not understand what is wrong with my code. The lengthP code is ok, so why the detectorP is wrong ? thanks for your help Frederic

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