How do I avoid writing this type of Haskell boilerplate code
I assume that we'd like to have a solution for the general case where the changing type parameter is not necessarily in the right position for DeriveFunctor. We can distinguish two cases. In the simple case out data type is not recursive. Here, prisms are a fitting solution: {-# LANGUAGE...
Pattern Match Vector Value in Data.Aeson
No, you cannot pattern match the way you are trying to do here. A JSON Array can contain values of different types, and you cannot pattern match on all values in a list like they where one. There are several ways to solve your actual problem here. There is a...
Combining Event and an attribute in threepenny-gui
This is intentional: The UI.checkedChange event only triggers when the user clicks the checkbox, but not when it is set programmatically. The intention is that the bBool behavior represents the canonical state of the checkbox and the UI.checkedChange event represents request from the user to change it, which may or...
Thread blocked indefinitely in an MVar operation
haskell,concurrency,network-programming
Three days later and its solved: Was actually unrelated to either the networking or concurrency code, and infact caused by my incorrect re-implementation of Yampas dpSwitch in Netwire. Corrected code posted below for anyone wishing to implement this function: dpSwitch :: (Monoid e, Applicative m, Monad m, T.Traversable col) =>...
attoparsec: succeeding on part of the input instead of failing
Depending on if consuming the whole input should be the property of parseNoteDocument or just the tests, I'd extend one or the other with endOfInput or atEnd. I'd suggest to define a proper Parser for your documents, like parseNoteDocument' :: Text -> Parsec NoteDocument parseNoteDocument' = many parseLine and then...
Specifying a constaint for a class
You don't want to do that. The simplest way of achieving what you want is the following: class HasId a where getId :: a -> UUID class HasId a => MyClass1 a where method1 = -- NOTE: Use "getId" instead of plain "id". when you define your data types you...
Define function in Haskell with computations in arguments
You can define this as follows: f 0 = 1 f i | even i = sin(k) * cos(k) | otherwise = cos(k/2) - tan(2*k) where k = fromIntegral $ div i 2 I replaced x by f because x looks more like a variable than a function. Furthermore I...
Haskell powerset function - How to avoid Couldn't match expected type `IO t0' with actual type `[[Integer]]'
the problem is main = ... main should have type IO () but you give an expression with type [[Integer]] (as the compiler tells you) - so as I think you want to output the result to the console I think you are looking for print this works for me:...
Fold over a heterogeneous, compile time, list
haskell,type-level-computation,hlist
Answering your comment: Actually, I can do if I can filter the heterogeneous list by type. Is that possible? You can filter the heterogeneous list by type if you add a Typeable constraint to b. The main idea is we will use Data.Typeable's cast :: (Typeable a, Typeable b) =>...
Tokenizer identifier in Haskell
For the Not in scope: data constructor 'Integer' part, the problem is that you have an extra Integer in the line isDigit c = TNumber Integer (read c) : tokenize cs which should be isDigit c = TNumber (read [c]) : tokenize cs The [c] part is needed because read...
How to properly use monadic expressions in Haskell without getting parse errors?
main = do let x = [0..10] print x works for me and so does main = do { let x = [0..10] in print x } I think you're trying to mix some different syntax options up....
Cabal sandbox is using a global dependency. Could not resolve
It is difficult for cabal-install to produce a complete description of the mutual incompatibilities of an installation problem. It instead decides to print the first path to failure. However, it prints Dependency tree exhaustively searched. which indicates that there actually is no solution to the given problem. One thing that...
What is haskellng? What is the difference between 'haskellPackages' and 'haskellngPackages'?
haskell,cabal,cabal-install,nix,haskell-ng
Could someone please explain what haskellng is in a simple, clear way? Well, haskellng is the next generation Nix Haskell package set made for Nix. I think most of the work was done by Peter Simons. But note that in the latest master version, haskellngPackages has been renamed back...
how to handle capital case in JSON?
deriveJSON and friends are Template Haskell functions which will generate instances for you. As such, you should not try to list them in the deriving clause. Instead, call them in a splice at the top level like this: {-# LANGUAGE TemplateHaskell #-} import Data.Aeson.TH data Person = Person { foo...
IO Monad Example
The code you posted desugars into the following. x >>= (\a -> print a >> return 500) Or, expanding out the definition of (>>) x >>= (\a -> print a >>= (\_ -> return 500)) Then, you can see that in the different calls to (>>=), the types a and...
Haskell do clause with multiple monad types
A do block is for a specific type of monad, you can't just change the type in the middle. You can either transform the action or you can nest it inside the do. Most times transformations will be ready for you. You can, for instance have a nested do that...
haskell beginner - my simple sort
The problem is you are only considering the same pairs of element on each pass. Consider: swap (a:b:t) | a <= b = a : b : (swap t) swap (a:b:t) | b < a = b : a : (swap t) What if we have a list [4,1,1,1]? swap...
Where to store API keys and other 'secrets' in a yesod app
There are many approaches to this, mostly depending on what flavor of devops/hosting your prefer. One option is to put a dummy value in the config file and override it with an environment variable at runtime (see: https://github.com/yesodweb/yesod/wiki/Configuration#overriding-configuration-values-with-environment-variables). You can also having an extra settings file for production that overrides...
How to bring a data kind to the value level?
valueKind :: SomeValue a -> SomeKind is not possible. The a type parameter isn't present runtime in any shape or form, so we can't branch on it. The standard method for making use of types both at runtime and compile time is to make so-called singleton types. Singletons are indexed...
Haskell make recipe fails for Paradox theorem prover using GHC
linux,haskell,make,ghc,theorem-proving
Looks like paradox was written for a rather old version of GHC. You can fix all these "Could not find module" errors by using GHC version 7.8 or older and setting GHC = ghc -hide-package base -package haskell98 in the Makefile, though you will likely encounter more errors after that....
Force GHC using local files
Yes, use ghc -hide-package evil-package. Or you can hide the package temporarily with ghc-pkg hide evil-package, and then undo it later with ghc-pkg expose evil-package.
From and ToJSON in Haskell - a nested data
You can use the same applicative notation to parse the nested values like this: instance FromJSON DataMain where parseJSON (Object v) = DataMain <$> v .: "a" <*> v .: "b" <*> (Data1 <$> v .: "c" <*> v .: "d") parseJSON _ = mzero ...
Running executable files using Haskell
shell,haskell,command-line-arguments,executable
The documentation for readProcess says: readProcess :: FilePath Filename of the executable (see RawCommand for details) -> [String] any arguments -> String standard input -> IO String stdout When it's asking for standard input it's not asking for a file to read the input from, but the actual contents of...
How to work with Cabal API
A exitcode-stdio-1.0 test suite is only an executable, which needs to implement main :: IO(). Cabal test just runs the executable and captures the output in a log file. So if you use a test framework like tasty, you'll get the output the tasty way. Of course, you can just...
Decremented value called in the recursion in Haskell
string,function,haskell,recursion,parameters
Yes, once you call again f with a new value of n, it has no way to reference the old value of n unless you pass it explicitly. One way to do it is to have an internal recursive function with its width parameter, as you have, but that can...
Infix operator with missing argument
function,haskell,infix-notation
Not really, no. On the other hand, you could perfectly well define a different operation that provides the default argument, e.g. withDef data2 = {- your default value -} <*> data2 If you really want to use a name that would otherwise be an operator, you can still name this...
Why is f <$> g <$> x equivalent to (f . g) <$> x although <$> is not right-associative?
haskell,syntax,infix-notation,applicative,infix-operator
Why is f <$> g <$> x equivalent to (f . g) <$> x ...well, this isn't so much a functor-thing as a Haskell-thing. The reason it works is that functions are functors. Both <$> operators work in different functors! f <$> g is in fact the same as...
When will travis-ci support ghc 7.10?
Using multi-ghc-travis, you can also set up Travis-CI for ghc 7.10 (apart from other versions).
Why does GHCI get “stuck” in an error state after an error?
The getStdRandom function basically looks up a StdGen value in a global variable, runs some function on it, puts the new seed back into the global variable, and returns the result to the caller. If the function in question returns with an error, that error gets put into the global...
Can't find defaultTimeLocale in Data.Time.Format
For some reason, cabal wasn't using the version I thought it was (1.5) but (1.4) probably from the haskell platform. Uprading fixed the problem.
apply a transformation with function inline
The read lambda applies to the first argument and the first argument to the function given to foldl is the accumulator. Those two arguments are the opposite for foldr. So, expanded, it looks like this: foldl (\acc element -> (read acc :: Int) + element) 0 ["10", "20", "30"] Since...
Haskell IO - read from standard input directly to list
You may write: main = readLn >>= print . subsequences You will need to nail down the type to be read, for example by having a monomorphic subsequences or by annotating readLn. In ghci: Data.List> (readLn :: IO [Integer]) >>= print . subsequences [1,2,3] [[],[1],[2],[1,2],[3],[1,3],[2,3],[1,2,3]] (I typed in the first...
Best practice for handling data types from 3rd party libraries in Haskell?
I am not ready to give a nice list of best practises, but for starters if you want to keep stuff sanely organized, use explicit exports instead of just exporting everything, e.g: module Parser ( parseConfig ) where ... Explicit exports also allow you to reexport your imports, e.g. module...
Haskell - generate and use the same random list
Here is a simple example (@luqui mentioned) you should be able to generalize to your need: module Main where import Control.Monad (replicateM) import System.Random (randomRIO) main :: IO () main = do randomList <- randomInts 10 (1,6) print randomList let s = myFunUsingRandomList randomList print s myFunUsingRandomList :: [Int] ->...
Stopping condition on a recursive function - Haskell
string,function,haskell,if-statement,recursion
Your code doesn't handle the case where a line is shorter than the maximum length. This is somewhat obscured by another bug: n is decremented until a whitespace is found, and then f is called recursively passing this decremented value of n, effectively limiting all subsequent lines to the length...
Constructor that lifts (via DataKinds) to * -> A
As Conor states, this is not directly possible. You can, however, define data K a = ... | C a Then this promotes to C :: a -> K a If you then use K *, you can achieve what you want. ...
Uniqueness and other restrictions for Arbitrary in QuickCheck
haskell,unique,predicate,quickcheck
Definitely you want the instance to produce only instances that match the intention of the data type. If you want all the variables to be distinct, the Arbitrary instance must reflect this. (Another question is if in this case it wouldn't make more sense to define Vars as a set,...
automatic typing of nested lists
Let's rewrite this: l = [l1, l2, l3] where l1 = [] l2 = [[]] l3 = [[[]]] Now we assign type variables: ab initio l1 :: b l2 :: b l3 :: b since they all have to have the same type, so l has some type [b]. Now,...
Instance of Show for Lambda
haskell,lambda,instance,show,typeclass
Add an instance declaration for the Show class. instance Show LExpr where show = show' And remove the deriving(Show) part data LExpr = Variable String -- variable | Apply LExpr LExpr -- function application | Lambda String LExpr -- Lambda abstraction deriving (Eq) ...
Why doesn't `iterate` from the Prelude tie the knot?
Tying the not like that doesn't appear to increase sharing. Contrast with: cycle xs = let x = xs ++ x in x Tying the knot here has the effect of creating a circular linked list in memory. x is its own tail. There's a real gain. Your suggested implementation...
Haskell list comprehension compilation error
list,haskell,list-comprehension
The comprehension syntax is [ <expression> | ... ] For <expression> you have ([x,z], i:j:s, j), b which is syntactically wrong. Did you mean ([x,z], i:j:s, b)?...
Haskell: `==' is not a (visible) method of class
haskell,compiler-errors,instance,equality,typeclass
It isn't clear what you are trying to achieve. Here are some thoughts: When you declare an instance of a class like instance (Eq a) => PartOrd a, you are expected to provide implementations for the functions in PartOrd a (ie partcmp, not == and \=). The compiler is telling...
Implementing map on a tree using fold
Try to write your last line as def map(tree:Tree[Int])(f:Int=>Int) : Tree[Int] = fold(tree , EmptyTree:Tree[Int])((l,x,r) => Node(f(x),l,r)) Scala's type inference is very limited compared to haskell, in this case it tries to infere type of fold from it's arguments left to right, and incorectly decides that result type of fold...
Haskell lens: let binding of Traversal'
The key point here is that the annotation should be in a separate line. If we do that, then we have a binding with an explicit type, as far as GHC is concerned. someFunction :: MyMonad () someFunction = do ... let x :: Traversal' MyState MyDataT x = myState.clients.ix...
How do I use zoom with a MonadState constraint of a newtype?
zoom has its own class for overloading, so no wonder just MonadState doesn't cut it. The Zoom class covers roughly the same ground as mtl, although it has somewhat more complicated machinery. In any case, you're not obligated to program in concrete monads. You can try enabling NoMonomorphismRestriction and inferring...
Hook into GHC runtime system
haskell,functional-programming,runtime,transactional-memory
(# s2#, TVar tvar# #) is an unboxed tuple. The name stg_newTVarzh is built from: The stg_ prefix, which is common to the whole GHC runtime, and stands for the spineless-tagless G-machine, an abstract machine to evaluate functional languages; newTVar which is the first part of newTVar#; the final zh,...
How to improve the performance of Haskell IO?
Your example is slow because it uses lazy IO with String-s. Both have their own overheads. In particular, String is a linked list of Char-s, therefore it has two words of space overhead for each character (one word for the constructor tag and one for the forward pointer), and each...
Generating Infinite List in Haskell
The problem is that x (-x) is not of type Int (thanks to @leftaroundabout removed non-sense about non-valid syntax here). You want to generate two values for every x. So I guess the easiest way is to create lots of list pairs [1, -1], [2, -2], ... and then to...
How to convert a Rational into a “pretty” String?
Here's one that I wrote a few weeks ago. You can specify the number of decimals you want (correctly rounded), or just pass Nothing in which case it will print the full precision, including marking the repeated decimals. module ShowRational where import Data.List(findIndex, splitAt) -- | Convert a 'Rational' to...
Type ambiguity of arithmetic expression when wrapped in a function
it's tricky but the thing is that the two 10s in (round (3/10))*10 don't have to be the same type but of course the bs in let roundBy b x = (round (x/b)) * b have to be. That`s why: λ> :t (round (3/10))*10 (round (3/10))*10 :: Integral a =>...
Unable to create a custom header to use it in “withManager”
Network.HTTP.Conduit and Network.HTTP.Headers are from incompatible HTTP libraries. Request headers for http-conduit/http-client can be created using the http-types library: import Data.Aeson import qualified Data.ByteString.Lazy as B import Network.HTTP.Conduit as HHeaders import qualified Network.HTTP.Types as HTypes import qualified Data.ByteString.Char8 as C8 import qualified Data.ByteString.Lazy.Char8 as LC8 myHeader1 = (HTypes.hAccept, C8.pack "some...
Is there any function in Haskell that applies a two argument function to two lists, element by element?
> :t zipWith zipWith :: (a -> b -> c) -> [a] -> [b] -> [c] > zipWith (*) [1,2,3] [4,5,6] [4,10,18] It's the eighth result provided by Hoogle when queried with your type (a -> a -> a) -> [a] -> [a] -> [a] Moreover, when you need to...
Idiomatic list construction
list,haskell,functional-programming,idiomatic
The multiple call to addPoints could be replaced by a fold. As suggested in a comment, reversing your addPoint function would make things easier: addPoint' :: Point -> Polyline -> Polyline addPoint' p line = p:line So then your constructLine function could build a temporary list of the points to...
What is the type of the variable in do-notation here in Haskell?
haskell,functional-programming,monads,do-notation
Well, you've stumbled upon a kind of weird monad. The monad in question is the Monad ((->) r). Now, what does that mean? Well, it's the monad of functions of the form r -> *. I.e., of functions that take the same type of input. You asked what the type...
Haskell Lempel Ziv 78 Compression
It's a parenthesization error: compress x newMap (i+1) "" out++[(newMap Map.! c, s)] parses as (compress x newMap (i+1) "" out) ++ [(newMap Map.! c, s)] But you wanted compress x newMap (i+1) "" (out++[(newMap Map.! c, s)]) Fun problem, by the way. I tried rewriting your code to make...
Can I generalize GADTs that use type tags?
It depends on what you mean by being able to plug in "any game". Is every game going to have a MoveTag denoting whose turn it is? If so, then you can give g a kind signature and define the type class as class Minimaxable (g :: MoveTag -> *)...
Where are line breaks allowed within Haskell expressions?
You can place a newline anywhere between lexical tokens of an expression. However, there are constraints about how much indentation may follow the newline. The easy rule of thumb is to indent the next line to start to the right of the line containing the expression. Beyond that, some style...
How to “wrap” monadic return value
createNotificationIdentifierItem :: APNSIdentifier -> APNSItem createNotificationIdentifierItem (Identifier identifier) = Item $ do putWord8 3 putWord16be 4 putWord32be identifier or createNotificationIdentifierItem :: APNSIdentifier -> APNSItem createNotificationIdentifierItem (Identifier identifier) = do Item $ putWord8 3 Item $ putWord16be 4 Item $ putWord32be identifier after making APNSItem an instance of Monad (you can...
What are the most common bugs in OCaml/Haskell/Scala programs? [closed]
This is a partial answer, written for Scala but applicable to other languages. A bug is something happening which was not expected. Thus, to solve bugs, the language should allow to let the user write any expectations and interact with them (proof, synthesis, etc.). There are several ways to address...
Rank N types in let bindings
You need explicit type annotations when GADTs are involved: abigail :: Person abigail = let x :: Person x Name = "Abigail" x Age = 27 in x Without it, GHC roughly sees let x Name = "Abigail" and says "OK, x is a function from the type of Name,...
Why is lazy evaluation in Haskell “not being lazy”?
take is of type Int -> [a] -> [a], i.e. it returns a list. It seems you’re looking for head, which returns one element. head $ head $ repeat [1..] ...
Refactor an IO recursive loop into a monad folding in Haskell
sockets,haskell,network-programming,io-monad
The idiomatic way to repeat the same action over and over again forever is forever serverLoop :: Socket -> IO () serverLoop sock = forever $ do (conn, _) <- accept sock forkIO $ handleConn conn ...
Changes in other elements based on listbox selections in threepenny-gui
First of all, there is a regression in Threepenny 0.6 which affects the code we are writing here. For the moment, if you are using 0.6 you will have either to go back to 0.5 or to cabal unpack and apply the temporary fix Heinrich suggests in the issue linked...
Haskell: Type mismatch in function using foldr
foldr :: (a -> b -> b) -> b -> [a] -> b as can be found on Hoogle expects a function of the form a -> b -> b, but your test function rather looks like a -> a -> b. As Carsten mentioned, the b of your function...
First three items of a list in Haskell
Well, foo (x:y:z:xs) plus a “too short clause” certainly wouldn't be a bad solution. Another would be foo xs = case splitAt 3 xs of ([x,y,z],xs') -> calc x y z : foo (y:z:xs') _ -> [] Or, perhaps nicest, import Data.List (tails) foo xs = [ calc x y...
importing haskell module says “not in scope”
First, module Tree2 ( Tree ) where only exports the Tree data type, not its constructors; you should use module Tree2 ( Tree(..) ) where instead. Second, as you're doing a qualified import, you need to use Tree2.EmptyTree instead of just EmptyTree....
How can I express the type of 'takeWhile for vectors'?
haskell,types,binding,dependent-type
The type you suggest can not be implemented, i.e. it is not inhabited: takeWhileVector :: (a -> Bool) -> Vector a n -> Vector a m Remember that the caller chooses the type variables a,n,m. This means that the caller can use your function as e.g. takeWhileVector :: (a ->...
Pipeline-like operation using TChan
It seems odd to me to be using STM and semaphores in the same code block... Why not do the entire thing in STM? In particular, why not a TChan (Maybe x), with Nothing indicating the end of the sequence? Also, notice that your fstPipe likely just generates a bunch...
Normal probability density function - GSL equivalent in Haskell
Here's an example which uses random-fu: import Data.Random -- for randomness import Text.Printf -- for printf import Data.Foldable -- for the for_ loop -- pdf and cdf are basically “Distribution -> Double -> Double” main = do -- defining normal distribution with mean = 10 and variation = 2 let...
compile-time vs. run-time cost of Hamlet templates
It's been a while since I worked on the code, so don't take this as authoratative (like Daniel mentioned, -ddump-simpl is a good call here). But I don't believe we're using the blaze-html combinators, just the data type. Hamlet itself concatenates strings as much as possible at compile time to...
Haskell: When declaring a class, how can I use a type variable that is not immediately in the constructors?
using TypeFamilies The problem is that you somehow have to connect b with your collection (the elements in it) - there are several ways to do this but I think a rather nice one is using TypeFamilies: {-# LANGUAGE TypeFamilies #-} module Test where import qualified Data.Map as Map import...
perform a function on first elements of list
Your first point-free attempts don't work because . composes functions with one argument, while take takes two. What really happens is that code like decrease_first = map (subtract1) . take is equivalent to decrease_first n = map (subtract1) (take n) but now take n is not a list, hence a...
Haskell - Could not deduce … from Context error - OpenGL AsUniform class type
class,haskell,opengl,types,uniform
Try adding -XFlexibleContexts and the constraint, literally, to your existing answer: {-# LANGUAGE FlexibleContexts #-} mvpUnif :: ( GL.UniformComponent a , Num a , Epsilon a , Floating a , AsUniform a , AsUniform (V4 (V4 a)) ) => (GLState a) -> ShaderProgram -> IO () Usually this is the...
Get each fibbonacci value in haskell
Consider the simpler problem of summing the first 100 positive integers: sum [x | x <- [1,2..], x <= 100] This doesn't work either. As a human, you know that once x <= 100 returns False, it will never return True again, because x is getting larger. But Haskell doesn't...
Generating instances for n-depth structures?
The other answers are quite nice in that they present a single (parameterized) data type that can handle structures of any depth; however, they use advanced type system features to achieve this. On the other hand, there are quite simple features that can be used instead to achieve the same...
How does Frege generalize number literals?
Simple decimal literals without type indicator (i.e. one of the letters lndf) do not automatically have type Int in Frege. They will get assigned the type you probably wanted, and the literal will get adapted accordingly. This is why they are called DWIM (do what I mean) literals. This is...
How can I parse fixed-length, non-delimited integers with attoparsec?
parsing,haskell,integer,attoparsec
Your code snippet is far from being self-contained (in particular, imports and the definition of your Packet data type are missing), but you seem to be overcomplicating things. First, define a parser for one-digit integers. Then, use the latter as a building block for a parser for two-digit integers. After...
Haskell return lazy string from file IO
haskell,file-io,lazy-evaluation
You're right, this is a pain. Avoid using the old standard file IO module, for this reason – except to simply read an entire file that won't change, as you did; this can be done just fine with readFile. readCsvContents :: Filepath -> IO String readCsvContents fileName = do contents...
issues with installing newer cabal version for haskell vim now
ubuntu,haskell,vim,ubuntu-14.04,cabal
Your $PATH variable seems to be broken. In a comment you said it was /home/me/google-cloud-sdk/bin:/.cabal/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games This means that your shell (assumed to be bash) will look in the following directories /home/me/google-cloud-sdk/bin /.cabal/bin /usr/local/sbin /usr/local/bin /usr/sbin /usr/bin /sbin /bin /usr/games /usr/local/games when looking for executable. If you look at the second...
Custom deriving(Read,Show) for enum type
haskell,template-haskell,deriving
The paper Invertible Syntax Descriptions: Unifying Parsing and Pretty Printing describes one particularly idiomatic solution. Your example looks like this, using the invertible-syntax package based on that paper: import Prelude hiding (Applicative(..), print) import Data.Maybe (fromJust) import Text.Syntax import Text.Syntax.Parser.Naive import Text.Syntax.Printer.Naive data TVShow = BobsBurgers | MrRobot | BatmanTAS...
Mapping with IO actions in Haskell
Here is a function f' which does what you describe. f' :: [(String,String)] -> IO [Bool] f' = mapM $ uncurry f Let me know if something is unclear! And, just to be clear, here is how you run it: main = do res <- f' [("a.txt", "b.txt"), ("c.txt", "d.txt")]...
logical expression evaluator Haskell
You're making eval a bit too low-level. By including Literals in the signature. A better way to do this is, is using recursion: eval :: Expression -> Bool eval (Literal x) = x eval (Operation AND x y) = (eval x) && (eval y) eval (Operation OR x y) =...
Easier way to apply multiple arguments in Haskell
haskell,functional-programming,composition,functor,applicative
The Applicative class has the <*> operator (usually pronounced "ap", and is equivalent to Control.Monad.ap for most Monads), which combined with the <$> operator (itself just an infix alias for fmap) lets you write code like -- f :: a -> b -> c -- fa :: Applicative f =>...
How do I reuse an intermediate value in chain of Haskell Either binds?
You have to thread it all the way back up to where you need it. repr <- case playerInput of -- now you have to return something of the appropriate type, -- say, `Left "didn't get it"`, from these other cases. _ -> do let repr = ... >>= whatHandRepresents...
How to extract the final part of the URL's path in Haskell
You can quite easily extract just the final part from a URI. Extract the uriPath from the parsed URI, and then just take in reverse until you hit a slash. For example, getFinalPart :: URI -> String getFinalPart = reverse . takeWhile (/='/') . reverse . uriPath ...
Applicative style understanding
<**> from Control.Applicative is flip <*>. Your example can work with that, slightly rearranged: >((+) <$> Just 3 <*> Just 5) <**> ((+) <$> Just 6) Just 14 ...
Replace all [ ] with {} - as short as possible [on hold]
All you need is love and to split print into putStrLn . show and then add a simple map in-between which does the conversion: main :: IO () main = let fn '[' = '{' fn ']' = '}' fn c = c in (readLn :: IO [Integer]) >>= putStrLn...
Why are takeR, dropR and splitAtR missing from Data.Sequence?
length is O(1), so splitAt suffices to define everything you need, in an efficient way. splitAtR i s = splitAt (length s - i) s takeR i s = snd $ splitAtR i s dropR i s = fst $ splitAtR i s According to the docs, splitAt costs O(log(min(i,length...
Recursion scheme in Haskell for repeatedly breaking datatypes into “head” and “tail” and yielding a structure of results
This looks like a special case of a (jargon here but it can help with googling) paramorphism, a generalisation of primitive recursion to all initial algebras. Reimplementing ListCase Let's have a look at how to reimplement your function using such a combinator. First we define the notion of paramorphism: a...
Keep track of loop without a counter
You can certainly do this without changing the type signature of func :: [Int] -> [Int]: have func call a different function, which takes an extra argument that is the counter you were talking about: func :: [Int] -> [Int] func = go 0 where go _ [] = []...
Haskell stripping function of syntactic sugar
First you need to understand the list data type. Here are the list data constructors: [] :: [a] (:) :: a -> [a] -> [a] where: [] create an empty list (:) takes an a , a list of a and returns a list of a with the new element...
How to use Proxy properly?
You need ScopedTypeVariables and forall introduction to get a scoped type variable: {-# LANGUAGE ScopedTypeVariables #-} bar :: forall a. Foo a => a -> Int bar _ = foo (Proxy :: Proxy a) ...
How can I express foldr in terms of foldMap for type-aligned sequences?
haskell,types,monoids,type-variables,foldable
I found that this typechecks: {-# LANGUAGE RankNTypes #-} module FoldableTA where import Control.Category import Prelude hiding (id, (.)) class FoldableTA fm where foldMapTA :: Category h => (forall b c . a b c -> h b c) -> fm a b d -> h b d foldrTA ::...
GHC complains about overlapping instances when in fact they are not
Instances are only matched by the instance head. So, for the purpose of overlapping or not, what you wrote is no better than instance ToAVector [] a instance ToAVector v a which is clearly overlapping. To argue another way: ... as [] does not match G.Vector [] a ... this...
Can I have an unknown KnownNat?
Here's something potentially interesting... {-# LANGUAGE DataKinds, KindSignatures, ScopedTypeVariables #-} import GHC.TypeLits import Data.Proxy data Bar (n :: Nat) = Bar String deriving Show bar :: KnownNat n => Bar n -> (String, Integer) bar [email protected](Bar s) = (s, natVal b) Ok, it's very pointless. But it's an example of...