https://www.sw4sd.com/hasksheet/ 2026-04-17T08:05:29+00:00 https://www.sw4sd.com/hasksheet/ https://www.sw4sd.com/hasksheet/lib/exe/fetch.php?media=wiki:dokuwiki.svg text/html 2025-10-07T22:48:04+00:00 Anonymous (anonymous@undisclosed.example.com) https://www.sw4sd.com/hasksheet/doku.php?id=codesnippets:accumulatingwithinmonadiccontextswithoutfolds&rev=1759877284&do=diff Accumulating within monadic contexts without folds * {-# OPTIONS_GHC -Wno-unrecognised-pragmas #-} {-# HLINT ignore "Use void" #-} {-# HLINT ignore "Use map" #-} {-# HLINT ignore "Use foldl" #-} main :: IO () main = do print (f1a [3, 2, 6, 8]) f1b [3, 2, 6, 8] f1c [3, 2, 6, 8] >>= print >> return () print (f2a [] [3, 2, 6, 8]) f2b [3, 2, 6, 8] f2c [] [3, 2, 6, 8] >> return () f1a :: [Int] -> [Int] f1a [] = [] f1a (n:rln) = (n + 1) : f1a … text/html 2025-10-07T22:48:04+00:00 Anonymous (anonymous@undisclosed.example.com) https://www.sw4sd.com/hasksheet/doku.php?id=codesnippets:applicative&rev=1759877284&do=diff Applicative * - to be written - * ... ✎ text/html 2025-10-07T22:48:04+00:00 Anonymous (anonymous@undisclosed.example.com) https://www.sw4sd.com/hasksheet/doku.php?id=codesnippets:avoidconflictprelude&rev=1759877284&do=diff Avoiding name conflict with Prelude The avoidance of prelude may make sense for the following reasons: * reprogramming prelude functions to understand what it does * reprogramming prelude functions to create improved versions * avoiding conflicts with same name text/html 2025-10-07T22:48:05+00:00 Anonymous (anonymous@undisclosed.example.com) https://www.sw4sd.com/hasksheet/doku.php?id=codesnippets:bindingandpatternmatching&rev=1759877285&do=diff Binding and pattern matching A binding binds a name or even several names with a function definition. In Haskell, a binding is defined by symbol “=” whereas * on the left hand side * the function name and its parameters * or a pattern is given, text/html 2025-10-07T22:48:05+00:00 Anonymous (anonymous@undisclosed.example.com) https://www.sw4sd.com/hasksheet/doku.php?id=codesnippets:codingconventions&rev=1759877285&do=diff Coding conventions * Derived from * Haskell - Programming guidelines * Haskell Style Guide * Cross-link * Haddock examples General rules Line length For source code, the line length is up to 80 characters, at the maximum. * Example: ------------------------------------------------------------------------------ |---... ... instance Formatting a => Formatting (Ds.Tree a) where format t = format' (Indent []) t where format' :: Formatting a => Indent -> (Ds.Tree… text/html 2025-10-07T22:48:05+00:00 Anonymous (anonymous@undisclosed.example.com) https://www.sw4sd.com/hasksheet/doku.php?id=codesnippets:compositionoperator&rev=1759877285&do=diff Composition operator * composition operator is “.” * type: (b -> c) -> (a -> b) -> a -> c * takes two functions as parameter and combines the functions * first function is applied to the result of the second function * example: import qualified Data.Char as Ch (ord) main :: IO () main = do print $ f3 'a' print $ f4 'a' f1 :: Char -> Integer f1 ch = toInteger $ Ch.ord(ch) f2 :: Integer -> Double f2 n = fromIntegral n :: Double f3 :: Char -> Double f3 = f2 . … text/html 2025-10-07T22:48:04+00:00 Anonymous (anonymous@undisclosed.example.com) https://www.sw4sd.com/hasksheet/doku.php?id=codesnippets:datastructuresfromsymboltrees&rev=1759877284&do=diff Creating data structures from symbol trees * example * data structures for * symbol tree * with Terminal Chars as leafs * container Construction as monadic class and instances * error handling * types for standard Construction instances text/html 2025-10-07T22:48:05+00:00 Anonymous (anonymous@undisclosed.example.com) https://www.sw4sd.com/hasksheet/doku.php?id=codesnippets:debugging&rev=1759877285&do=diff Debugging Example with trace module Main where import Debug.Trace (trace) main :: IO () main = do putStrLn $ reverse' "1234abcd" reverse' :: Show a => [a] -> [a] reverse' l = rev l [] where rev [] acc = acc rev (e:r) acc = trace (show acc) (rev r (e : acc)) text/html 2025-10-07T22:48:05+00:00 Anonymous (anonymous@undisclosed.example.com) https://www.sw4sd.com/hasksheet/doku.php?id=codesnippets:either&rev=1759877285&do=diff Either * example code main :: IO () main = do print $ f "" print $ f "2" print $ f "23" print $ f "235" print $ f "2345" print $ f "2345a" print $ f "a2345" print $ f "a" print $ f "1a" print $ f "a1" f :: String -> Either String Int f lch = f' lch 0 where f' :: String -> Int -> Either String Int f' [] niAccu = Left "empty" f' [ch] niAccu = f'' ch >>= (\ni -> return ((niAccu * 10)… text/html 2025-10-07T22:48:05+00:00 Anonymous (anonymous@undisclosed.example.com) https://www.sw4sd.com/hasksheet/doku.php?id=codesnippets:equivalentfunctioncompositions&rev=1759877285&do=diff Equivalent function compositions * assertion: f1 (f2 . f3) equals (f1 . (f2 . )) f3 * example code: main :: IO () main = do print $ f f1' f2' f3' 6.4 print $ f' f1' f2' f3' 6.4 print $ f'' f1' f2' f3' 6.4 f :: ((Integer -> String) -> Double -> [String]) -> (Int -> String) -> (Integer -> Int) -> Double -> [String] f f1 f2 f3 x' = f1 (f2 . f3) x' f' :: ((Integer -> String) -> Double -> [String]) -> (Int -> String) -> (Integer -> Int) -> Double -> [String] f'… text/html 2025-10-07T22:48:04+00:00 Anonymous (anonymous@undisclosed.example.com) https://www.sw4sd.com/hasksheet/doku.php?id=codesnippets:filesystemioandstreams&rev=1759877284&do=diff Filesystem IO and streams * example: * code, module FileSystem {-| Description : is to provides a generalised, abstract, and save interface to the files system. Copyright : (c) Jörg K.-H. W. Brüggmann, 2021 License : CC0 1.0 Universal Maintainer : ...@... Stability : experimental Portability : POSIX * traits: * abstractions: * uniform types for identification of input and output streams ('Inp', 'Out') * higher order functions encapsulated in data types ('Da… text/html 2025-10-07T22:48:05+00:00 Anonymous (anonymous@undisclosed.example.com) https://www.sw4sd.com/hasksheet/doku.php?id=codesnippets:firsthaskellprogram&rev=1759877285&do=diff First Haskell programs * main function with output, as a complete code example * compiles, error and warning free, with compiler: GHC 8.10.4, using compiler option -Wall * main :: IO () main = print 'x' * executes with the following output: text/html 2025-10-07T22:48:05+00:00 Anonymous (anonymous@undisclosed.example.com) https://www.sw4sd.com/hasksheet/doku.php?id=codesnippets:flexibleinstances&rev=1759877285&do=diff {-# LANGUAGE FlexibleInstances #-} * allows to implement instances that are of form (T a1 ... an) where a1 ... an are particlar types (not type variables) * [Double] instead of [a] * implies the application of extension TypeSynonymInstances * example: * code: text/html 2025-10-07T22:48:04+00:00 Anonymous (anonymous@undisclosed.example.com) https://www.sw4sd.com/hasksheet/doku.php?id=codesnippets:fnowarnmissingsignatures&rev=1759877284&do=diff {-# OPTIONS_GHC -fno-warn-missing-signatures #-} * Prevents warnings of top level functions which have no type signature. * NOTE: For better readability, sometimes it makes sense not to declate the type of top level functions and letting the compiler to infer the type. text/html 2025-10-07T22:48:04+00:00 Anonymous (anonymous@undisclosed.example.com) https://www.sw4sd.com/hasksheet/doku.php?id=codesnippets:fnowarnorphans&rev=1759877284&do=diff {-# OPTIONS_GHC -fno-warn-orphans #-} * prevents warnings of orphan instance, meaning: * instance declaration of a type is not declared in the module of the class * NOTE: Orphan instances may be useful. * see also Haskell wiki - Orphan instance * NOTE: The purpose of the examples here is only to demonstrate the impact on the compiler. The examples may not make sense as a use case of type classes. text/html 2025-10-07T22:48:03+00:00 Anonymous (anonymous@undisclosed.example.com) https://www.sw4sd.com/hasksheet/doku.php?id=codesnippets:folds&rev=1759877283&do=diff Folds * inspired by Haskell wiki - Fold * fold functions are used to fold lists * folding means combining list elements * fold functions take at least the following parameters: * a function that combines two elements of the list * the list of elements to be combined text/html 2025-10-07T22:48:04+00:00 Anonymous (anonymous@undisclosed.example.com) https://www.sw4sd.com/hasksheet/doku.php?id=codesnippets:ghcextensions&rev=1759877284&do=diff text/html 2025-10-07T22:48:05+00:00 Anonymous (anonymous@undisclosed.example.com) https://www.sw4sd.com/hasksheet/doku.php?id=codesnippets:haddockexamples&rev=1759877285&do=diff Haddock examples Markups Paragraphs One or more blank lines separates two paragraphs in a documentation comment. Special characters The following characters have special meanings in documentation comments: \, /, ', `, “, @, <. To insert a literal occurrence of one of these special characters, precede it with a backslash (\). text/html 2025-10-07T22:48:04+00:00 Anonymous (anonymous@undisclosed.example.com) https://www.sw4sd.com/hasksheet/doku.php?id=codesnippets:hint&rev=1759877284&do=diff Interpreted Haskell code by package hint * Hint is a package containing a Haskell interpreter built on top of the GHC API. * example, modified from here, creates a source code file, interprets it and finally deletes it import Data.List import qualified Control.Monad as M import qualified Language.Haskell.Interpreter as Itpr import qualified System.Directory as Dir import qualified FileSystem as FS main :: IO () main = do Dir.setCurrentDirectory "app" createSomeModul… text/html 2025-10-07T22:48:04+00:00 Anonymous (anonymous@undisclosed.example.com) https://www.sw4sd.com/hasksheet/doku.php?id=codesnippets:letandwhereconstructs&rev=1759877284&do=diff Let and where constructs Hard differences Inside expression * let constructs are legal * example code: main = print (1 + (let i = 10 in 2 * i + 1)) * where constructs are ilegal * example code: main = print (1 + (2 * i + 1 where i = 10)) text/html 2025-10-07T22:48:03+00:00 Anonymous (anonymous@undisclosed.example.com) https://www.sw4sd.com/hasksheet/doku.php?id=codesnippets:listcomprehension&rev=1759877283&do=diff List comprehension * syntactic sugar to apply set operations to lists * syntax := “[” output expression “|” variable / binding = input set “,” predicate “]” * e.g. [ 1 / x | x <- [-1, -0.75 .. 1], x /= 0 ] * creates a list text/html 2025-10-07T22:48:05+00:00 Anonymous (anonymous@undisclosed.example.com) https://www.sw4sd.com/hasksheet/doku.php?id=codesnippets:minimalcompletedefinition&rev=1759877285&do=diff Annotation for a minimal complete definition Format: {-# MINIMAL function1 | function2 #-} Minimal pragmas are boolean expressions. For instance, with | as logical OR, either definition of the above functions must be defined. Comma indicates logical AND where both definitions must be defined. text/html 2025-10-07T22:48:04+00:00 Anonymous (anonymous@undisclosed.example.com) https://www.sw4sd.com/hasksheet/doku.php?id=codesnippets:monads&rev=1759877284&do=diff Monads Example 1a * this example shows how alternative results (sum type, algebraic type, see type A, B and C in the example) can be handled and propagated in a computation by means of 'case' / 'of' constructions * it works but it is quite some code text/html 2025-10-07T22:48:05+00:00 Anonymous (anonymous@undisclosed.example.com) https://www.sw4sd.com/hasksheet/doku.php?id=codesnippets:octets&rev=1759877285&do=diff Octets * type class Octetable * with type Octet, and Octets * with instances for Word32, Char, Utf8Char, Integer, and Int * example * code, of module Octetable: {-# LANGUAGE BinaryLiterals #-} module Octetable ( Octetable(..), Octet, Octets, getOctets, putOctets ) where import qualified Data.Word as W import qualified Data.Bits as Bts import qualified Data.Char as Chr import qualified System.IO as SysIo import qualified Data.… text/html 2025-10-07T22:48:04+00:00 Anonymous (anonymous@undisclosed.example.com) https://www.sw4sd.com/hasksheet/doku.php?id=codesnippets:parametrictypeforanintermediatevaluewhencombiningfunctions&rev=1759877284&do=diff Parametric type for an intermediate value when combining functions * example with the folling problem(s): * function buildNRender has an ambigious intermediate parametric type * the type of buildNRender has no parametric type g involved at all text/html 2025-10-07T22:48:05+00:00 Anonymous (anonymous@undisclosed.example.com) https://www.sw4sd.com/hasksheet/doku.php?id=codesnippets:patternmatching&rev=1759877285&do=diff Pattern matching, again There was already some pattern matching explained, in the context of bindings here. Here we explain pattern matching in other contexts, which are: * function definitions * list comprehensions * do notations * case constructs text/html 2025-10-07T22:48:05+00:00 Anonymous (anonymous@undisclosed.example.com) https://www.sw4sd.com/hasksheet/doku.php?id=codesnippets:prelude&rev=1759877285&do=diff Prelude * see also Prelude overview * Composition operator * Folds * Scans * Applicative * Monads * State monads text/html 2025-10-07T22:48:04+00:00 Anonymous (anonymous@undisclosed.example.com) https://www.sw4sd.com/hasksheet/doku.php?id=codesnippets:randomvalues&rev=1759877284&do=diff Random values * example * code, module Main: {-# OPTIONS_GHC -fno-warn-orphans #-} import qualified Randomisable as Rnd main :: IO () main = do prng0 <- Rnd.newStdGen print $ processMyData ((Rnd.random prng0) :: MyData) processMyData :: MyData -> (MyData, MyData, MyData2, ScaleA) processMyData (MyData prng0 nd1 nd2 mydt2) = let (prng1, prng2, prng3, prng4, prng5) = Rnd.tplPrngSplit5 prng0 in ((MyData prng1 nd1 nd2 mydt2), (MyData prng2 (Rnd.randomise prng3 … text/html 2025-10-07T22:48:04+00:00 Anonymous (anonymous@undisclosed.example.com) https://www.sw4sd.com/hasksheet/doku.php?id=codesnippets:recordsyntax&rev=1759877284&do=diff Record syntax * algebraic sum type )1 * provides access functions * fields in curly brackets, comma separated * short example: * data MatrixCursor = MatrixCursor { rnRow :: Integer, rnCol :: Integer } ---------- )1 An algebraic sum type (see also text/html 2025-10-07T22:48:03+00:00 Anonymous (anonymous@undisclosed.example.com) https://www.sw4sd.com/hasksheet/doku.php?id=codesnippets:scans&rev=1759877283&do=diff Scans * example * code: main :: IO () main = do print $ scanl (\nAcc -> \(_, n) -> nAcc + n) (0 :: Integer) (zip "halleluja" [1..]) * executes, with output: [0,1,3,6,10,15,21,28,36,45] ✎ text/html 2025-10-07T22:48:05+00:00 Anonymous (anonymous@undisclosed.example.com) https://www.sw4sd.com/hasksheet/doku.php?id=codesnippets:statefuldecoder&rev=1759877285&do=diff Stateful decoder * example * code, module Main: import qualified Control.Monad.State as S import qualified Safer as Sfr main :: IO () main = do -- MyData -- + - take the next 4 characters converted to Integer -------------------------------> 4736 -- / + - take the next 3 Integer -----------------------------… text/html 2025-10-07T22:48:04+00:00 Anonymous (anonymous@undisclosed.example.com) https://www.sw4sd.com/hasksheet/doku.php?id=codesnippets:statemonads&rev=1759877284&do=diff State monads * inspired by Brandon Simmons: The State Monad: A Tutorial for the Confused? Control.Monad.State applied, input from list * inspired from <https://wiki.haskell.org/State_Monad> * example compiles warning free with * compiler: GHC 8.10.4 using -Wall * package: mtl-2.2.2 * example: * import qualified Control.Monad.State as S main :: IO () main = do print $ S.evalState (playGame "abcaaacbbcabbab") startState print $ S.evalState playGame2 s… text/html 2025-10-07T22:48:04+00:00 Anonymous (anonymous@undisclosed.example.com) https://www.sw4sd.com/hasksheet/doku.php?id=codesnippets:templatehaskell&rev=1759877284&do=diff TemplateHaskell Creating code at compile time * Example, creates a curry function for arbitrary large curry functions module Curry ( curryN ) where import Control.Monad import Language.Haskell.TH curryN :: Int -> Q Exp curryN n = do f <- newName "f" xs <- replicateM n (newName "x") let args = map VarP (f:xs) ntup = TupE (map (pure . VarE) xs) return $ LamE args (AppE (VarE f) ntup) text/html 2025-10-07T22:48:04+00:00 Anonymous (anonymous@undisclosed.example.com) https://www.sw4sd.com/hasksheet/doku.php?id=codesnippets:testingconventions&rev=1759877284&do=diff Testing conventions Setup with stack package.yaml * package.yaml: ... tests: Test4-test: main: Spec.hs source-dirs: test ghc-options: - -threaded - -rtsopts - -with-rtsopts=-N dependencies: - Test4 - tasty - tasty-quickcheck - tasty-hunit text/html 2025-10-07T22:48:04+00:00 Anonymous (anonymous@undisclosed.example.com) https://www.sw4sd.com/hasksheet/doku.php?id=codesnippets:twopagesgenericparser&rev=1759877284&do=diff A generic parser on two pages * usage in a main program: module Main where import Parser main :: IO () main = do putStrLn "" putStrLn "parse \"0\" synNumber:" putStr " "; print $ parse "0" synNumber putStrLn "" putStrLn "parse \"2\" synNumber" putStr " "; print $ parse "2" synNumber putStrLn "" putStrLn "parse \"90\" synNumber" putStr " "; print $ parse "90" synNumber putStrLn "" putStrLn "… text/html 2025-10-07T22:48:05+00:00 Anonymous (anonymous@undisclosed.example.com) https://www.sw4sd.com/hasksheet/doku.php?id=codesnippets:typeapplications&rev=1759877285&do=diff {-# LANGUAGE TypeApplications #-} * allows to specify types to apply * example, without type application * code: module Main where main :: IO () main = print (read "123") * compiles with error: app\Main.hs:4:15: error: * Ambiguous type variable `a0' arising from a use of `read' prevents the constraint `(Read a0)' from being solved. Probable fix: use a type annotation to specify what `a0' should be. These potential instances exist: instance Read Orderi… text/html 2025-10-07T22:48:04+00:00 Anonymous (anonymous@undisclosed.example.com) https://www.sw4sd.com/hasksheet/doku.php?id=codesnippets:typeclasses&rev=1759877284&do=diff Type classes and its instances, and the deriving mechanism * A type class (keyword: class) is a set of functions declarations that * can have generic default bindings, and * have instances (keyword: instance) with type specific bindings. text/html 2025-10-07T22:48:05+00:00 Anonymous (anonymous@undisclosed.example.com) https://www.sw4sd.com/hasksheet/doku.php?id=codesnippets:typedefaulting&rev=1759877285&do=diff Type defaulting Stil not clear how this works: * module Main where main :: IO () main = do print $ negate 4 Output: * -4 ...or this: * module Main where main :: IO () main = do print $ sin 3 Output: * 0.1411200080598672 text/html 2025-10-07T22:48:04+00:00 Anonymous (anonymous@undisclosed.example.com) https://www.sw4sd.com/hasksheet/doku.php?id=codesnippets:typesynonyminstances&rev=1759877284&do=diff {-# LANGUAGE TypeSynonymInstances #-} * allows to use type synonyms as instances * example: * code: main :: IO () main = print x1 class MyClass x where fMy :: x -> x -> x type Speed = Double instance MyClass Speed where fMy xA xB = xA + xB x1 :: Speed x1 = fMy 1.3 2.5 text/html 2025-10-07T22:48:05+00:00 Anonymous (anonymous@undisclosed.example.com) https://www.sw4sd.com/hasksheet/doku.php?id=codesnippets:verificationandvalidation&rev=1759877285&do=diff Verification and Validation Plan The goals of this plan is to document all steps to release a product like “genc³” as an online-product aiming to satisfiy the endusers expectations regarding download, installation, use, and further information.