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HASKELL QUESTIONS

Can't observe speedup on parallel Haskell code but profiling indicates otherwise
Can't observe speedup on parallel Haskell code but profiling indicates otherwise
should help you out When par compute it's first argument it just force top level node to compute, but it can still refer to some lazy computation.If you change it to
TAG : haskell
Date : October 24 2020, 06:10 PM , By : Hang Fu
Binary representation of a string
Binary representation of a string
This might help you You can get where you need to go with fromEnum (or its specialized version ord) and showIntAtBase.
TAG : haskell
Date : October 23 2020, 06:10 PM , By : Jimis o armanis
Sorting a part of a list without using library functions
Sorting a part of a list without using library functions
With these it helps You don't need to involve tuples. You can write what you wrote for tuples as patterns to match instead.
TAG : haskell
Date : October 23 2020, 06:10 PM , By : Jack Stasney
Is there an efficient, lazy way to fuse foldMap with traverse?
Is there an efficient, lazy way to fuse foldMap with traverse?
fixed the issue. Will look into that further I don't believe it's possible. Avoiding fmaps at the elements seems to require some knowledge of the structure of the container. For example, the Traversable instance for lists can be written
TAG : haskell
Date : October 23 2020, 06:10 PM , By : NeeCheats
Do IO operations run in green threads?
Do IO operations run in green threads?
will help you From the documentation of Control.Concurrent.Async
TAG : haskell
Date : October 23 2020, 06:10 AM , By : Mamun
Manipulating a value within several layers of monads
Manipulating a value within several layers of monads
I wish this help you Apart from all the syntax issues, I think what you are trying to do is the following.This is how you declare a data type:
TAG : haskell
Date : October 23 2020, 06:10 AM , By : Israel Montes
Haskell Bool to Char function seems to have the wrong type
Haskell Bool to Char function seems to have the wrong type
wish help you to fix your issue Variable names must start with lower-case letters (or non-: symbols if they are infix names), so that the compiler can easily distinguish them from constructors.
TAG : haskell
Date : October 22 2020, 06:10 AM , By : Mahdi Nsaif Jasim
Define a function replic lt in Haskell
Define a function replic lt in Haskell
This might help you What you could do is solve the problem using explicit recursion and then try to detect places where you can replace certain parts of the recursion with a higher-order function. Here are some hints for creating an explicitly recurs
TAG : haskell
Date : October 21 2020, 06:10 PM , By : Rudy Flores
The relationship between generic-lens and DataKinds GHC extension
The relationship between generic-lens and DataKinds GHC extension
it fixes the issue DataKinds is an extension that allows data types to be promoted to the type-level. What this means precisely is kind of a can of worms (it's one step on the way to dependent types), so I'll try and keep this explanation focused on
TAG : haskell
Date : October 20 2020, 06:10 AM , By : JCYoky
(new to haskell) if statment generating parse error
(new to haskell) if statment generating parse error
I wish did fix the issue. In Haskell, if clauses are expressions. That implies that they need to always evaluate to a value, and also to have a definite type. More specifically, if clauses without an else are syntax errors. Also, the then and else ca
TAG : haskell
Date : October 19 2020, 06:10 PM , By : Eduard Herzog
Default type of id function
Default type of id function
should help you out In short: id [] has type [a], the exact type will depend on the context of the caller.We can perform the reasoning of the types ourselves. We basically have two items here: id and [], id has as type id :: a -> a, and [] is just a
TAG : haskell
Date : October 19 2020, 06:10 PM , By : tongfu
How to get element's index in list when using foldl
How to get element's index in list when using foldl
this will help We can implement this in a more simple way. We can consider a infinite list where we repeat two functions: id :: a -> a, and negate :: Num a => a -> a, and use cycle :: [a] -> [a] to construct an infinite list. So cycle [id, negate] wi
TAG : haskell
Date : October 19 2020, 06:10 PM , By : Iconic Coin
Implementing monad on recursive type
Implementing monad on recursive type
it should still fix some issue To overcome your specific challenge, you should try pattern matching on the result of f x.
TAG : haskell
Date : October 17 2020, 06:10 PM , By : helpcentre.us
Why doesn't Haskell allow more complex infix expressions?
Why doesn't Haskell allow more complex infix expressions?
this one helps. Why doesn't Haskell allow more complex infix expressions, eg. , It can be faked
TAG : haskell
Date : October 17 2020, 06:10 PM , By : ธงชัย บุญอินทร์
Catamorphism that allows looking at part of the final result
Catamorphism that allows looking at part of the final result
Hope this helps Though this is not necessarily what you were looking for, we can encode the laziness trick with a hylomorphism:
TAG : haskell
Date : October 17 2020, 06:10 AM , By : Zoe Williams
Good examples of not a Contravariant/Contravariant/Divisible/Decidable?
Good examples of not a Contravariant/Contravariant/Divisible/Decidable?
should help you out The Contravariant family of typeclasses represents standard and fundamental abstractions in the Haskell ecosystem: , (partial answer)Not contravariant
TAG : haskell
Date : October 16 2020, 06:10 PM , By : Marie Swarzenski
Replace element in list
Replace element in list
hope this fix your issue you don't do mutation on the list but create a new list from the given list.an example, without any error handling...
TAG : haskell
Date : October 14 2020, 06:10 PM , By : oliverfjenkins
How to display hashmap in Haskell
How to display hashmap in Haskell
To fix the issue you can do Given that the keys k and the values v are instances of Show, then a HashMap is an instance of Show as well. Indeed in the source code, we see:
TAG : haskell
Date : October 14 2020, 03:00 AM , By : Blije Komne
How can I concisely match a list according to its latter half, and bind it's former half accordingly?
How can I concisely match a list according to its latter half, and bind it's former half accordingly?
I hope this helps you . On built-in String, this is a very bad idea, since your pattern match turns out to be quite expensive. On other string types, like Text or ByteString, you can use pattern guards:
TAG : haskell
Date : October 14 2020, 01:00 AM , By : Daike Chibueze
Insertion in a binary search tree(data stored at leaf node only)
Insertion in a binary search tree(data stored at leaf node only)
should help you out The technique you require to solve this problem is called pattern matching combined with conditional branching (if / pattern guards).You can do this either by using a case statement or by defining alternative function definitions
TAG : haskell
Date : October 13 2020, 11:00 PM , By : Sreekumar Hariharan
Listing Items in Haskell
Listing Items in Haskell
I hope this helps you . Assuming Item.Item is the name of a data constructor with record syntax, you simply need to use it twice.
TAG : haskell
Date : October 13 2020, 10:00 PM , By : Diana
Custom Filter Function with Predicate using List Comprehension
Custom Filter Function with Predicate using List Comprehension
This might help you I need to develop my own filter function similar to how filter works in Haskell but by using list comprehension and a predicate. So I would put lcFilter (>3) [1,2,3,4,5,6,10,444,3] in ghci and it would print all numbers greater th
TAG : haskell
Date : October 13 2020, 09:00 PM , By : Robin
How can I read mutiple lines from stdin within GHCI more than once?
How can I read mutiple lines from stdin within GHCI more than once?
around this issue In contrast to the information in "learn you a haskell", on my windows system, ghci translates CTRL-D to EOT, not EOF. , Here is the function I was looking for:
TAG : haskell
Date : October 13 2020, 08:00 PM , By : Nick R
How to make type conversion in Haskell?
How to make type conversion in Haskell?
wish help you to fix your issue In your call truncate (sqrt 5) + mod 5 2, the 5s have different types. Indeed the 5 in the sqrt 5 should have as type Floating a => a, whereas the 5 in mod 5 2 has type Integral b => b. Although it is, strictly speakin
TAG : haskell
Date : October 13 2020, 08:00 AM , By : Malcolm Wright
Inferred generic function typechecks as a return type but not an argument type
Inferred generic function typechecks as a return type but not an argument type
this one helps. I'm learning about SYB and rank n types, and came across a confusing case of what seems like the monomorphism restriction. , In your second snippet, op is actually not polymorphic.
TAG : haskell
Date : October 13 2020, 05:00 AM , By : Aladdin Adel
screen doesn't show, Console.ReadLine() in Haskell?
screen doesn't show, Console.ReadLine() in Haskell?
This might help you getLine reads a whole line (returning that as a string, but you don't need that).
TAG : haskell
Date : October 13 2020, 12:00 AM , By : jenny sk
Using "Either" in Data Validation
Using "Either" in Data Validation
wish helps you Either's Monad and Applicative instances can't accumulate errors: by law, they must stop at the first Left value. So if you want to use Either to accumulate errors, you have to do it by hand, not through the Applicative or Monad instan
TAG : haskell
Date : October 12 2020, 10:00 PM , By : Paulo Vinicius Silva
making permutation from two list but not using full number
making permutation from two list but not using full number
it helps some times Making a guess at what your current code does, here's a proposal for an algorithm: nondeterministically pick a permutation of the vowels assign indices to the permutation use the standard tails trick to nondeterministically choose
TAG : haskell
Date : October 12 2020, 07:00 PM , By : Sri
haskell : calculated fields in custom data types
haskell : calculated fields in custom data types
it helps some times Is it possible to do something that is equivalent to having a field in a data type that is automatically calculated from other fields in the data type. For example: , Without any additional requirement, that's simply a function.
TAG : haskell
Date : October 12 2020, 04:00 PM , By : Lacey Lee Huang
Defining append using foldr
Defining append using foldr
To fix this issue foldr needs three arguments: an accumulator function, an initial accumulator value, and a list (well, a Foldable).foldr (:), therefore, requires two more arguments: initial accumulator, and list.
TAG : haskell
Date : October 12 2020, 01:00 PM , By : Mr_Ex
Example of non-trivial functors
Example of non-trivial functors
wish of those help Here's kind of a fun argument I just stumbled on. (It's late so I wonder if it will be sensical tomorrow)We can construct the type of proofs of SK reducibility as a GADT:
TAG : haskell
Date : October 12 2020, 12:00 PM , By : Josh
Quantified constraints vs. (closed) type families
Quantified constraints vs. (closed) type families
seems to work fine I don't think you can do such thing, but I could certainly be wrong. In your example you are missing an extra constraint Show (f a) in order for it to be complete:
TAG : haskell
Date : October 12 2020, 05:00 AM , By : Caner Oktem
Increment positive numbers and decrement negative number all by 1
Increment positive numbers and decrement negative number all by 1
fixed the issue. Will look into that further $ is going to bind looser than any other operator, so you need explicit parentheses. Also, you don't need to deconstruct the list since you're just putting it back together (the x:xs pattern is unnecessary
TAG : haskell
Date : October 12 2020, 05:00 AM , By : Peppedoc Brown
Why doesn't Gloss render in native resolution?
Why doesn't Gloss render in native resolution?
it should still fix some issue Gloss can be used with native resolution under OSX with hdpi-display when the default window management backend GLUT is exchanged for GLFW. To do this rebuild Gloss with the appropriate flags:cabal install -f -GLUT -f G
TAG : haskell
Date : October 11 2020, 10:00 PM , By : Stiven Baci
Is there anything wrong with wrapping Data.Sequence in my own Queue type?
Is there anything wrong with wrapping Data.Sequence in my own Queue type?
To fix this issue Nothing wrong at all - in fact, that's precisely what newtype is for!Ideally, your types should reflect your intentions - whoever reads your code (including yourself, in the future, when you have forgotten what you did and why) shou
TAG : haskell
Date : October 11 2020, 08:00 PM , By : FunCod
Is Traversable different for breadth-first and depth-first trees?
Is Traversable different for breadth-first and depth-first trees?
may help you . Traversable has to agree with Foldable. Specifically, if Monoid m, then Applicative (Const m), causing a consistency law foldMap f = getConst . traverse (Const . f). It is thus impossible for Breadth and Depth to share a Traversable. T
TAG : haskell
Date : October 11 2020, 08:00 PM , By : Masaka Frances
Haskell: Order of evaluation for (++) and (:) operators
Haskell: Order of evaluation for (++) and (:) operators
I think the issue was by ths following , Both the (++) :: [a] -> [a] -> [a] and (:) :: a -> [a] -> [a] function have 5 as precedence, as we can read in the Haskell '10 report, and are right associative.We can also obtain this data with :i in the ghci
TAG : haskell
Date : October 11 2020, 10:00 AM , By : 姣哥哥哥
Is it possible to design Stack (Haskell) just like NPM (NodeJS)?
Is it possible to design Stack (Haskell) just like NPM (NodeJS)?
This might help you Yes, it is possible. In fact, cabal used to work that way by default. It was abandoned in the move to nix-style builds, but as far as I know there's no technical problem standing in the way of doing it again.That said, my experien
TAG : haskell
Date : October 11 2020, 08:00 AM , By : Staybyme Belleza
Is there a standard Haskell function with type: (Floating a, RealFrac b) => a -> b?
Is there a standard Haskell function with type: (Floating a, RealFrac b) => a -> b?
wish of those help I need to call floor() on a value, which is only constrained to be of class Floating, but floor() requires RealFrac. , Consider the following instance of Floating:
TAG : haskell
Date : October 11 2020, 07:00 AM , By : egghead
How values are parsed to this insert function in haskell
How values are parsed to this insert function in haskell
With these it helps This represents a set as a function that, given an Int, tells you if it is in the et (by returning True) or not (by returning False).The empty set is thus the function that returns False no matter what argument you give.
TAG : haskell
Date : October 11 2020, 05:00 AM , By : WG Choi
"Generalized arrows" and proc notation?
"Generalized arrows" and proc notation?
should help you out I've wondered about that before, too. But – proc notation is so widely considered a silly oddball that there's probably not much interest in generalisation either (though I daresay this is what would make it actually useful!)Howev
TAG : haskell
Date : October 11 2020, 03:00 AM , By : cxcx_cxy
Why provide the `HasCallStack` mechanism since we already have `ghc -prof -fprof-auto-top` in GHC?
Why provide the `HasCallStack` mechanism since we already have `ghc -prof -fprof-auto-top` in GHC?
will help you AFAIK, there are two ways to get a call stack for debugging in Haskell: , HasCallStack has a few basic advantages:
TAG : haskell
Date : October 11 2020, 12:00 AM , By : Buchi
How to write this code shorter and so that it may be used for a wider purpose?
How to write this code shorter and so that it may be used for a wider purpose?
hop of those help? It does not make much sense to use a list in the first place. A list should be used for a colleciton of items. But here "Mathematics" (the Name), and "BSc" (the Type) are orthogonal items. They only "happen" to have the same type.
TAG : haskell
Date : October 10 2020, 11:00 PM , By : Esteban Ruiz Romero
"cabal new-install Cabal cabal-install" answered "cabal.exe: Could not read index. Did you call 'checkFor
"cabal new-install Cabal cabal-install" answered "cabal.exe: Could not read index. Did you call 'checkFor
will help you I have tried the "cabal -v3 v2-update" and it showed me the file that was missing and its path. At first I tried to restore the file by hand, but someone suggested erasing all the files related to the missing one, and, voilá, cabal rege
TAG : haskell
Date : October 10 2020, 09:00 PM , By : Sandra Nicole
How to run Haskell on GraalVM?
How to run Haskell on GraalVM?
it helps some times This is not a complete answer, but it's most of the way there. It is not too complicated:The Haskell runtime is just a library that you can find in your GHC installation. On my Mac it's in $PREFIX/lib/ghc-$VERSION/rts, where $PREF
TAG : haskell
Date : October 10 2020, 06:00 PM , By : Shambhunath Yadav
How to accumulate changes without renaming variable?
How to accumulate changes without renaming variable?
like below fixes the issue I have a list of Strings, errors. I do some checks and if any fail, I append a message to errors. Something like this: , You could group the conditions and messages together
TAG : haskell
Date : October 10 2020, 11:00 AM , By : Noxx
Do monad transformers, generally speaking, arise out of adjunctions?
Do monad transformers, generally speaking, arise out of adjunctions?
Hope this helps The three constructions in this answer are also available, in a reproducible form, in this Gist.Simon C's construction...
TAG : haskell
Date : October 10 2020, 11:00 AM , By : Jenna Steller
How do you include a multiline number in a Haskell source file?
How do you include a multiline number in a Haskell source file?
Does that help You could make a quasiquoter. Might even want to upload it to hackage:
TAG : haskell
Date : October 10 2020, 10:00 AM , By : Marie-Flo Adj
doing trivial search and replace via a regex
doing trivial search and replace via a regex
Hope that helps You can use the (=~) form that returns the "before", "matched", and "after" parts. To replace only the first occurrence, use:
TAG : haskell
Date : October 10 2020, 08:00 AM , By : Santhosh N
Haskell List functions
Haskell List functions
help you fix your problem Haskell's function has two couples of list functions: head :: [a] -> a, and tail :: [a] -> [a]; and init :: [a] -> [a] and last :: [a] -> a.
TAG : haskell
Date : October 10 2020, 07:00 AM , By : Alexandru Cimpean
How do I import a package I have installed from stackage?
How do I import a package I have installed from stackage?
wish helps you to Willem Van Onsem for pointing out that I need to be importing modules, not packages. In this case, import Data.Text.Prettyprint.Doc was the appropriate choice.
TAG : haskell
Date : October 10 2020, 06:00 AM , By : bitsby
Difference between -> and => symbols. What do they mean?
Difference between -> and => symbols. What do they mean?
hop of those help? -> is for explicit functions. I.e. when f is something that can be written in an expression of the form f x, the signature must have one of these arrows in it†. Specifically, the type of x (the argument) must appear to the left of
TAG : haskell
Date : October 10 2020, 06:00 AM , By : b1550659
How to get better error messages with type-families?
How to get better error messages with type-families?
wish of those help Sure it's possible to provide better compile-time error message using custom type errors. I've described how to use them in my blog post: A story told by Type Errors
TAG : haskell
Date : October 09 2020, 01:00 PM , By : Serhii Marchuk
Retrieve values from datatype
Retrieve values from datatype
I wish did fix the issue. The only way to get values out of a data constructor is indeed to pattern-match on it. If you want, you can define a function that does it, as you have done with nodeId, or you can just do the pattern matching inline. For ex
TAG : haskell
Date : October 09 2020, 11:00 AM , By : nirvana20
How to "extend" classes in Haskell
How to "extend" classes in Haskell
I wish this helpful for you I want to create two typeclasses, A and B, where A is a superclass of B. The functions defined in B are sufficient to implement those in A. Then, if I have a function with the constraint fun :: (A thing) => ... an instance
TAG : haskell
Date : October 09 2020, 05:00 AM , By : Muhiddin Qurbonov
Get the source directory of a Haskell Stack & Turtle script from within the script itself
Get the source directory of a Haskell Stack & Turtle script from within the script itself
I hope this helps you . I think you can use the executable-path package to get this information:https://www.stackage.org/haddock/nightly-2019-07-07/executable-path-0.0.3.1/System-Environment-Executable.html
TAG : haskell
Date : October 09 2020, 04:00 AM , By : Anderson Ferraz
what is the use of "either" type constructor in hakell
what is the use of "either" type constructor in hakell
this will help Either is a tagged union [wiki]. It means it can take values from (in this case) two types. If Code is for example not a string, your function will need some sort of sum type. How will it otherwise communicate a Code or a String? Eithe
TAG : haskell
Date : October 09 2020, 04:00 AM , By : Nikhil Pore
Couldn't match expected type [a] with actual type Maybe[a]
Couldn't match expected type [a] with actual type Maybe[a]
hope this fix your issue please pardon for very beginner level quesion. , So you have
TAG : haskell
Date : October 09 2020, 03:00 AM , By : Ngcebo Ngcebo Ndwand
How to use toSing from singletons library?
How to use toSing from singletons library?
wish helps you I'm a newbie when it comes to the singleton library, and may have bitten off more than I can chew here. , If you compile the code:
TAG : haskell
Date : October 09 2020, 02:00 AM , By : Regis
How to access data in list of elements of custom type?
How to access data in list of elements of custom type?
seems to work fine There is a data type Ticket with four attributes and then i created Trip that is a list of Tickets. I want to check if a trip is valid by checking if for any consecutive tickets the destination city of the first ticket is same as t
TAG : haskell
Date : October 09 2020, 01:00 AM , By : sutance

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