April 2014 - Beginners

Data type definition for a list of elements of alternating types?
by Jacek Dudek 04 Apr '14

04 Apr '14

-- As an exercise I wanted to define a datatype that is an alternating list of elements of two different types. The best that I could do are the type definitions below: module BiList ( BiList (..) , AList (EmptyA) , BList (EmptyB) ) where data BiList a b = Empty | BA (AList a b) | BB (BList a b) data AList a b = EmptyA | AL a (BList a b) data BList a b = EmptyB | BL b (AList a b) (<#) :: a -> (BList a b) -> (AList a b) a <# bs = AL a bs (<@) :: b -> (AList a b) -> (BList a b) b <@ as = BL b as infixr 5 <# infixr 5 <@ example :: BiList Int Char example = BA $ 1 <# 'a' <@ 2 <# 'b' <@ 3 <# 'c' <@ 4 <# 'd' <@ EmptyA -- There are two things that I don't like about this implementation. -- (1) The BA and BB constructors that must be applied on top of instances of (AList a b) and (BList a b) to lift them to be of type (BiList a b). -- (2) Having three different constructors for an empty list: Empty, EmptyA, EmptyB, where ideally I would just have one. -- Is it possible to get around either of these annoyances with some type theory gymnastics? Maybe something like the function fromIntegral (the mechanics of which I don't really understand at this point)?

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How do you pronounce these operators?
by John M. Dlugosz 04 Apr '14

04 Apr '14

I'm having some difficult reading because I don't have "names" for many of the operators, such as <*>. I know that >>= is pronounced “bind”, but what about the others? Is there some common consensus, a list somewhere, or at least the proper mathematical names to serve as a starting point? I'd hate to have to make something up (see < http://www.muppetlabs.com/~breadbox/intercal-man/tonsila.html >) —John

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Help installing Esqueleto (for Yesod)!
by Gilberto Melfe 03 Apr '14

03 Apr '14

Hello community! I'm trying to install Yesod on a freshly setup openSUSE 13.1 virtual machine! I installed the Haskell Platform and three other (ghc) packages required (for yesod-bin), from the standard online repositories! Then i issued: cabal update cabal install cabal-install (because cabal told me so) export PATH=$HOME/.cabal/bin:$PATH cabal install yesod-platform After churning for a while, that last command stated that some packages, more precisely the Esqueleto package, failed to install. I tried to install it alone and got these error messages: -- Begin Command Output Resolving dependencies... Configuring esqueleto-1.3.5... Building esqueleto-1.3.5... Preprocessing library esqueleto-1.3.5... [1 of 4] Compiling Database.Esqueleto.Internal.PersistentImport ( src/Database/Esqueleto/Internal/PersistentImport.hs, dist/build/Database/Esqueleto/Internal/PersistentImport.o ) [2 of 4] Compiling Database.Esqueleto.Internal.Language ( src/Database/Esqueleto/Internal/Language.hs, dist/build/Database/Esqueleto/Internal/Language.o ) [3 of 4] Compiling Database.Esqueleto.Internal.Sql ( src/Database/Esqueleto/Internal/Sql.hs, dist/build/Database/Esqueleto/Internal/Sql.o ) src/Database/Esqueleto/Internal/Sql.hs:531:46: Not in scope: type constructor or class `C.ResourceT' src/Database/Esqueleto/Internal/Sql.hs:559:42: Not in scope: type constructor or class `C.ResourceT' src/Database/Esqueleto/Internal/Sql.hs:619:31: Not in scope: type constructor or class `C.ResourceT' src/Database/Esqueleto/Internal/Sql.hs:634:24: Not in scope: type constructor or class `C.ResourceT' src/Database/Esqueleto/Internal/Sql.hs:636:17: Not in scope: `C.runResourceT' Failed to install esqueleto-1.3.5 cabal: Error: some packages failed to install: esqueleto-1.3.5 failed during the building phase. The exception was: ExitFailure 1 -- End Command Output Can someone figure out what might be wrong? (I guess it's not a dependencies issue...) Thank You Very Much, in advance, for your time! Gilberto PS: Should I have included the command output in an attached file? Which "method" is preferred?

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Functor on Tree data constructor
by Nishant 03 Apr '14

03 Apr '14

Hi, Can some explain me the error and a possibly a fix : data Tree a = NULL | Node (Tree a) a (Tree a) deriving Show instance Functor Tree where ............fmap f NULL = NULL ............fmap f (Node left x right) | (left == NULL) && (right == NULL) = Node left (f x) right ................................................... | (left /= NULL) = Node (fmap f left) x right ................................................... | (right /= NULL) = Node left x (fmap f right) ghci gives: Prelude> :load programs\tree.hs [1 of 1] Compiling Main ( programs\tree.hs, interpreted ) programs\tree.hs:32:78: Couldn't match type `a' with `b' `a' is a rigid type variable bound by the type signature for fmap :: (a -> b) -> Tree a -> Tree b at programs\tree.hs:31:7 `b' is a rigid type variable bound by the type signature for fmap :: (a -> b) -> Tree a -> Tree b at programs\tree.hs:31:7 Expected type: Tree b Actual type: Tree a In the first argument of `Node', namely `left' In the expression: Node left (f x) right In an equation for `fmap': fmap f (Node left x right) | (left == NULL) && (right == NULL) = Node left (f x) right | (left /= NULL) = Node (fmap f left) x right | (right /= NULL) = Node left x (fmap f right) Failed, modules loaded: none. Regards. Nishant

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Mapping inside State
by martin 01 Apr '14

01 Apr '14

Hello all, I found myselft writing this ugly piece of code silenceAll :: TI -> State (M.Map CH StringState,[DtzEvent]) TI silenceAll t = silence t 0 >> silence t 1 >> silence t 2 >> silence t 3 >> silence t 4 >> silence t 5 >> silence t 6 >> return t because I couldn't figure out how to map (silence t) over [0..6]. What is the standard way of doing this?

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